[gimp-web/testing] Added tutorial 'Tone Mapping using GIMP Levels' by Elle Stone
- From: Pat David <patdavid src gnome org>
- To: commits-list gnome org
- Cc:
- Subject: [gimp-web/testing] Added tutorial 'Tone Mapping using GIMP Levels' by Elle Stone
- Date: Mon, 2 May 2016 20:57:58 +0000 (UTC)
commit 3c5acb8cb1d3196789743ef7a608783b54094a59
Author: Pat David <patdavid gmail com>
Date: Mon May 2 15:56:57 2016 -0500
Added tutorial 'Tone Mapping using GIMP Levels' by Elle Stone
Ported tutorial from Elle's site + assets to work on wgo.
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+Title:Tone mapping and shadow recovery using GIMP Levels
+Date: 2016-03
+Modified: 2016-03
+Author: Elle Stone
+canonical: http://ninedegreesbelow.com/photography/gimp-tone-map-with-levels.html
+
+<small>
+<a href='http://creativecommons.org/licenses/by-sa/3.0/'>
+<img class='cc-badge' src='http://i.creativecommons.org/l/by-sa/3.0/80x15.png' alt='Creative Commons By
Share Alike'/>
+</a>
+<span xmlns:dct="http://purl.org/dc/terms/";>GIMP Tutorial - Tone Mapping Using GIMP Levels (text &
images)</span> by [Elle Stone](http://ninedegreesbelow.com/) is licensed under a [Creative Commons
Attribution-ShareAlike 3.0 Unported License](http://creativecommons.org/licenses/by-sa/3.0/).
+</small>
+
+
+<!-- BEGIN ARTICLE -->
+
+A very common editing problem is how to lighten the shadows and midtones of an image while retaining
highlight details, a task sometimes referred to as "shadow recovery" and more generally speaking as "tone
mapping". This step-by-step tutorial shows you how to use GIMP's unbounded Levels operations to add one or
more stops of positive exposure compensation to an image's shadows and midtones while retaining highlight
details.
+
+**Page Contents**
+
+[TOC]
+
+
+## High bit depth GIMP unbounded Levels: much better than Curves for lightening the shadows and midtones of
an image without blowing the highlights
+
+A very common editing problem is how to lighten the shadows and midtones of an image without blowing out the
highlights, which problem is very often encountered when dealing with photographs of scenes lit by direct
sunlight. Precanned algorithms for accomplishing this task are often referred to as "shadow recovery"
algorithms. But really these algorithms are special-purpose tone-mapping algorithms, which sometimes work
pretty well, and sometimes not so well, depending on the algorithm, the image, and your artistic intentions
for the image.
+
+This step-by-step tutorial shows you how to use GIMP's unbounded floating point Levels operations to recover
shadow information — that is, add one or more stops of positive exposure compensation to an image's
shadows and midtones — without blowing out or unduly compressing the image highlights. The procedure is
completely "hand-tunable" using masks and layers, and is as close as you can get to non-destructive image
editing using high bit depth GIMP 2.9/2.10.
+
+<figure>
+<img src='power-lines.jpg' alt='power-lines'>
+<figcaption>
+<cite>Power lines at noon</cite>, before and after tone mapping (aka "shadow recovery") using high bit depth
GIMP's unbounded floating point Levels.<br>
+<span class='toggle-swap' data-fig-swap='before-auto-stretch-contrast.jpg'>Scene-referred interpolated raw
file.</span><br>
+<span class='toggle-swap' data-fig-swap='power-lines.jpg'>After tone mapping/shadow recovery using GIMP
unbounded Levels.</span>
+</figcaption>
+</figure>
+
+<noscript>
+<figure>
+<img src='before-auto-stretch-contrast.jpg' alt='before-auto-stretch-contrast'>
+<figcaption>
+Scene-referred interpolated raw file.
+</figcaption>
+</figure>
+</noscript>
+
+<!--
+Accepted wisdom is that Curves provides more versatility than Levels for modifying image tonality. Of course
you can use Curves to accomplish somewhat similar results to the procedure described in this tutorial. But
GIMP Curves is a highly destructive editing operation, immediately "baking" the resulting tonality into the
layer on which Curves is done, clipping any out of gamut RGB values, and leaving you with no way to fine-tune
the results without starting over. -->
+
+Hight bit depth GIMP is my primary image editor, and I've used the procedure described below for the last
couple of years as my "go to" way to modify image tonality. The same general procedure can be used to darken
as well as lighten portions of an image, again controlling the effect using a layer mask. This isn't exactly
nondestructive editing because at some point you need to make a "New from Visible" layer. But unlike using
Curves, using high bit depth GIMP's Unbounded Levels doesn't clip RGB channel values and allows you to
fine-tune the results by modifying and remodifying the layer mask until you are completely happy with the
resulting tonality.
+
+
+
+## A worked example showing how to recover shadow information using high bit depth GIMP's unbounded Levels
+
+<figure>
+<img src='tree-exposed-for-sky-ground-is-dark.jpg' alt='tree-exposed-for-sky-ground-is-dark'>
+<figcaption>
+Using high bit depth GIMP Levels to lighten the ground by one stop without blowing out portions of the sky.
+
+<ul>
+<li><em>Left:</em> The original image, an interpolated camera raw file that was deliberately underexposed in
camera to avoid blowing out the sky. It might not appear to be the case, but this image is already very close
to having out of gamut RGB channel values in the sky, and a simple Auto Stretch Contrast won't lighten the
image at all.</li>
+<li><em>Right:</em> The same image after using high bit depth GIMP Levels at 32-bit floating point linear
precision to add one stop of positive exposure compensation. An inverse grayscale mask was used to keep the
highlights from blowing out.</li>
+</ul>
+
+</figcaption>
+</figure>
+
+
+This worked example provides a sample image and is broken down into five steps, starting with downloading
the image. Steps 3, 4, and 5 describe the actual procedure, so here's an overview:
+
+<ol start='3' style='padding-left: 3rem; font-style: italic;' markdown=1 >
+
+3. Duplicate the base layer and then use GIMP's unbounded Levels to add one stop of positive exposure
compensation to the duplicated layer.
+4. Add an inverse grayscale layer mask to the now much brighter duplicated layer.
+5. Do "Auto/Stretch Contrast" on the layer mask and then fine-tune the mask until you are happy. Then make a
"New from Visible" layer.
+
+</ol>
+
+---
+
+<ol style='max-width: 35rem;' markdown=1>
+
+1. **Download [tree.png](tree.png)**, which is a 16-bit integer sRGB image. High bit depth GIMP really is an
"sRGB only" image editor, so it's best if you don't even try to edit in other RGB working spaces (if you
prefer a larger RGB working space, my [patched version of GIMP uses the Rec.2020 color space instead of
sRGB](http://ninedegreesbelow.com/photography/patch-gimp-in-prefix-for-artists.html)).
+
+2. **Open tree.png and convert it to 32-bit floating point linear precision**: Go to "Image/Precision",
select "32-bit floating point", and when the Dither dialog pops up, select "Linear light".
+
+<li markdown=1>
+Make a copy of the "tree.png" layer, and label it "Levels: +1 stop exp. comp". Then **use Levels to add one
stop of positive exposure compensation** — Figure 3 below shows the proper settings for the Levels
dialog, and Figure 4 shows the result:
+
+<figure>
+<img src="levels-add-one-stop-positive-exposure-compensation.jpg"
alt="levels-add-one-stop-positive-exposure-compensation" >
+<figcaption>
+Using Levels to add one stop of positive exposure compensation — <i>make sure the "Gamma hack" box is
checked!</i>
+</figcaption>
+</figure>
+
+When using Levels to add one stop of positive exposure compensation, make sure the image really is at
floating point precision, because integer precision will clip the highlights. Also make sure the "Gamma hack"
box is checked — <a title="GIMP bug report: Curves and Levels should operate by default on linear RGB
and present linear RGB Histograms." href="https://bugzilla.gnome.org/show_bug.cgi?id=757444";>otherwise the
Levels operation will operate on perceptually uniform RGB</a>, despite the fact that the image is at "Linear
light" precision. Trying to add one stop of positive exposure compensation using RGB values encoded using the
sRGB TRC would require moving the upper right Levels Value slider to the entirely unobvious and nonintuitive
value of 73.53, and in the process the shadow and midtone tonalities would be distorted because the sRGB TRC
is not a true gamma TRC. (An easier alternative might be to use the GEGL Exposure operation:
"Colors/Exposure", which always
operates on linearized RGB).
+
+<figure>
+<img width="772" src="one-stop-positive-exposure-compensation-added.jpg"
alt="one-stop-positive-exposure-compensation-added" >
+<figcaption>At floating point precision, GIMP's Levels operations are unbounded. This means you can use
Levels to add positive exposure compensation without blowing out the highlights.<br>
+Notice the RGB channel values for the four sample points: the channel information that would have been
clipped using integer precision is encoded using channel values that are greater than 1.0 floating point.
+</figcaption>
+</figure>
+
+The image in Figure 4 clearly has "blown" highlights in the sky. But the highlights aren't really blown
(that is, clipped to 1.0 in one or more channels). Instead the highlight information is still there, but the
RGB channel values fall outside the RGB <a title="Models for image editing: Display-referred and
scene-referred." href="http://ninedegreesbelow.com/photography/display-referred-scene-referred.html";>display
channel value range</a> of 0.0f to 1.0f. The sample points dialog in Figure 4 above shows four sample points
that have RGB channel values that are greater than 1.0. As shown in Figure 5 below, adding a mask allows you
to recover these highlights by bringing them back down into the display range.
+
+If you had used integer precision instead of floating point, the highlights really would be blown: The
sample points would have a maximum channel values of 255, 65535 or 4294967295, depending on the bit depth.
And masking would only "recover" a solid expanse of gray, completely lacking any details (try for yourself
and see what happens).
+</li>
+
+<li><img style='float: right;' src="add-inverse-grayscale-mask.jpg" alt="add-inverse-grayscale-mask" >
+<b>Add an inverse grayscale layer mask</b>: Right-click on the layer and select "Layer/Mask/Add Layer Mask",
and when the "Add a mask to the Layer" dialog pops up, choose "Grayscale copy of layer" and check the "Invert
mask" box. <p>As shown in Figure 5 below, at this point the highlights will be brought back into the display
range, meaning all RGB channel values are between 0.0f and 1.0f. But the image will probably look a little
odd (sort of cloudy and flat), and depending on the image, the brightest highlights might actually have dark
splotches — don't worry! this is temporary.</p>
+
+
+<figure >
+<img width="772" src="inverse-grayscale-mask-added.jpg" alt="inverse-grayscale-mask-added" >
+<figcaption>Result of adding an inverse grayscale layer mask to bring the highlights back into the display
range.<br>
+Adding an inverse grayscale layer mask brings the highlights back into the display range, but at this point
most images will look flat and cloudy, and some images will have dark splotches in the highlights. The next
step — "Auto Stretch Contrast" performed on the mask — will take care of this problem.
+</figcaption>
+</figure>
+</li>
+
+
+<li>
+<img src="auto-stretch-contrast-mask.jpg" alt="auto-stretch-contrast-mask" style='float:right; margin-left:
1rem;' >
+<b>Click on the layer mask to select it for editing, and then select "Colors/Auto/Stretch Contrast"</b>:
+<p>"Keep Colors" should be checked (though it doesn't really matter on a layer mask) and <i>the "Gamma hack"
box should <b>not</b> be checked!</i> Unlike Levels, Stretch Contrast operates on linear RGB by default.
Figure 6 below shows the final result:</p>
+
+<figure>
+<img src="mask-auto-stretched.jpg" alt="mask-auto-stretched" >
+<figcaption>Doing "Auto/Stretch Contrast" on the layer mask removes the "cloudy" appearance, leaving a
nicely brightened image with intact highlights.
+<p>"Auto/Stretch Contrast" on the mask is necessary because just like the image layer has out of gamut RGB
channel values, the inverted grayscale mask contains out of gamut grayscale values. "Auto/Stretch Contrast"
brings all the mask grayscale values back into the display range, allowing the mask to proportionately
compensate for the layer's otherwise out-of-gamut RGB channel values, masking more in the layer highlights
and less/not at all in the image's shadows and midtones.</p>
+
+<p>Notice that one of the sample points still has a blue RGB channel value that is slightly out of gamut.
The easiest way to deal with this is to do a Levels Gamma=0.45 adjustment <i>on the mask</i>, not on the
actual image layer. You can make this gamma adjustment either on the entire mask (works well, less effort).
Or else you can make the adjustment just on the mask shadows (which correspond to the layer highlights), in
which case you'd load the mask as a selection, invert the selection, and make the Levels gamma=0.45 slider
adjustment. Or if the remaining out of gamut channel values are only very slightly out of gamut, make a "New
from Visible" layer and then "Auto/Stretch Contrast" the result to bring the remaining channel values back
into gamut.</p>
+</figcaption>
+</figure>
+</li>
+
+</ol>
+
+That's the whole procedure for using Levels to add a stop of positive exposure compensation to the shadows
without blowing out the highlights. Now you can either fine-tune the mask, or else just make a "New from
Visible" layer and continue editing your nicely brightened image. Depending on the image and also on your
artistic intentions for the image, the mask might not need fine-tuning. But very often you'll want to modify
the resulting tonal distribution by doing a Levels gamma correction or Curves operation on the mask, or even
by painting directly on the mask. And sometimes you'll want to blur the mask to <a
href="#blur-to-restore-micro-contrast">restore micro contrast</a>.
+
+
+
+
+## Use Notes
+<ol class="double-space">
+
+<li>Depending on your particular artistic intentions for an image, <b>some images are more likely than
others to benefit from being tone mapped using unbounded Levels.</b> Your mileage may vary, but typically the
procedure described on this page works best for photographs of scenes with a pronounced tonal difference
between the highlights and shadows, as per typical sunny day "sky-ground" photographs.</li>
+
+<li><b>For adding just one stop of positive exposure compensation, the procedure described on this page
works really well.</b> Depending on the image you might want to blur the mask using an edge-respecting blur
algorithm, and/or tweak the mask using the Levels gamma slider, Curves, etc. But only modify the mask
<i>after</i> using Auto Stretch Contrast on the mask. Otherwise results will be unpredictable: <a
href="http://ninedegreesbelow.com/photography/unbounded-srgb-levels-gamma-slider.html";>Levels gamma slider
adjustments produce odd results when operating on out of gamut values</a>, and Curves will summarily clip out
of gamut values.</li>
+
+
+<li><b>For adding more than one stop of exposure compensation, you can use one or several
positive-exposure-compensation layers.</b> Either way the layer mask(s) will need careful tweaking that's
very image-specific and also specific to your intended result. An example using two layers to add two stops
of exposure compensation is show in Figure 7 below:
+
+<figure>
+<img src="orchard-truck.jpg" alt="orchard-truck" >
+<figcaption>Using GIMP's floating point unbounded Levels plus layer masks to add two stops of positive
exposure compensation to the shadows and midtones of a photograph of an apple orchard truck that was taken in
bright sunshine.
+
+<ol>
+<li><i>Top:</i> the final image after using high bit depth GIMP's floating point unbounded Levels to add two
stops of positive exposure compensation to the image shadows and midtones, plus a bit of GEGL saturation
masked to mostly affect the sky.</li>
+<li><i>Lower left:</i> the original image from the camera, underexposed to avoid blowing out highlights.</li>
+<li><i>Lower right:</i> for comparision, the result of using GEGL's Mantiuk tone mapping on the original
image, using the default settings. IMHO, Mantiuk and other "automagic" tone mapping algorithms tend to
produce rather artificial results compared to what you can achieve using masks and layers combined with
GIMP's unbounded floating point Levels.</li>
+</ol>
+<p style='text-align: center;'>
+ <img class="imgcenter" src="orchard-truck-layer-stack.jpg" alt="orchard-truck-layer-stack" >
+</p>
+<div style='clear:both;'></div>
+<p>This screenshot of the layer stack shows the individual layers and masks that were used to add the two
stops of positive exposure compensation to the shadows and midtones of the photograph of the apple orchard
truck.</p>
+</figcaption>
+</figure>
+</li>
+
+<li>Before using Levels to add positive exposure compensation, <b>the base layer should already be stretched
to its maximum dynamic range.</b> The easiest way to stretch the base layer to its maximum dynamic range is
to do "Colors/Auto/Stretch Contrast" and make sure that "Keep colors" is checked (and <i>don't</i> use the
Gamma hack).
+
+<p>If you've never used an unbounded floating point image editor before, "Colors/Auto/Stretch Contrast" can
produce an unexpected result: The image might actually end up with a severely reduced dynamic range, having
either lighter shadows or darker highlights or both:</p>
+
+<figure>
+<img src='before-auto-stretch-contrast.jpg' alt='before-auto-stretch-contrast'>
+<figcaption>
+Before and after doing "Colors/Auto/Stretch Contrast" on the base layer, plus the final image after tone
mapping using Levels.<br>
+(Click to view):<br>
+<span class='toggle-swap' data-fig-swap='before-auto-stretch-contrast.jpg'>1. Image from the
camera.</span><br>
+<span class='toggle-swap' data-fig-swap='after-auto-stretch-contrast.jpg'>2. After doing
"Colors/Auto/Stretch Contrast".</span><br>
+<span class='toggle-swap' data-fig-swap='power-lines.jpg'>3. Final "Power lines" image.</span>
+
+<ol style='margin-top: 1rem;'>
+<li>
+This scene-referred interpolated raw file from the PhotoFlow raw processor (which provides a GIMP plug-in
for easy opening of raw files) has out-of-display-range RGB channel values that will be brought back into the
display range by doing "Colors/Auto/Stretch Contrast".</li>
+<li>
+After doing "Colors/Auto/Stretch Contrast", shadows are lighter and highlights are darker because the
dynamic range has been compressed to fit within the display range. This looks like an editing step in the
wrong direction! but actually it's necessary.</li>
+<li>Here's the final "Power lines" image after tone mapping the scene-referred interpolated raw file using
the procedure described in this tutorial.</li>
+</ol>
+
+<p>As captured by the raw file, this picture of power lines marching into the distance is a typical result
of taking a photograph at noon on a bright sunny day: The sky and clouds looked pretty good right out of the
camera, but the ground was far too dark. So the image could benefit from some tone mapping to raise the
shadows and midtones. The first step is to do "Colors/Auto/Stretch Contrast" to bring any channel values that
are less than 0.0f or greater than 1.0f back within the display range of 0.0 to 1.0 floating point.</p>
+
+<p>Performing "Auto/Stretch Contrast" to bring the channel values back inside the display range doesn't
exactly look like an editing step in the right direction for tone-mapping this particular image! but really
it is. Using Levels to add positive exposure compensation to the shadows and midtones won't work if the image
has channel values that fall outside the display range.</p>
+
+</figcaption>
+</figure>
+</li>
+
+
+<li> <b>Dispensing with "useless" shadow and highlight information:</b> Sometimes interpolated raw files of
photographs of high dynamic range scenes end up with a sprinkling of highlight and shadow pixels that
contains essentially no useful information. The easiest thing to do with such pixels is to use Levels to set
the desired black and white points, and then clip the resulting out of gamut channel information.
+ <ol style="list-style-type: lower-roman">
+ <li><i><b>Useless highlight information:</b></i>
+<p><img src="histogram-specular-highlights.jpg" alt="histogram-specular-highlights"><br>
+For the "Power lines" picture shown in Figure 8 above, after doing "Color/Auto/Stretch Contrast", a measly
48 pixels occupied nearly half the tonal range (see the histogram to the right). A little investigation with
GIMP's Threshold tool revealed that all 48 pixels are the peak values of specular highlights on the ceramic
insulators on the power line pole in the foreground.</p>
+
+<p>In cases where nearly half the histogram is occupied by a sprinkling of specular highlights, you could
use Curves to compress these specular highlights so they occupy a smaller proportion of the tonal range. But
for the "Power lines" image, the 48 pixels in question carried essentially zero information. So I used Levels
to raise the white point by setting the upper right Levels Value slider to 67.00, and then used "Tools/GEGL
Operation/Clip RGB" to actually clip the channel information in the highlights (this time making sure the
"Clip high pixel values" box was checked). </p>
+ </li>
+
+ <li><i><b>Useless shadow information:</b></i>
+ <p>Some raw processors can output images with negative channel values. And previous edits using high bit
depth GIMP might have produced negative channel values. If doing an "Auto/Stretch Contrast" on your base
image layer makes the image a whole lot lighter in the shadows, the problem is negative RGB channel values.
One solution is to use Levels to move the black point to where you want it to be, and then clip the negative
channel values. Here are two ways to clip negative channel values:</p>
+ <ul class="disc"><li>Use "Tools/GEGL Operation/Clip RGB", making sure to uncheck the "Clip high pixel
values" box. </li>
+ <li>Or else create a solid black layer above your base image layer, set the blend mode to "Lighten
only", and make a "New from Visible" layer.</li>
+ </ul>
+ <p>If you don't want to clip the shadow information, Curves can be used to compress the shadow channel
values. Using Curves immediately clips all out of gamut channel values, so Curves should be used with caution
unless you've already auto-stretched the image layer, and/or you are sure that there's no highlight
information worth retrieving.</p>
+ </li>
+ </ol>
+</li>
+
+
+<li id='blur-to-restore-micro-contrast'><b>Blurring the mask to restore micro contrast:</b> Putting an
inverse mask on a layer that's used to add positive exposure compensation necessarily slightly flattens micro
contrast. Depending on your artistic intentions for the image, you might want to blur the mask to restore
micro contrast. The trick is how to blur the mask without introducing "halos" around the edges of objects in
the image. Small radius gaussian blurs produce small but distressingly obvious halos around dark edges. A
large radius gaussian blur sometimes works but just as often produces a large obvious halo separating the
brighter and darker portions of the image. For many images a better solution is to blur the mask use an
edge-respecting filter such as the GIMP <a href="https://github.com/dtschump/gmic";>G'MIC</a> bilateral smooth
filter:
+
+<figure>
+<img src='without-bilateral-smoothing-of-mask.jpg' alt='without-bilateral-smoothing-of-mask'>
+<figcaption>Adding exposure compensation with and without bilateral smoothing of the mask.
+(Click to view):<br>
+<span class='toggle-swap' data-fig-swap='without-bilateral-smoothing-of-mask.jpg'>Without applying bilateral
smoothing to the mask, micro contrast is flattened.</span><br>
+<span class='toggle-swap' data-fig-swap='with-bilateral-smoothing-of-mask.jpg'>After applying bilateral
smoothing to the mask, micro contrast is restored.</span>
+<p>Adding exposure compensation combined with an inverse grayscale mask does flatten micro contrast, which
might or might not be desireable depending on your artistic intentions for the image. To restore micro
contrast, try using an edge-respecting blur such as G'MIC's bilateral smoothing filter. GIMP G'MIC doesn't
work on layer masks. A workaround is to to turn the unblurred mask into a selection, save the selection as a
channel, and then drag the channel to the layer stack for blurring.<br>
+</p>
+</figcaption>
+</figure>
+</li>
+
+
+<li>An essential component of the procedure for using Levels to add positive exposure compensation to images
with dark shadows and midtones needs to be explicitly mentioned: Not only is the high bit depth GIMP's Levels
operation unbounded at floating point precision — <a title="GIMP bug report: Some filters / operations
provide float values superior to 1.0." href="https://bugzilla.gnome.org/show_bug.cgi?id=737925";>layer masks
are also unbounded</a>. <p>If the inverted grayscale masks were summarily clipped (as is the case when
editing at integer precision), then the procedure described in this tutorial wouldn't work. However, if you
are trying this procedure in a floating point image editor that doesn't allow for retaining out of display
range values on a layer mask, you can make an inverse grayscale mask from the base layer, auto-stretch the
resulting mask, turn it into a selection, add it to the "Levels positive exposure compensation" copy of the
base layer, and then
delete or disable the mask on the base layer.</p></li>
+
+</ol>
+
+
+## Conclusion
+
+<p>Photographs taken in bright direct sunlight typically are of high dynamic range scenes, and the resulting
camera file usually requires careful tone mapping to produce a satisfactory final image. High bit depth
GIMP's floating point unbounded Levels provides a very useful tool for dealing with this type of image, and
of course is equally useful for any image where the goal is to raise the shadows and midtones without blowing
out the highlights. </p>
+
+
+<p>High bit depth GIMP's unbounded Levels combined with a suitable layer mask can also be used to darken
portions of the image, either by moving the upper left Value slider to the right (darkens the image by
increasing contrast and also increases saturation; requires careful masking to avoid producing regions of
solid black), or moving the lower right Value slider to the left (darkens the image by decreasing contrast,
useful for de-emphasizing portions of the image).</p>
+
+<p>This is a GIMP-specific tutorial. However, the same technique can be employed using the <a
href="http://photoflowblog.blogspot.com/";>PhotoFlow raw processor</a> and possibly other image editors that
allow for 32-bit floating point processing using unbounded RGB channel values. The neat thing about using
this technique in PhotoFlow is that PhotoFlow uses nodes, which allows for completely non-destructive editing
of the inverted grayscale mask that's used to recover the highlight detail after applying positive exposure
compensation to raise the tonality of the shadows and midtones (even if you close and reopen the image, if
you save the image's PFI file).</p>
+
+<!-- END ARTICLE -->
+
+<small>
+<a href='http://creativecommons.org/licenses/by-sa/3.0/'>
+<img class='cc-badge' src='http://i.creativecommons.org/l/by-sa/3.0/80x15.png' alt='Creative Commons By
Share Alike'/>
+</a>
+<span xmlns:dct="http://purl.org/dc/terms/";>GIMP Tutorial - Tone Mapping Using GIMP Levels (text &
images)</span> by [Elle Stone](http://ninedegreesbelow.com/) is licensed under a [Creative Commons
Attribution-ShareAlike 3.0 Unported License](http://creativecommons.org/licenses/by-sa/3.0/).
+</small>
+
+
+<script type='text/javascript' src='/js/img-swap.js'></script>
+
+<style media='screen' type='text/css'>
+.toggle-swap {
+ cursor: pointer;
+ color: #497bad;
+}
+body {
+ counter-reset: figure 0;
+}
+ol li {
+ margin-bottom: 1rem;
+}
+figure figcaption {
+ text-align: left;
+}
+figcaption::before {
+ counter-increment: figure;
+ content: "Figure " counter(figure) ". ";
+ font-weight: bold;
+}
+.toc ul {
+ list-style-type: decimal;
+}
+.toc li {
+ margin-bottom: 1rem;
+}
+</style>
diff --git a/content/tutorials/Tone_Mapping_Using_GIMP_Levels/index.md.orig
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+Title:Tone mapping and shadow recovery using GIMP Levels
+Date: 2016-03
+Modified: 2016-03
+Author: Elle Stone
+canonical: http://ninedegreesbelow.com/photography/gimp-tone-map-with-levels.html
+
+<small>
+<a href='http://creativecommons.org/licenses/by-sa/3.0/'>
+<img class='cc-badge' src='http://i.creativecommons.org/l/by-sa/3.0/80x15.png' alt='Creative Commons By
Share Alike'/>
+</a>
+<span xmlns:dct="http://purl.org/dc/terms/";>GIMP Tutorial - Tone Mapping Using GIMP Levels (text &
images)</span> by [Elle Stone](http://ninedegreesbelow.com/) is licensed under a [Creative Commons
Attribution-ShareAlike 3.0 Unported License](http://creativecommons.org/licenses/by-sa/3.0/).
+</small>
+
+
+<!-- BEGIN ARTICLE -->
+
+A very common editing problem is how to lighten the shadows and midtones of an image while retaining
highlight details, a task sometimes referred to as "shadow recovery" and more generally speaking as "tone
mapping". This step-by-step tutorial shows you how to use GIMP's unbounded Levels operations to add one or
more stops of positive exposure compensation to an image's shadows and midtones while retaining highlight
details.
+
+<div id="pagecontents" role="navigation">
+<p>Page Contents</p>
+<ol>
+<li><a href="#introduction">High bit depth GIMP unbounded Levels</a>: much better than Curves for lightening
the shadows and midtones of an image without blowing the highlights</li>
+<li><a href="#step-by-step">A worked example showing how to recover shadow information using high bit depth
GIMP's unbounded Levels</a></li>
+<li><a href="#use-notes">Use Notes</a></li>
+<li><a href="#conclusion">Conclusion</a></li>
+</ol>
+</div>
+
+
+## High bit depth GIMP unbounded Levels: much better than Curves for lightening the shadows and midtones of
an image without blowing the highlights
+
+A very common editing problem is how to lighten the shadows and midtones of an image without blowing out the
highlights, which problem is very often encountered when dealing with photographs of scenes lit by direct
sunlight. Precanned algorithms for accomplishing this task are often referred to as "shadow recovery"
algorithms. But really these algorithms are special-purpose tone-mapping algorithms, which sometimes work
pretty well, and sometimes not so well, depending on the algorithm, the image, and your artistic intentions
for the image.
+
+This step-by-step tutorial shows you how to use GIMP's unbounded floating point Levels operations to recover
shadow information — that is, add one or more stops of positive exposure compensation to an image's
shadows and midtones — without blowing out or unduly compressing the image highlights. The procedure is
completely "hand-tunable" using masks and layers, and is as close as you can get to non-destructive image
editing using high bit depth GIMP 2.9/2.10.
+
+<figure>
+<img src='power-lines.jpg' alt='power-lines'>
+<figcaption>
+<cite>Power lines at noon</cite>, before and after tone mapping (aka "shadow recovery") using high bit depth
GIMP's unbounded floating point Levels.<br>
+<span class='toggle-swap' data-fig-swap='before-auto-stretch-contrast.jpg'>Scene-referred interpolated raw
file.</span><br>
+<span class='toggle-swap' data-fig-swap='power-lines.jpg'>After tone mapping/shadow recovery using GIMP
unbounded Levels.</span>
+</figcaption>
+</figure>
+
+<noscript>
+<figure>
+<img src='before-auto-stretch-contrast.jpg' alt='before-auto-stretch-contrast'>
+<figcaption>
+Scene-referred interpolated raw file.
+</figcaption>
+</figure>
+</noscript>
+
+<!--
+Accepted wisdom is that Curves provides more versatility than Levels for modifying image tonality. Of course
you can use Curves to accomplish somewhat similar results to the procedure described in this tutorial. But
GIMP Curves is a highly destructive editing operation, immediately "baking" the resulting tonality into the
layer on which Curves is done, clipping any out of gamut RGB values, and leaving you with no way to fine-tune
the results without starting over. -->
+
+Hight bit depth GIMP is my primary image editor, and I've used the procedure described below for the last
couple of years as my "go to" way to modify image tonality. The same general procedure can be used to darken
as well as lighten portions of an image, again controlling the effect using a layer mask. This isn't exactly
nondestructive editing because at some point you need to make a "New from Visible" layer. But unlike using
Curves, using high bit depth GIMP's Unbounded Levels doesn't clip RGB channel values and allows you to
fine-tune the results by modifying and remodifying the layer mask until you are completely happy with the
resulting tonality.
+
+
+
+## A worked example showing how to recover shadow information using high bit depth GIMP's unbounded Levels
+
+<figure>
+<img src='tree-exposed-for-sky-ground-is-dark.jpg' alt='tree-exposed-for-sky-ground-is-dark'>
+<figcaption>
+Using high bit depth GIMP Levels to lighten the ground by one stop without blowing out portions of the sky.
+
+<ul>
+<li><em>Left:</em> The original image, an interpolated camera raw file that was deliberately underexposed in
camera to avoid blowing out the sky. It might not appear to be the case, but this image is already very close
to having out of gamut RGB channel values in the sky, and a simple Auto Stretch Contrast won't lighten the
image at all.</li>
+<li><em>Right:</em> The same image after using high bit depth GIMP Levels at 32-bit floating point linear
precision to add one stop of positive exposure compensation. An inverse grayscale mask was used to keep the
highlights from blowing out.</li>
+</ul>
+
+</figcaption>
+</figure>
+
+
+This worked example provides a sample image and is broken down into five steps, starting with downloading
the image. Steps 3, 4, and 5 describe the actual procedure, so here's an overview:
+
+<ol start='3' style='padding-left: 3rem; font-style: italic;' markdown=1 >
+
+3. Duplicate the base layer and then use GIMP's unbounded Levels to add one stop of positive exposure
compensation to the duplicated layer.
+4. Add an inverse grayscale layer mask to the now much brighter duplicated layer.
+5. Do "Auto/Stretch Contrast" on the layer mask and then fine-tune the mask until you are happy. Then make a
"New from Visible" layer.
+
+</ol>
+
+
+<ol style='max-width: 34rem;' markdown=1>
+
+<li> **Download [tree.png](tree.png)**, which is a 16-bit integer sRGB image. High bit depth GIMP really is
an "sRGB only" image editor, so it's best if you don't even try to edit in other RGB working spaces (if you
prefer a larger RGB working space, my [patched version of GIMP uses the Rec.2020 color space instead of
sRGB](http://ninedegreesbelow.com/photography/patch-gimp-in-prefix-for-artists.html)).</li>
+
+<li> **Open tree.png and convert it to 32-bit floating point linear precision**: Go to "Image/Precision",
select "32-bit floating point", and when the Dither dialog pops up, select "Linear light".</li>
+
+<li>Make a copy of the "tree.png" layer, and label it "Levels: +1 stop exp. comp". Then **use Levels to add
one stop of positive exposure compensation** — Figure 3 below shows the proper settings for the Levels
dialog, and Figure 4 shows the result:
+
+
+<figure style="width: 386px;"><img width="386"
+src="levels-add-one-stop-positive-exposure-compensation.jpg"
+alt=""
+>
+<figcaption>Using Levels to add one stop of positive exposure compensation — <i>make sure the "Gamma
hack" box is checked!</i>
+</figcaption></figure>
+
+<p style="text-indent: 0;">When using Levels to add one stop of positive exposure compensation, make sure
the image really is at floating point precision, because integer precision will clip the highlights. Also
make sure the "Gamma hack" box is checked — <a title="GIMP bug report: Curves and Levels should operate
by default on linear RGB and present linear RGB Histograms."
href="https://bugzilla.gnome.org/show_bug.cgi?id=757444";>otherwise the Levels operation will operate on
perceptually uniform RGB</a>, despite the fact that the image is at "Linear light" precision. Trying to add
one stop of positive exposure compensation using RGB values encoded using the sRGB TRC would require moving
the upper right Levels Value slider to the entirely unobvious and nonintuitive value of 73.53, and in the
process the shadow and midtone tonalities would be distorted because the sRGB TRC is not a true gamma TRC.
(An easier alternative might be to use the GEGL Exposure operation: "Colo
rs/Exposure", which always operates on linearized RGB).</p>
+
+<figure style="width: 772px;margin-left: -1rem;"><img width="772"
+src="one-stop-positive-exposure-compensation-added.jpg"
+alt=""
+>
+<figcaption>At floating point precision, GIMP's Levels operations are unbounded. This means you can use
Levels to add positive exposure compensation without blowing out the highlights. <p>Notice the RGB channel
values for the four sample points: the channel information that would have been clipped using integer
precision is encoded using channel values that are greater than 1.0 floating point.</p></figcaption>
+</figure>
+<p style="text-indent: 0;">The image in Figure 4 clearly has "blown" highlights in the sky. But the
highlights aren't really blown (that is, clipped to 1.0 in one or more channels). Instead the highlight
information is still there, but the RGB channel values fall outside the RGB <a title="Models for image
editing: Display-referred and scene-referred."
href="/photography/display-referred-scene-referred.html">display channel value range</a> of 0.0f to 1.0f. The
sample points dialog in Figure 4 above shows four sample points that have RGB channel values that are greater
than 1.0. As shown in Figure 5 below, adding a mask allows you to recover these highlights by bringing them
back down into the display range. </p>
+
+<p>If you had used integer precision instead of floating point, the highlights really would be blown: The
sample points would have a maximum channel values of 255, 65535 or 4294967295, depending on the bit depth.
And masking would only "recover" a solid expanse of gray, completely lacking any details (try for yourself
and see what happens).</p>
+</li>
+
+<li><img width="292" class="imgright"
+src="add-inverse-grayscale-mask.jpg"
+alt=""
+><b>Add an inverse grayscale layer mask</b>: Right-click on the layer and select "Layer/Mask/Add Layer
Mask", and when the "Add a mask to the Layer" dialog pops up, choose "Grayscale copy of layer" and check the
"Invert mask" box. <p>As shown in Figure 5 below, at this point the highlights will be brought back into the
display range, meaning all RGB channel values are between 0.0f and 1.0f. But the image will probably look a
little odd (sort of cloudy and flat), and depending on the image, the brightest highlights might actually
have dark splotches — don't worry! this is temporary.</p>
+
+
+<figure style="width: 772px;margin-left: -1rem;"><img width="772"
+src="inverse-grayscale-mask-added.jpg"
+alt=""
+>
+<figcaption>Result of adding an inverse grayscale layer mask to bring the highlights back into the display
range.
+<p style="text-indent: 0;">Adding an inverse grayscale layer mask brings the highlights back into the
display range, but at this point most images will look flat and cloudy, and some images will have dark
splotches in the highlights. The next step — "Auto Stretch Contrast" performed on the mask — will
take care of this problem.</p></figcaption>
+</figure>
+</li>
+
+<li><img width="301" class="imgright"
+src="auto-stretch-contrast-mask.jpg"
+alt=""
+><b>Click on the layer mask to select it for editing, and then select "Colors/Auto/Stretch Contrast"</b>:
+<p>"Keep Colors" should be checked (though it doesn't really matter on a layer mask) and <i>the "Gamma hack"
box should <b>not</b> be checked!</i> Unlike Levels, Stretch Contrast operates on linear RGB by default.
Figure 6 below shows the final result:</p>
+
+<figure style="width: 772px;margin-left: -1rem;"><img width="772"
+src="mask-auto-stretched.jpg"
+alt=""
+>
+<figcaption>Doing "Auto/Stretch Contrast" on the layer mask removes the "cloudy" appearance, leaving a
nicely brightened image with intact highlights. <p>"Auto/Stretch Contrast" on the mask is necessary because
just like the image layer has out of gamut RGB channel values, the inverted grayscale mask contains out of
gamut grayscale values. "Auto/Stretch Contrast" brings all the mask grayscale values back into the display
range, allowing the mask to proportionately compensate for the layer's otherwise out-of-gamut RGB channel
values, masking more in the layer highlights and less/not at all in the image's shadows and midtones.</p>
+
+<p>Notice that one of the sample points still has a blue RGB channel value that is slightly out of gamut.
The easiest way to deal with this is to do a Levels Gamma=0.45 adjustment <i>on the mask</i>, not on the
actual image layer. You can make this gamma adjustment either on the entire mask (works well, less effort).
Or else you can make the adjustment just on the mask shadows (which correspond to the layer highlights), in
which case you'd load the mask as a selection, invert the selection, and make the Levels gamma=0.45 slider
adjustment. Or if the remaining out of gamut channel values are only very slightly out of gamut, make a "New
from Visible" layer and then "Auto/Stretch Contrast" the result to bring the remaining channel values back
into gamut.</p>
+</figcaption>
+</figure>
+</li>
+</ol>
+
+<p>That's the whole procedure for using Levels to add a stop of positive exposure compensation to the
shadows without blowing out the highlights. Now you can either fine-tune the mask, or else just make a "New
from Visible" layer and continue editing your nicely brightened image. Depending on the image and also on
your artistic intentions for the image, the mask might not need fine-tuning. But very often you'll want to
modify the resulting tonal distribution by doing a Levels gamma correction or Curves operation on the mask,
or even by painting directly on the mask. And sometimes you'll want to blur the mask to <a
href="#blur-to-restore-micro-contrast">restore micro contrast</a>.
+
+
+
+
+## Use Notes
+<ol class="double-space">
+
+<li>Depending on your particular artistic intentions for an image, <b>some images are more likely than
others to benefit from being tone mapped using unbounded Levels.</b> Your mileage may vary, but typically the
procedure described on this page works best for photographs of scenes with a pronounced tonal difference
between the highlights and shadows, as per typical sunny day "sky-ground" photographs.</li>
+
+<li><b>For adding just one stop of positive exposure compensation, the procedure described on this page
works really well.</b> Depending on the image you might want to blur the mask using an edge-respecting blur
algorithm, and/or tweak the mask using the Levels gamma slider, Curves, etc. But only modify the mask
<i>after</i> using Auto Stretch Contrast on the mask. Otherwise results will be unpredictable: <a
href="/photography/unbounded-srgb-levels-gamma-slider.html">Levels gamma slider adjustments produce odd
results when operating on out of gamut values</a>, and Curves will summarily clip out of gamut values.</li>
+
+
+<li><b>For adding more than one stop of exposure compensation, you can use one or several
positive-exposure-compensation layers.</b> Either way the layer mask(s) will need careful tweaking that's
very image-specific and also specific to your intended result. An example using two layers to add two stops
of exposure compensation is show in Figure 7 below:
+
+<figure style="width: 770px;margin-left: -1rem;"><img width="770"
+src="orchard-truck.jpg"
+alt=""
+>
+<figcaption>Using GIMP's floating point unbounded Levels plus layer masks to add two stops of positive
exposure compensation to the shadows and midtones of a photograph of an apple orchard truck that was taken in
bright sunshine.
+
+<ol class="single-space">
+<li><i>Top:</i> the final image after using high bit depth GIMP's floating point unbounded Levels to add two
stops of positive exposure compensation to the image shadows and midtones, plus a bit of GEGL saturation
masked to mostly affect the sky.</li>
+<li><i>Lower left:</i> the original image from the camera, underexposed to avoid blowing out highlights.</li>
+<li><i>Lower right:</i> for comparision, the result of using GEGL's Mantiuk tone mapping on the original
image, using the default settings. IMHO, Mantiuk and other "automagic" tone mapping algorithms tend to
produce rather artificial results compared to what you can achieve using masks and layers combined with
GIMP's unbounded floating point Levels.</li>
+</ol>
+<img width="560" class="imgcenter"
+src="orchard-truck-layer-stack.jpg"
+alt=""
+>
+<div class="clear"></div>
+<p>This screenshot of the layer stack shows the individual layers and masks that were used to add the two
stops of positive exposure compensation to the shadows and midtones of the photograph of the apple orchard
truck.</p>
+</figcaption>
+</figure>
+</li>
+
+<li>Before using Levels to add positive exposure compensation, <b>the base layer should already be stretched
to its maximum dynamic range.</b> The easiest way to stretch the base layer to its maximum dynamic range is
to do "Colors/Auto/Stretch Contrast" and make sure that "Keep colors" is checked (and <i>don't</i> use the
Gamma hack).
+
+<p>If you've never used an unbounded floating point image editor before, "Colors/Auto/Stretch Contrast" can
produce an unexpected result: The image might actually end up with a severely reduced dynamic range, having
either lighter shadows or darker highlights or both:</p>
+
+<figure style="width: 770px;margin-left: -1rem;">
+<div class="container">
+ <div class="imghover">
+ <ol class="none single-space">
+
+ <li>
+ <a href="#">1. Image from the camera.</a>
+ <img src="before-auto-stretch-contrast.jpg" alt="">
+ </li>
+
+ <li>
+ <a href="#">2. After doing "Colors/Auto/Stretch Contrast".</a>
+ <img src="after-auto-stretch-contrast.jpg" alt="">
+ </li>
+
+ <li>
+ <a href="#">3. Final "Power lines" image.</a>
+ <img src="power-lines.jpg" alt="">
+ </li>
+
+ </ol>
+ </div>
+
+ <img class="default" src="before-auto-stretch-contrast.jpg" alt="">
+</div>
+<figcaption style="margin-top: -0.5rem;">Before and after doing "Colors/Auto/Stretch Contrast" on the base
layer, plus the final image after tone mapping using Levels:
+
+<ol>
+<li>This scene-referred interpolated raw file from the PhotoFlow raw processor (which provides a GIMP
plug-in for easy opening of raw files) has out-of-display-range RGB channel values that will be brought back
into the display range by doing "Colors/Auto/Stretch Contrast".</li>
+
+<li>After doing "Colors/Auto/Stretch Contrast", shadows are lighter and highlights are darker because the
dynamic range has been compressed to fit within the display range. This looks like an editing step in the
wrong direction! but actually it's necessary.</li>
+
+<li>Here's the final "Power lines" image after tone mapping the scene-referred interpolated raw file using
the procedure described in this tutorial.</li>
+</ol>
+
+<p>As captured by the raw file, this picture of power lines marching into the distance is a typical result
of taking a photograph at noon on a bright sunny day: The sky and clouds looked pretty good right out of the
camera, but the ground was far too dark. So the image could benefit from some tone mapping to raise the
shadows and midtones. The first step is to do "Colors/Auto/Stretch Contrast" to bring any channel values that
are less than 0.0f or greater than 1.0f back within the display range of 0.0 to 1.0 floating point.</p>
+<p>Performing "Auto/Stretch Contrast" to bring the channel values back inside the display range doesn't
exactly look like an editing step in the right direction for tone-mapping this particular image! but really
it is. Using Levels to add positive exposure compensation to the shadows and midtones won't work if the image
has channel values that fall outside the display range.</p></figcaption>
+</figure>
+</li>
+
+<li> <b>Dispensing with "useless" shadow and highlight information:</b> Sometimes interpolated raw files of
photographs of high dynamic range scenes end up with a sprinkling of highlight and shadow pixels that
contains essentially no useful information. The easiest thing to do with such pixels is to use Levels to set
the desired black and white points, and then clip the resulting out of gamut channel information.
+ <ol class="lower-roman">
+ <li><i><b>Useless highlight information:</b></i>
+<p><img class="imgright" src="histogram-specular-highlights.jpg" alt="">For the "Power lines" picture shown
in Figure 8 above, after doing "Color/Auto/Stretch Contrast", a measly 48 pixels occupied nearly half the
tonal range (see the histogram to the right). A little investigation with GIMP's Threshold tool revealed that
all 48 pixels are the peak values of specular highlights on the ceramic insulators on the power line pole in
the foreground.</p>
+
+<p>In cases where nearly half the histogram is occupied by a sprinkling of specular highlights, you could
use Curves to compress these specular highlights so they occupy a smaller proportion of the tonal range. But
for the "Power lines" image, the 48 pixels in question carried essentially zero information. So I used Levels
to raise the white point by setting the upper right Levels Value slider to 67.00, and then used "Tools/GEGL
Operation/Clip RGB" to actually clip the channel information in the highlights (this time making sure the
"Clip high pixel values" box was checked). </p>
+ </li>
+
+ <li><i><b>Useless shadow information:</b></i>
+ <p>Some raw processors can output images with negative channel values. And previous edits using high bit
depth GIMP might have produced negative channel values. If doing an "Auto/Stretch Contrast" on your base
image layer makes the image a whole lot lighter in the shadows, the problem is negative RGB channel values.
One solution is to use Levels to move the black point to where you want it to be, and then clip the negative
channel values. Here are two ways to clip negative channel values:</p>
+ <ul class="disc"><li>Use "Tools/GEGL Operation/Clip RGB", making sure to uncheck the "Clip high pixel
values" box. </li>
+ <li>Or else create a solid black layer above your base image layer, set the blend mode to "Lighten
only", and make a "New from Visible" layer.</li>
+ </ul>
+ <p>If you don't want to clip the shadow information, Curves can be used to compress the shadow channel
values. Using Curves immediately clips all out of gamut channel values, so Curves should be used with caution
unless you've already auto-stretched the image layer, and/or you are sure that there's no highlight
information worth retrieving.</p>
+ </li>
+ </ol>
+</li>
+
+
+<li> <a id="blur-to-restore-micro-contrast"></a><b>Blurring the mask to restore micro contrast:</b> Putting
an inverse mask on a layer that's used to add positive exposure compensation necessarily slightly flattens
micro contrast. Depending on your artistic intentions for the image, you might want to blur the mask to
restore micro contrast. The trick is how to blur the mask without introducing "halos" around the edges of
objects in the image. Small radius gaussian blurs produce small but distressingly obvious halos around dark
edges. A large radius gaussian blur sometimes works but just as often produces a large obvious halo
separating the brighter and darker portions of the image. For many images a better solution is to blur the
mask use an edge-respecting filter such as the GIMP <a href="https://github.com/dtschump/gmic";>G'MIC</a>
bilateral smooth filter:
+
+<figure style="width: 770px;margin-left: -1rem;">
+<div class="container">
+ <div class="imghover">
+ <ol class="none single-space">
+ <li>
+ <a href="#">Without applying bilateral smoothing to the mask, micro contrast is flattened.</a>
+ <img src="without-bilateral-smoothing-of-mask.jpg" alt="">
+ </li>
+ <li>
+ <a href="#">After applying bilateral smoothing to the mask, micro contrast is restored.</a>
+ <img src="with-bilateral-smoothing-of-mask.jpg" alt="">
+ </li>
+ </ol>
+ </div>
+
+ <img class="default" src="without-bilateral-smoothing-of-mask.jpg" alt="">
+</div>
+<figcaption style="margin-top: -0.5rem;">Adding exposure compensation with and without bilateral smoothing
of the mask. <p>Adding exposure compensation combined with an inverse grayscale mask does flatten micro
contrast, which might or might not be desireable depending on your artistic intentions for the image. To
restore micro contrast, try using an edge-respecting blur such as G'MIC's bilateral smoothing filter. GIMP
G'MIC doesn't work on layer masks. A workaround is to to turn the unblurred mask into a selection, save the
selection as a channel, and then drag the channel to the layer stack for blurring.</p></figcaption>
+</figure>
+</li>
+
+
+<li>An essential component of the procedure for using Levels to add positive exposure compensation to images
with dark shadows and midtones needs to be explicitly mentioned: Not only is the high bit depth GIMP's Levels
operation unbounded at floating point precision — <a title="GIMP bug report: Some filters / operations
provide float values superior to 1.0." href="https://bugzilla.gnome.org/show_bug.cgi?id=737925";>layer masks
are also unbounded</a>. <p>If the inverted grayscale masks were summarily clipped (as is the case when
editing at integer precision), then the procedure described in this tutorial wouldn't work. However, if you
are trying this procedure in a floating point image editor that doesn't allow for retaining out of display
range values on a layer mask, you can make an inverse grayscale mask from the base layer, auto-stretch the
resulting mask, turn it into a selection, add it to the "Levels positive exposure compensation" copy of the
base layer, and then
delete or disable the mask on the base layer.</p></li>
+
+</ol>
+
+
+## Conclusion
+
+<p>Photographs taken in bright direct sunlight typically are of high dynamic range scenes, and the resulting
camera file usually requires careful tone mapping to produce a satisfactory final image. High bit depth
GIMP's floating point unbounded Levels provides a very useful tool for dealing with this type of image, and
of course is equally useful for any image where the goal is to raise the shadows and midtones without blowing
out the highlights. </p>
+
+
+<p>High bit depth GIMP's unbounded Levels combined with a suitable layer mask can also be used to darken
portions of the image, either by moving the upper left Value slider to the right (darkens the image by
increasing contrast and also increases saturation; requires careful masking to avoid producing regions of
solid black), or moving the lower right Value slider to the left (darkens the image by decreasing contrast,
useful for de-emphasizing portions of the image).</p>
+
+<p>This is a GIMP-specific tutorial. However, the same technique can be employed using the <a
href="http://photoflowblog.blogspot.com/";>PhotoFlow raw processor</a> and possibly other image editors that
allow for 32-bit floating point processing using unbounded RGB channel values. The neat thing about using
this technique in PhotoFlow is that PhotoFlow uses nodes, which allows for completely non-destructive editing
of the inverted grayscale mask that's used to recover the highlight detail after applying positive exposure
compensation to raise the tonality of the shadows and midtones (even if you close and reopen the image, if
you save the image's PFI file).</p>
+
+
+
+<!-- END ARTICLE -->
+
+<a href='http://creativecommons.org/licenses/by-sa/3.0/'>
+<img class='cc-badge' src='http://i.creativecommons.org/l/by-sa/3.0/80x15.png' alt='Creative Commons By
Share Alike'/>
+</a>
+<span xmlns:dct="http://purl.org/dc/terms/";>GIMP Tutorial - Tone Mapping Using GIMP Levels (text &
images)</span> by [Elle Stone](http://ninedegreesbelow.com/).
+Licensed under a [Creative Commons Attribution-ShareAlike 3.0 Unported
License](http://creativecommons.org/licenses/by-sa/3.0/).
+
+
+<script type='text/javascript' src='/js/img-swap.js'></script>
+
+<style media='screen' type='text/css'>
+.toggle-swap {
+ cursor: pointer;
+ color: #497bad;
+}
+body {
+ counter-reset: figure 0;
+}
+figure figcaption {
+ text-align: left;
+}
+figcaption::before {
+ counter-increment: figure;
+ content: "Figure " counter(figure) ". ";
+ font-weight: bold;
+}
+</style>
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