[gimp-help-2] Bug 780065 - Docs need an update: changes in Desaturate tool
- From: Elle Stone <ellestone src gnome org>
- To: commits-list gnome org
- Cc:
- Subject: [gimp-help-2] Bug 780065 - Docs need an update: changes in Desaturate tool
- Date: Tue, 28 Mar 2017 13:25:22 +0000 (UTC)
commit 1b42002d4894e713dd7180b738320d695ff63549
Author: Elle Stone <ellestone ninedegreesbelow com>
Date: Tue Mar 28 08:55:28 2017 -0400
Bug 780065 - Docs need an update: changes in Desaturate tool
Update Colors/Desaturate/Desaturate documentation and images.
.../colors-desaturate-average-red-globe.png | Bin 0 -> 54448 bytes
.../colors-desaturate-average-sunflower.png | Bin 0 -> 58614 bytes
.../colors-desaturate-lightness-red-globe.png | Bin 0 -> 55184 bytes
.../colors-desaturate-lightness-sunflower.png | Bin 0 -> 50937 bytes
.../colors-desaturate-luma-red-globe.png | Bin 0 -> 51916 bytes
.../colors-desaturate-luma-sunflower.png | Bin 0 -> 84872 bytes
.../colors-desaturate-luminance-red-globe.png | Bin 0 -> 53765 bytes
.../colors-desaturate-luminance-sunflower.png | Bin 0 -> 86687 bytes
.../colors-desaturate-original-red-globe.png | Bin 0 -> 87553 bytes
.../colors-desaturate-original-sunflower.png | Bin 0 -> 60399 bytes
.../colors-desaturate-value-red-globe.png | Bin 0 -> 60829 bytes
.../colors-desaturate-value-sunflower.png | Bin 0 -> 22493 bytes
images/C/menus/colors/desaturate/desaturate.png | Bin 19253 -> 15492 bytes
images/C/toolbox/colors-desaturate-average.png | Bin 22061 -> 0 bytes
images/C/toolbox/colors-desaturate-brightness.png | Bin 21949 -> 0 bytes
images/C/toolbox/colors-desaturate-luminosity.png | Bin 22707 -> 0 bytes
images/C/toolbox/colors-desaturate-orig.png | Bin 36230 -> 0 bytes
src/menus/colors/desaturate/desaturate.xml | 330 ++++++++++++++++----
18 files changed, 277 insertions(+), 53 deletions(-)
---
diff --git a/images/C/menus/colors/desaturate/colors-desaturate-average-red-globe.png
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diff --git a/images/C/menus/colors/desaturate/colors-desaturate-lightness-red-globe.png
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diff --git a/images/C/menus/colors/desaturate/colors-desaturate-lightness-sunflower.png
b/images/C/menus/colors/desaturate/colors-desaturate-lightness-sunflower.png
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diff --git a/images/C/menus/colors/desaturate/colors-desaturate-luma-red-globe.png
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diff --git a/images/C/menus/colors/desaturate/colors-desaturate-luma-sunflower.png
b/images/C/menus/colors/desaturate/colors-desaturate-luma-sunflower.png
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diff --git a/images/C/menus/colors/desaturate/colors-desaturate-luminance-red-globe.png
b/images/C/menus/colors/desaturate/colors-desaturate-luminance-red-globe.png
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diff --git a/images/C/menus/colors/desaturate/colors-desaturate-luminance-sunflower.png
b/images/C/menus/colors/desaturate/colors-desaturate-luminance-sunflower.png
new file mode 100644
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diff --git a/images/C/menus/colors/desaturate/colors-desaturate-original-red-globe.png
b/images/C/menus/colors/desaturate/colors-desaturate-original-red-globe.png
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diff --git a/images/C/menus/colors/desaturate/colors-desaturate-original-sunflower.png
b/images/C/menus/colors/desaturate/colors-desaturate-original-sunflower.png
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diff --git a/images/C/menus/colors/desaturate/colors-desaturate-value-red-globe.png
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diff --git a/images/C/menus/colors/desaturate/colors-desaturate-value-sunflower.png
b/images/C/menus/colors/desaturate/colors-desaturate-value-sunflower.png
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diff --git a/images/C/menus/colors/desaturate/desaturate.png b/images/C/menus/colors/desaturate/desaturate.png
index 9f74bd1..b2ada7f 100644
Binary files a/images/C/menus/colors/desaturate/desaturate.png and
b/images/C/menus/colors/desaturate/desaturate.png differ
diff --git a/src/menus/colors/desaturate/desaturate.xml b/src/menus/colors/desaturate/desaturate.xml
index 06c69cc..6c2c194 100644
--- a/src/menus/colors/desaturate/desaturate.xml
+++ b/src/menus/colors/desaturate/desaturate.xml
@@ -2,15 +2,8 @@
<!DOCTYPE sect2 PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN"
"http://www.docbook.org/xml/4.3/docbookx.dtd";>
<!-- section history:
+ 2017-03-26 elle stone: update desaturate help and add new images
2017-01-29 desaturate moved to Colors/Desaturate, changed id to plug-in-desaturate
- 2009-04-11 ude: changed id (gimp-layer-desaturate -> gimp-tool-desaturate)
- 2009-03-12 j.h: moved from menus:colors/. revised for v2.6
- 2008-01-21 ude: updated de; added equations
- 2007-12-31 j.h updated to v2.4. Options added en;fr
- 2007-10-16 ude: copied here from "menus/layer-desaturate.xml"
- 2007-06-6 Added Spanish translation by AntI
- 2007-02-22 KoSt: added no
- 2005/11/12 split of from layer-colors.xml by axel.wernicke
-->
<sect2 id="gimp-filter-desaturate">
<title>Desaturate</title>
@@ -23,13 +16,13 @@
<primary>Desaturate</primary>
</indexterm>
<para>
- By using the <guimenuitem>Desaturate</guimenuitem> command, you can
- convert all of the colors on the active layer to corresponding shades
- of gray. This differs from converting the image to grayscale in two
- respects. First, it only operates on the active layer and second,
- the colors on the layer are still RGB values with three components. This
- means that you can paint on the layer, or individual parts of it,
- using color at a later time.
+ By using the <guimenuitem>Desaturate</guimenuitem> command, you can convert
+ all of the colors on the active layer to corresponding shades of gray. This
+ differs from converting the image to grayscale in two respects. First, it
+ only operates on the active layer and second, the colors on the layer are
+ still RGB values with three components, with R=G=B, which makes gray.
+ This means that you then can paint on the layer, or individual parts of it,
+ using non-gray colors.
</para>
<note>
@@ -42,7 +35,7 @@
<sect3>
<title>Activating the Command</title>
- <para>You can find this command through
+ <para>You can find this command through
<menuchoice>
<guimenu>Colors</guimenu>
<guisubmenu>Desaturate</guisubmenu>
@@ -57,22 +50,30 @@
<title>The <quote>Desaturate</quote> option dialog</title>
<mediaobject>
<imageobject>
- <imagedata fileref="images/menus/colors/desaturate/desaturate.png" format="PNG"/>
+ <imagedata fileref="images/menus/colors/desaturate/desaturate.png"
+ format="PNG"/>
</imageobject>
</mediaobject>
</figure>
- <para>Three options are available:</para>
+ <para><guilabel>Mode:</guilabel> Five options are available for converting
+ from color to black and white:</para>
<variablelist>
- <title>Choose shade of gray based on</title>
<varlistentry>
- <term>Lightness</term>
+ <term><link linkend="More-information-about-Luminance">Luminance</link>
+ </term>
<listitem>
- <para>The graylevel will be calculated as</para>
+ <para>The shades of gray will be calculated using
+ <ulink url="http://brucelindbloom.com/index.html?Eqn_RGB_to_XYZ.html";>
+ linearized sRGB</ulink> as
+ </para>
<informalequation>
<mediaobject>
<textobject>
<phrase>
- Lightness = ½ × (max(R,G,B) + min(R,G,B))
+ Luminance =
+ (0.22 × R) +
+ (0.72 × G) +
+ (0.06 × B)
</phrase>
</textobject>
</mediaobject>
@@ -80,14 +81,15 @@
</listitem>
</varlistentry>
<varlistentry>
- <term>Luma</term>
+ <term><link linkend="More-information-about-Luma">Luma</link></term>
<listitem>
- <para>The graylevel will be calculated as</para>
+ <para>The shades of gray will be calculated using non-linearized sRGB
+ </para>
<informalequation>
<mediaobject>
<textobject>
<phrase>
- Luma = 0.21 × R + 0.72 × G + 0.07 × B
+ Luma = (0.22 × R) + (0.72 × G) + (0.06 × B)
</phrase>
</textobject>
</mediaobject>
@@ -95,29 +97,49 @@
</listitem>
</varlistentry>
<varlistentry>
- <term>Average</term>
+ <term>
+ <link linkend="More-information-about-Lightness-Average-and-Value">
+ Lightness (HSL)</link></term>
<listitem>
- <para>The graylevel will be calculated as</para>
+ <para>The shades of gray will be calculated as</para>
<informalequation>
<mediaobject>
<textobject>
- <phrase>Average Brightness = (R + G + B) ÷ 3</phrase>
+ <phrase>
+ Lightness (HSL)) = ½ × (max(R,G,B) + min(R,G,B))
+ </phrase>
</textobject>
</mediaobject>
</informalequation>
</listitem>
</varlistentry>
<varlistentry>
- <term>Luminance</term>
+ <term>
+ <link linkend="More-information-about-Lightness-Average-and-Value">
+ Average (HSI Intensity)</link></term>
<listitem>
- <para>The graylevel will be calculated as</para>
+ <para>The shades of gray will be calculated as</para>
+ <informalequation>
+ <mediaobject>
+ <textobject>
+ <phrase>Average (HSI Intensity) = (R + G + B) ÷ 3
+ </phrase>
+ </textobject>
+ </mediaobject>
+ </informalequation>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>
+ <link linkend="More-information-about-Lightness-Average-and-Value">
+ Value (HSV)</link></term>
+ <listitem>
+ <para>The shades of gray will be calculated as</para>
<informalequation>
<mediaobject>
<textobject>
<phrase>
- Luminance = 0.21 × R +
- 0.72 × G +
- 0.07 × B
+ Value (HSV) = max(R,G,B)
</phrase>
</textobject>
</mediaobject>
@@ -125,13 +147,14 @@
</listitem>
</varlistentry>
</variablelist>
+
<figure>
- <title>Comparing the three options</title>
+ <title>Using the five modes to convert two very different color images
+ to black and white</title>
<mediaobject>
<imageobject>
- <!--was: "images/menus/sunflower.png"-->
<imagedata format="PNG"
- fileref="images/toolbox/colors-desaturate-orig.png"/>
+ fileref="images/menus/colors/desaturate/colors-desaturate-original-sunflower.png"/>
</imageobject>
<caption>
<para>Original image</para>
@@ -140,55 +163,256 @@
<mediaobject>
<imageobject>
<imagedata format="PNG"
- fileref="images/toolbox/colors-desaturate-brightness.png"/>
+ fileref="images/menus/colors/desaturate/colors-desaturate-original-red-globe.png"/>
</imageobject>
<caption>
- <para><quote>Lightness</quote> applied</para>
+ <para>Original image</para>
</caption>
</mediaobject>
+ <literallayout>
+ </literallayout>
<mediaobject>
<imageobject>
<imagedata format="PNG"
- fileref="images/toolbox/colors-desaturate-luminosity.png"/>
+ fileref="images/menus/colors/desaturate/colors-desaturate-luminance-sunflower.png"/>
+ </imageobject>
+ <caption>
+ <para><quote>Luminance</quote> applied</para>
+ </caption>
+ </mediaobject>
+ <mediaobject>
+ <imageobject>
+ <imagedata format="PNG"
+ fileref="images/menus/colors/desaturate/colors-desaturate-luminance-red-globe.png"/>
+ </imageobject>
+ <caption>
+ <para><quote>Luminance</quote> applied</para>
+ </caption>
+ </mediaobject>
+ <literallayout>
+ </literallayout>
+ <mediaobject>
+ <imageobject>
+ <imagedata format="PNG"
+ fileref="images/menus/colors/desaturate/colors-desaturate-luma-sunflower.png"/>
</imageobject>
<caption>
<para>
<quote>Luma</quote> applied.
- <!-- The result is brighter. -->
</para>
</caption>
</mediaobject>
<mediaobject>
<imageobject>
<imagedata format="PNG"
- fileref="images/toolbox/colors-desaturate-average.png"/>
+ fileref="images/menus/colors/desaturate/colors-desaturate-luma-red-globe.png"/>
</imageobject>
<caption>
<para>
- <quote>Average</quote> applied.
- <!-- The result is in between Brightness and Luminosity. -->
+ <quote>Luma</quote> applied.
</para>
</caption>
</mediaobject>
-<!-- Find out what Luminance and Luma really do
+ <literallayout>
+ </literallayout>
<mediaobject>
<imageobject>
<imagedata format="PNG"
- fileref="images/toolbox/colors-desaturate-luminance.png"/>
+ fileref="images/menus/colors/desaturate/colors-desaturate-lightness-sunflower.png"/>
</imageobject>
<caption>
<para>
- <quote>Luminance</quote> applied.
+ <quote>Lightness (HSL)</quote> applied.
+ </para>
+ </caption>
+ </mediaobject>
+ <mediaobject>
+ <imageobject>
+ <imagedata format="PNG"
+ fileref="images/menus/colors/desaturate/colors-desaturate-lightness-red-globe.png"/>
+ </imageobject>
+ <caption>
+ <para>
+ <quote>Lightness (HSL)</quote> applied.
+ </para>
+ </caption>
+ </mediaobject>
+ <literallayout>
+ </literallayout>
+ <mediaobject>
+ <imageobject>
+ <imagedata format="PNG"
+ fileref="images/menus/colors/desaturate/colors-desaturate-average-sunflower.png"/>
+ </imageobject>
+ <caption>
+ <para>
+ <quote>Average (HSI Intensity)</quote> applied.
+ </para>
+ </caption>
+ </mediaobject>
+ <mediaobject>
+ <imageobject>
+ <imagedata format="PNG"
+ fileref="images/menus/colors/desaturate/colors-desaturate-average-red-globe.png"/>
+ </imageobject>
+ <caption>
+ <para>
+ <quote>Average (HSI Intensity)</quote> applied.
+ </para>
+ </caption>
+ </mediaobject>
+ <literallayout>
+ </literallayout>
+ <mediaobject>
+ <imageobject>
+ <imagedata format="PNG"
+ fileref="images/menus/colors/desaturate/colors-desaturate-value-sunflower.png"/>
+ </imageobject>
+ <caption>
+ <para>
+ <quote>Value (HSV)</quote> applied.
+ </para>
+ </caption>
+ </mediaobject>
+ <mediaobject>
+ <imageobject>
+ <imagedata format="PNG"
+ fileref="images/menus/colors/desaturate/colors-desaturate-value-red-globe.png"/>
+ </imageobject>
+ <caption>
+ <para>
+ <quote>Value (HSV)</quote> applied.
</para>
</caption>
</mediaobject>
--->
- <!-- XXX:
- The conclusions above are true only for this special example;
- e.g. try
- lightness vs. luminosity with (0, 255, 0) or (0, 0, 255),
- lightness vs. average with (255, 255, 0) or (255, 0, 0).
- -->
</figure>
</sect3>
+
+ <sect3>
+ <title>Comparing results from using different options for
+ converting from color to black and white:</title>
+ <orderedlist>
+ <listitem>
+ <para>The degree and direction from which the various ways to convert
+ an image to black and white diverge from a straight Luminance conversion
+ to black and white depends on:</para>
+ <itemizedlist>
+ <listitem><para>The conversion method you choose.</para></listitem>
+ <listitem><para>The RGB color space in which the conversion is done.
+ </para></listitem>
+ <listitem><para>How saturated the colors in the original image happen to
+ be, with more saturated starting colors (such as the red globe and the
+ bright yellow sunflower) producing greater amounts of deviation from a
+ straight Luminance conversion.</para></listitem>
+ <listitem><para>The hues (for example yellows vs reds) of the various
+ saturated colors in an image also make a difference.</para></listitem>
+ <listitem><para>Being the maximum of the RGB channel values for each
+ pixel, an HSV Value conversion to black and white is always lighter than
+ the original color image, and also lighter than all the other ways to
+ convert to black and white.</para></listitem>
+ </itemizedlist>
+ </listitem>
+ <listitem>
+ <para>Comparing the red globe to the yellow sunflower:</para>
+ <itemizedlist>
+ <listitem><para>For the red globe Lightness (HSL) produces a result very
+ similar to Luminance, and Luma produces a conversion that is much darker.
+ </para></listitem>
+ <listitem><para>For the sunflower, Luma produces a result very similar to
+ Luminance, and Lightness (HSL) produces a conversion that is much darker.
+ </para></listitem>
+ <listitem><para>Notice that the less saturated parts of each image look
+ more or less the same, regardless of which method is chosen for converting
+ from color to black and white.
+ </para></listitem>
+ </itemizedlist>
+ </listitem>
+ </orderedlist>
+ </sect3>
+
+ <sect3>
+ <title>More information about the five options for
+ converting from color to black and white:</title>
+ <orderedlist>
+ <listitem id="More-information-about-Luminance">
+ <para><emphasis>More information about Luminance:</emphasis></para>
+ <itemizedlist>
+ <listitem><para>"Luminance" is the only physically meaningful way to
+ convert a color image to black and white, as the resulting black and
+ white image has the same relative luminance (reflects the same
+ percentage of light from the various shades of gray) as the colors in
+ the original color image.</para></listitem>
+ <listitem><para>Luminance must be calculated using linearized RGB
+ values.</para></listitem>
+ <listitem><para>For convenience we say "Luminance", but what we really
+ mean is "Relative Luminance". For more information, see
+ <ulink url="https://en.wikipedia.org/wiki/Relative_luminance";>Relative
+ Luminance</ulink> and
+ <ulink url="https://en.wikipedia.org/wiki/CIE_1931_color_space#Meaning_of_X.2C_Y.2C_and_Z";>
+ CIE 1931 [XYZ] color space</ulink>.</para></listitem>
+ <listitem>
+ <para>GIMP 2.10 uses hard-coded sRGB values to do Luminance
+ conversions to black and white. "Future GIMP" will support correct
+ conversions for images in other color spaces.</para>
+ </listitem>
+ </itemizedlist>
+ </listitem>
+ <listitem id="More-information-about-Luma">
+ <para><emphasis>More information about Luma:</emphasis></para>
+ <itemizedlist>
+ <listitem><para>"Luma" is what you get if you use the formula for
+ Luminance on RGB values that haven't been properly linearized. Luma
+ corresponds to GIMP 2.8's "Luminosity" method of converting to black and
+ white.</para></listitem>
+ <listitem><para>Compared to GIMP 2.8, GIMP 2.10's "Luma" option uses
+ slightly different multipliers. Unlike the GIMP 2.8 multipliers, the
+ GIMP 2.10 multipliers have been properly Bradford-adapted from D65 to
+ D50, which is required for use in an ICC profile color-managed editing
+ application (at least until the next version of the ICC specs is
+ released and people figure out how to deal with the new freedom to use
+ non-D50 reference white points).</para></listitem>
+ <listitem>
+ <para>GIMP 2.10 uses hard-coded sRGB values to do Luma
+ conversions to black and white. "Future GIMP" will support correct
+ conversions for images in other color spaces.</para>
+ </listitem>
+ </itemizedlist>
+ </listitem>
+ <listitem id="More-information-about-Lightness-Average-and-Value">
+ <para><emphasis>More information about Lightness, Average, and Value:
+ </emphasis></para>
+ <para>The "Lightness (HSL)", "Average (HSI Intensity)", and "Value (HSV)"
+ ways to to convert a color image to black and white use color space models
+ that were invented for fast processing on consumer-grade computers from the
+ 1990s. For details see
+ <ulink url="https://en.wikipedia.org/wiki/HSL_and_HSV";>HSV and HSV</ulink>,
+ paying particular attention to the section on
+ <ulink url="https://en.wikipedia.org/wiki/HSL_and_HSV";>Disadvantages
+ </ulink>.</para>
+ </listitem>
+ <listitem>
+ <para><emphasis>In case you are wondering why LAB Lightness</emphasis> is
+ not among the options
+ for converting an RGB image to black and white, a properly calculated
+ conversion from RGB to LAB Lightness, and then back to RGB, produces exactly
+ the same result as the Luminance conversion to black and white. Here is why:
+ </para>
+ <itemizedlist>
+ <listitem><para>In the XYZ color space, Y is Luminance. So if you
+ convert a color RGB image to XYZ, the "Y" of XYZ is the same number as
+ the R=G=B values that you get when you calculate RGB Luminance.</para></listitem>
+ <listitem><para>LAB is a perceptually uniform transform of XYZ. If you
+ convert from RGB to XYZ and then to LAB, and set A=B=0.0 (or 0.5 for
+ image editors that put the midpoint of the A and B axes as 0.5 instead
+ of at 0.0), and then convert back to XYZ, the X and Z values will
+ change, but Y will not change.</para></listitem>
+ </itemizedlist>
+ <para>Tutorials that produce anything other than Relative Luminance when
+ converting an RGB image to black and white using LAB Lightness, very sadly
+ are trading on various mathematical mistakes in the conversion routines.
+ </para>
+ </listitem>
+ </orderedlist>
+ </sect3>
+
</sect2>
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