[Gimp-developer] Soft proofing and the GIMP Display Filters and Color Management settings

[Elle Stone <ellestone ninedegreesbelow com>]

I've been reading through the various sections of the online GIMP 
documentation that relate to color management, which prompted me to take 
a closer look at the Display Filters.

Most of the observations below are related to the Color Management and 
Color Proof ("Soft Proof") Display filters. As background for people who 
haven't used soft proofing, soft proofing isn't just for preparing an 
image to be printed on paper. Rather it's useful any time an image is 
destined for display on a device other than the one on which it was 
created (eg a digital photo frame on a wall). It's also useful when an 
image is to be converted from one ICC profile to another (eg when 
converting from ProPhoto or a camera or scanner input profile to sRGB).

"Edit/Preferences/Color Management" settings are global, affecting all 
open images. Choices made in "View/Color Display Filters" are applied on 
a per image basis. At present "View/Color Display Filters" and 
"Edit/Preferences/Color Management" interact in sometimes very 
unexpected ways, especially when the user sends multiple copies of the 
various Display filters over to the Active filter list. Some expected 
and unexpected results of using the Display Filters are:

a. If "Edit/Preferences/Color Management" is set to "Color managed 
display" and "View/Color Display Filters" has the Color Management 
Filter active (which it is by default), then unchecking the Color 
Management Display filter disables color management for the image for 
which the "View/Color Display Filters" dialog was opened. This makes 
sense and is an example of using the per image Display Filters to 
override the Color Management Settings global settings.

b. If "Edit/Preferences/Color Management" is set to "No color 
management" and "View/Color Display Filters" has the Color Management 
Filter active, then "View/Color Display Filters" Active Color Management 
filter doesn't override the "No color management" choice made in the 
Preference/Color Management dialog. In this case the per image choice 
doesn't override the global choice.

c. The Color Proof Display Filter works when color management is 
disabled. This provides accurate results if (and only if) the monitor is 
calibrated to exactly match sRGB - not too likely given today's LCD 
monitors, but still theoretically possible.

d. The Color Proof Display Filter doesn't provide the option to enable 
and disable marking out of gamut colors.

e. If the "Mode of operation" is set to "Print simulation" in the 
globally applied "Edit/Preferences/Color Management" settings dialog, 
and if the user also chooses to activate the per image "Color Proof" 
display filter, then the global choices made in the Color Management 
settings are applied on top of the per image choices made in the Color 
Proof Display filter, and the results are wrong.

f. Multiple active copies of the Color Proof Display Filter in turn 
apply each copy's soft proofing choices, with increasingly wrong results.

g. One active copy of the Color Management display filter acts as 
expected. But multiple active copies of the Color Management display 
filter each in turn apply the conversion from the image ICC profile to 
the monitor ICC profile. If the monitor ICC profile is "none" or is 
sRGB, the multiple active Color Management filters make no difference. 
But when using an actual monitor profile (sRGB doesn't adequately 
describe LCD monitors), successively applying multiple copies of the 
Color Management filter results in increasingly noticeable and wrong 
color and tonality changes.

h. Multiple active copies of the Contrast and Gamma filters also are 
applied successively. This seems unlikely to be useful.

i. Multiple active copies of the Color Deficient Vision filter also add 
one to the other, but only if different Color deficiency types are 
chosen. Once one each of insensitivity to red, green, and blue are 
chosen, additional active Color Deficient Vision filters make no 
difference in what's shown on the screen (for example, removing green 
twice doesn't make magenta).


I don't know how difficult it would be to change the way the per image 
Display Filters currently interact with the global Color Management 
settings, but I put together some suggestions in hope of starting a 
discussion:

1. In the Display Filters dialog, the user should only be able to send 
one copy of the Available Filters over to the Active Filters panel. To 
compensate, the Active Color Deficient Vision filter should have a list 
with check boxes rather than a drop-down menu, so the user could check 
one, two, or all three types at the same time.

2. The Color Management and Color Proof display filters should both be 
in the Active Filter list by default, with Color Management checked by 
default and Color Proofing unchecked by default.

3. If the user has already chosen to be in "Print Simulation Mode of 
operation" in the "Preferences/Color Management" dialog, then that 
choice affects all open images. But for those images for which the user 
has also activated the Color Proof display filter, choices made in the 
display filter should completely override choices made in the Color 
Management Preferences dialog. This includes not only the proofing 
profile but also the rendering intent and whether or not to use black 
point compensation. In no case should a choice made in the display 
filter "add to" the effect of a choice made in the "Preferences/Color 
Management" dialog.

4. The Color Proof display filter should include the option to enable 
and disable marking out of gamut colors, overriding any choice made in 
"Preferences/Color Management".

5. Currently the Active Color Proof display filter opens with no choices 
having been preselected. Instead the Color Proof display filter settings 
should default to showing the soft proofing choices that were already 
made in the global Color Management Preferences. That way, if the user 
wants to change all of the global settings for just the one image, the 
same number of clicks are required, but if the user only wants to change 
one or two settings, fewer clicks are required.

6. The "Color Proof" display filter should be labelled "Soft Proof" to 
be consistent with terminology used in the global Color Management 
Settings. And the Color Management Settings "Print simulation" Mode of 
operation should read "Device/Profile simulation", to provide a hint to 
the user that soft proofing covers more than just preparing an image for 
printing.

7. Users who have color management disabled in "Preferences/Color 
Management" should have the option to enable color management on a per 
image basis using the Color Management display filter dialog, parallel 
to the current option to use the display filters to disable color 
management on a per image basis.

8. At present, when Color Management is activated, which rendering 
intent to use and whether or not to use black point compensation are 
only controllable via the "Preferences/Color Management" dialog, which 
affects all open images. These same options should also be available in 
the Active Color Management display filter (defaulting to the same 
choices that were made in the Preferences/Color Management dialog), so 
they can be changed on a per image basis.

9. Some people have more than one monitor profile, for example a matrix 
profile for image editing and one or more LUT profiles for soft 
proofing. So it would be useful to be able to use the Active Color 
Management display filter to override the globally set monitor profile 
on a per image basis.

10. Because the Display Filters apply on a per image basis, the user 
needs to be able to tell at a glance which open image is affected by 
which Display Filter dialog. Perhaps the Display Filter title bar could 
display the image file name so the user knows exactly which image which 
be affected by a change in an open Display Filter dialog. Or perhaps 
clicking on an image could also highlight the corresponding Display 
Filter dialog. Or perhaps the Display Filter dialog really should be in 
an Image drop-down menu rather than a separately opened, stand-alone 
dialog box.

11. I'm not clear on the intended use cases for the Contrast, Gamma, and 
Color deficiency Display Filters (emulation? compensation?). But if 
there are users who need these Filters applied routinely for all images, 
then these Filters should also be in the global Preferences/Color 
Management Settings dialog.

Cheers,
Elle