[Gimp-developer] GIMP useability - choosing linear vs perceptually uniform RGB

[Elle Stone <ellestone ninedegreesbelow com>]

The single biggest useability issue with GIMP 2.9 is the mechanism for 
allowing the user to switch between linear and perceptually uniform RGB.

In the current UI, the user has no way to know whether any given editing 
operation is performed on linear or perceptually uniform RGB. For most 
operations there are four possibilities for the user to try:

1. Switch to linear precision and don't use the gamma hack.
2. Switch to linear precision and use the gamma hack.
3. Switch to "gamma" precision and don't use the gamma hack.
4. Switch to "gamma" precision and use the gamma hack.

I realize the above situation is supposed to be temporary. However, 
given the current UI, the only way the user can know what's happening to 
her RGB data is to:

1. Compile a version of GIMP with the babl flips disabled and have two 
versions of an image open, one in regular sRGB and one in linear gamma sRGB.
2. Perform the editing operation in the version of GIMP with the babl 
flips disabled on both the regular and the linear gamma version of the 
image.
3. Compare results to what happens in default GIMP when editing the 
regular sRGB image, by systematically trying all four combinations 
listed above.

The suggestion was made on IRC that it doesn't matter to the user what's 
happening to the RGB data because the user can see what the image looks 
like on the screen and if s/he doesn't like the result, s/he can do 
something else. This view on image editing is just plain wrong.

A person who is new to image editing might be content to just push 
sliders around and "see what happens". But there is a difference in the 
editing needs of slider-pushing beginners (everyone starts out as a 
slider pusher) and people who have sufficiently mastered the art of 
image editing to know what they want to accomplish and how to make it 
happen.

People who use advanced workflows with previsualized editing goals need 
to be able to control whether any given editing operation happens on 
linear or perceptually uniform RGB.

In PhotoShop, switching between linear and perceptually uniform RGB 
required doing an actual ICC profile conversion, which is quite 
time-consuming once the image has more than a couple layers.

Converting between linear and gamma precision in GIMP is just as slow 
and a good deal less predictable from the user's point of view. At least 
in PhotoShop, once you convert to linear RGB, you know all operations 
are performed on linear RGB, and the same for converting to perceptually 
uniform RGB. But you don't have that kind of assurance in GIMP - rather 
you are left to guess what's happening to your RGB data.

The babl flips ought to make it possible for GIMP users to instantly 
flip between linear and perceptually uniform RGB, at the *user's* own 
discretion. Such fundamentally important editing decisions should not be 
dictated by developers who can't possibly know the user's particular 
artistic or technical goals.

The current UI for selecting whether an operation is done on linear or 
perceptually uniform RGB works isn't useable for anyone except a slider 
pusher. Here's a suggestion for improving the situation:

1. Eliminate the "(linear)" and "(gamma)" options in the 
"Image/Precision" menu.

2. Double the number of layer blend options, so that the user can choose 
whether each layer blend mode should be done using perceptually uniform 
or linear RGB. Deal with the increased number of layer blend modes by 
allowing the user to "collapse" the less-used blend modes and expand the 
more-used blend modes, as Krita does.

3. For all editing operations, in the UI include a check box so the user 
can select whether the operation is done on linear or perceptually 
uniform RGB.

4. Have the technically correct option preselected. For example, for 
Gaussian blur, "linear RGB" would be preselected. And for adding RGB 
noise, "perceptually uniform RGB" would be preselected. And so on.

Best,
Elle