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

[Elle Stone <ellestone ninedegreesbelow com>]

On 04/19/2015 06:44 AM, Elle Stone wrote:
> The single biggest useability issue with GIMP 2.9 is the mechanism for
> allowing the user to switch between linear and perceptually uniform RGB.

On 04/19/2015 03:40 PM, Liam R. E. Quin wrote:
> In the meantime in my own workflow the lack of "repeat last filter
> used" is a much bigger usability issue than anything to do with gamma
> or clipping. So phrases like "everyone" and "the biggest usability
> problen" don't carry as much weight as specific use cases, I think.

I will guess that most current GIMP users, just like most current 
PhotoShop users, don't have any notion of whether any given RGB 
operation should be done on linear vs perceptually uniform RGB. They 
rely on what they see on the screen, and they are used to seeing wrong 
results.

Anyone who understands and intends to take advantage of high bit depth 
linear gamma image editing needs *control* over whether an operation is 
done using linear or perceptually uniform RGB:

* The user shouldn't have to guess whether the RGB values have been 
linearized or not.

* The user shouldn't be forced to rely on the developer's "decisions 
from afar" regarding whether any given RGB operation should be done 
using linear or perceptually uniform RGB.

* The GIMP UI should provide easy user choice for all RGB editing 
operations, with good default settings. For example the default for 
gaussian blur should be linear RGB and the default for adding RGB noise 
should be perceptually uniform RGB. But the user needs the ability to 
*easily* make the other choice.

Back when I used PhotoShop for linear gamma image editing I encountered 
three useability issues, which apparently still exist in the latest 
version of PhotoShop:

1. PhotoShop couldn't display a true linear gamma image without causing 
posterization in the shadows, at least until the image was zoomed in to 
nearly 100%.

2. PhotoShop Curves and Levels provided far too few tonal steps in the 
shadows to allow good control over edits made in linear gamma color spaces.

3. Switching between linear gamma and perceptually uniform RGB required 
doing an ICC profile conversion.

GIMP 2.9 from git used to have (but doesn't any longer have) the first 
two problems (well, there seems to be some kind of problem with 
Curves/Levels shadow adjustments, but it's not related to having too few 
tonal steps).

The babl flips are a brilliant solution to the third problem. But the 
current User Interface for switching between linear gamma and 
perceptually uniform RGB is a useability nightmare.

Currently the only way a use could know how to get an operation to 
operate on linear vs perceptually uniform RGB is if someone supplied a 
table that includes all operations, with a column for how to get linear 
RGB and another column for how to get perceptually uniform RGB. For 
examples of how very inconsistent the current UI is, see this table: 
http://ninedegreesbelow.com/photography/users-guide-to-high-bit-depth-gimp.html#linear-or-perceptual-table. 
The table explains how to get selected operations to work on linear RGB.

Best,
Elle