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
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