Red channel in histogram impossible to see for colourblind people

[eduardo-heuvel]

When editing an image in Pix using the curves, the rendering of the red channel (when editing separate RGB channels) is nearly impossible to see for me (against the dark gray background). I imagine that (nearly) all other users with the same type of coulourblindness (aprox. 5% of the male population) suffer the same experience. Note: I don't have an exotic form of colourblindness, just the very common red-green colourblindness.
Proposed solution: use a brighter tone of red that stands out against the dark gray background of the histogram.

[darkowlzz]

Hi, I just came across this issue and looked around the code and found the locations to change the colors. I don't have much experience with accessibility design and if there's any specific design guideline followed by pix. But since I played around with the code a bit, I thought I would share a few things. It may help anyone else who may know better how to address the issue.

The histogram's background color is set in a CSS file, see

pix/pix/resources/pix.css

Line 33 in 3a7dd7f

background-color: #333333;

.

The red channel color is set in

pix/pix/gth-histogram-view.c

Line 331 in 3a7dd7f

cairo_set_source_rgba (cr, 0.68, 0.18, 0.19, 1.0); /* #af2e31 */

.

Looking at the git history, I see that it used to be (1, 0, 0), ignoring the alpha value, at some point. But then there was an update to what we have today with a lot of other changes. I'm unable to find the context to determine how the new value was determined.

Regardless, just to get some feedback around what kind of change would be preferable, following are a few images I took with color changes. It would be helpful to know what change is better to help steer towards a good value.

Without any change, the histogram colors appear:

Increasing the red in the red channel by 0.10:

Increasing all red, blue and greed in the red channel by 0.10:

Reverting to the default channel colors and making the background color lighter:

I'm curious to know if changing the background color makes it any better.

I can help submit the code change once we get some feedback from the project maintainers as well.

[eduardo-heuvel]

Hi, thanks for diving into this, but I think we are talking about two different histograms. Looking at your attached proposals, seeing all three channels in a single histogram suggests they are from the Image Properties tab rather than the Histogram Editing tool. (In Pix, I can easily distinguish the channels in the Properties tab).
​The problem occurs specifically when editing the histogram. As someone who is color blind, I find the current UI choices particularly challenging. I used a color picker to analyze the display, and these are the HTML values:
​Background: #333333
​Red Histogram: #4c3233 (very dark/unsaturated)
​Red Curve: #a22e30
​Green Histogram: #3a5138
​Green Curve: #50b44a
​Blue Histogram: #304552
​Blue Curve: #237fba
​Technical Suggestion regarding Accessibility (WCAG):
I noticed that the contrast ratio between the background (#333333) and the Red Histogram (#4c3233) is approximately 1.2:1. For meaningful UI elements, the WCAG 2.1 guidelines generally recommend a minimum contrast ratio of 3:1 for graphics and 4.5:1 for text to ensure readability for all users, including those with visual impairments.
​The color scheme from the Properties tab works well for me. I hope this can be resolved soon.

[darkowlzz]

Thanks for correcting me. Now that I read the issue description again, I see that you mentioned editing using curves.
I'll spend some more time looking at the history of the code later and also researching about the accessibility details you shared. But at a quick glance, I just noticed that the issue could be more due to transparency/alpha value. For some reason, the transparency is set to 0.20

pix/extensions/file_tools/gth-curve-editor.c

Line 205 in 3a7dd7f

#define HISTOGRAM_TRANSPARENCY 0.20

This results in faded color, which is even a little hard for me to see for long:

I just raised it to .80 and that made it so much better:

If it was just due the transparency/alpha, maybe we don't need to change the colors. It need not be .80, but any reasonable higher value could work. In the properties histogram the alpha value is set to 1.0. I'm not sure why it is set to a low value here. But maybe the information about the minimum contrast ratio could be a good reason to change it.

Does the second image looks good for you too?

[eduardo-heuvel]

Thank you for your quick response. The second image definitely looks better to me. Although I would say this value of 0.80 for me (and other colourblind people) is the absolute minimum. Why not choose 0.90?

[eduardo-heuvel]

By the way if this change is implemented, I think that also the brightness and/or opacity of the curve-lines should be increased, otherwise they're likely to get hard to see against the histogram background (especially in logarithmic view as the histogram easily occupies most of the surface of the graph-window).