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<p>I just extended the <code>modus-themes-diffs</code> customisation option with the
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ability to read a <code>deuteranopia</code> value.</p>
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<p><em>Deuteranopia</em> describes insensitivity to the green channel of light.
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This includes the possibility of not distinguishing between red and
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green; the two colours that are typically codified in diff buffers as
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denoting “removed” and “added” states respectively.</p>
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<p>Evaluating <code>(setq modus-themes-diffs 'deuteranopia)</code> and then re-loading
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the theme of choice will replace all instances of green in diff buffers
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with variants of blue. This affects the standard <code>diff-mode</code>, Ediff,
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Smerge, Magit, Vdiff, and any other library that inherits from those.</p>
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<p>Below are illustrations of the phenomenon while using Magit diffs as an
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example with word-wise differences enabled (“refined” diffs). We try to
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simulate how deuteranopia is experienced. Those with it see red and
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green as variants of brown/yellow and interpret shades of magenta
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without its red channel’s intensity (magenta consists primarily of red
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and blue, so it looks closer to a desaturated blue).</p>
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<h2>Red VS Green (default)</h2>
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<p><a href="https://protesilaos.com/assets/images/attachments/2021-02-25-modus-operandi-diff-default.png"><img alt="Modus Operandi default" src="https://protesilaos.com/assets/images/attachments/2021-02-25-modus-operandi-diff-default.png" /></a></p>
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<p><a href="https://protesilaos.com/assets/images/attachments/2021-02-25-modus-operandi-diff-default-simulated.png"><img alt="Modus Operandi default simulated" src="https://protesilaos.com/assets/images/attachments/2021-02-25-modus-operandi-diff-default-simulated.png" /></a></p>
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<p><a href="https://protesilaos.com/assets/images/attachments/2021-02-25-modus-vivendi-diff-default.png"><img alt="Modus Vivendi default" src="https://protesilaos.com/assets/images/attachments/2021-02-25-modus-vivendi-diff-default.png" /></a></p>
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<p><a href="https://protesilaos.com/assets/images/attachments/2021-02-25-modus-vivendi-diff-default-simulated.png"><img alt="Modus Vivendi default simulated" src="https://protesilaos.com/assets/images/attachments/2021-02-25-modus-vivendi-diff-default-simulated.png" /></a></p>
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<h2>Red VS Blue (‘deuteranopia’ option)</h2>
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<p><a href="https://protesilaos.com/assets/images/attachments/2021-02-25-modus-operandi-diff-deuteranopia.png"><img alt="Modus Operandi deuteranopia" src="https://protesilaos.com/assets/images/attachments/2021-02-25-modus-operandi-diff-deuteranopia.png" /></a></p>
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<p><a href="https://protesilaos.com/assets/images/attachments/2021-02-25-modus-operandi-diff-deuteranopia-simulated.png"><img alt="Modus Operandi deuteranopia simulated" src="https://protesilaos.com/assets/images/attachments/2021-02-25-modus-operandi-diff-deuteranopia-simulated.png" /></a></p>
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<p><a href="https://protesilaos.com/assets/images/attachments/2021-02-25-modus-vivendi-diff-deuteranopia.png"><img alt="Modus Vivendi deuteranopia" src="https://protesilaos.com/assets/images/attachments/2021-02-25-modus-vivendi-diff-deuteranopia.png" /></a></p>
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<p><a href="https://protesilaos.com/assets/images/attachments/2021-02-25-modus-vivendi-diff-deuteranopia-simulated.png"><img alt="Modus Vivendi deuteranopia simulated" src="https://protesilaos.com/assets/images/attachments/2021-02-25-modus-vivendi-diff-deuteranopia-simulated.png" /></a></p>
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<h2>Accessibility broadly understood</h2>
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<p>While <a href="https://protesilaos.com/emacs/modus-themes/">the Modus themes</a> are not
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designed with the express goal of optimising for deuteranopia across all
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of Emacs’ interfaces, we can always make steps towards improving that
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experience. The fact that we already conform with the highest standard
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for colour contrast (WCAG AAA) puts us in a good position to expand our
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commitment to accessibility so that it accounts for variations in colour
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perception.</p>
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<p>Designing for such cases is no mean feat. I had to take screenshots of
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all relevant configurations, display them in the GNU Image Manipulation
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Program, and set a filter that simulates deuteranopia vision: from GNU
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IMP’s “View” menu, search for “Display Filters” and then apply “Color
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Deficient Vision”. That had to be done tens of times during the testing
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process.</p>
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<p>What inspired me to add support for <code>(setq modus-themes-diffs
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'deuteranopia)</code> is the blog post of Sam Kleinman (aka “tycho garen”) on
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<a href="https://tychoish.com/post/editor-themes/">Editor Themes</a> (2021-02-24),
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which echoes the relevant wishlist item on my project’s issue tracker
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(<a href="https://gitlab.com/protesilaos/modus-themes/-/issues/118">issue 118</a>)
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where I got feedback from Hörmetjan Yiltiz.</p>
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<p>Apart from its immediate utility, consider this new feature a statement
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of intent: I want to make the themes as accessible as possible and am
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willing to act on user feedback to that end.</p>
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<p>If you believe something does not work properly while using either
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<code>modus-operandi</code> or <code>modus-vivendi</code>, please do not hesitate to bring the
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issue to my attention. Colour is a complex phenomenon: even when we
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formally conform with the lofty target of a minimum 7:1 contrast in
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relative luminance, we may still encounter cases where things do not
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“feel right”. We can always fix those outright or, where necessary,
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benefit from the power of Elisp to provide the relevant customisation
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options.</p>
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