Image Diff for Accessibility and Contrast Testing

Image Diff for Accessibility Testing

Image diff can be applied to accessibility testing by comparing normal renderings against simulated views that mimic various vision impairments. This approach reveals whether your UI remains usable and distinguishable under different visual conditions.

Color Blindness Simulation

Approximately 8% of men and 0.5% of women have some form of color vision deficiency. Simulating these conditions and comparing against the original reveals which UI elements lose distinction:

• Protanopia (red-blind) — Red and green appear similar; affects ~1% of males
• Deuteranopia (green-blind) — Red and green appear similar; affects ~6% of males
• Tritanopia (blue-blind) — Blue and yellow appear similar; rare (~0.01%)
• Achromatopsia (total color blindness) — Only shades of gray; very rare

Using Diff for Contrast Verification

After applying accessibility improvements (increasing contrast ratios, changing colors):

1. Capture the page before the change
2. Apply the accessibility improvement
3. Capture the page after the change
4. Use pixel diff to verify the change affects only the intended elements
5. Check that the change does not inadvertently reduce contrast elsewhere

High Contrast Mode Testing

Compare your UI in normal mode against Windows High Contrast Mode or forced-colors mode:

• Text visibility — Ensure all text remains readable
• Border visibility — Verify that interactive elements have visible borders
• Icon visibility — Check that icons are visible against high-contrast backgrounds
• Focus indicators — Confirm that focus rings are visible in high contrast mode

Dark Mode vs Light Mode Comparison

Diff between dark mode and light mode screenshots to verify:

• All text is legible in both modes
• No elements become invisible against the background
• Sufficient contrast ratios are maintained in both modes
• Images and icons adapt appropriately

Zoom Testing

Compare screenshots at 100%, 150%, and 200% zoom to detect:

• Text overlap at higher zoom levels
• Elements that do not scale properly
• Scrollbar behavior changes
• Touch target size adequacy at different zoom levels