New check: com.google.fonts/check/color_cpal_brightness

Author: yanone

CHANGELOG.md

@@ -7,6 +7,7 @@ A more detailed list of changes is available in the corresponding milestones for
 #### Added to the Google Fonts Profile
   - **[com.google.fonts/check/colorfont_tables]:** Fonts must have neither or both the tables `COLR` and `SVG`. (issue #3886)
   - **[com.google.fonts/check/description/noto_has_article]:** Noto fonts must have an ARTICLE.en_us.html file. (issue #3841)
+  - **[com.google.fonts/check/color_cpal_brightness]:** Warn if COLRv0 layers are colored too dark or too bright instead of foreground color
 
 ### BugFixes
   - **[com.adobe.fonts/check/varfont/valid_default_instance_nameids]:** The check did not account for nameID 17. (issue #3895)

Lib/fontbakery/profiles/googlefonts.py

@@ -200,6 +200,7 @@
     'com.google.fonts/check/render_own_name',
     'com.google.fonts/check/STAT',
     'com.google.fonts/check/colorfont_tables',
+    'com.google.fonts/check/color_cpal_brightness',
 ]
 
 GOOGLEFONTS_PROFILE_CHECKS = \
@@ -6181,6 +6182,50 @@ def com_google_fonts_check_colorfont_tables(ttFont):
     else:
         yield PASS, "Looks good!"
 
+@check(
+    id = "com.google.fonts/check/color_cpal_brightness",
+    rationale = """
+        Layers of a COLRv0 font should not be too dark or too bright. When layer colors are
+        set explicitly, they can't be changed and they may turn out illegible against dark or bright backgrounds.
+        While traditional color-less fonts
+        can be colored in design apps or CSS, a black color definition in a COLRv0
+        font actually means that that layer will be rendered in black regardless of the
+        background color. This leads to text becoming invisible against a dark background,
+        for instance when using a dark theme in a web browser or operating system.
+
+        This check ensures that layer colors are at least 10% bright and at most 90% bright, when not 
+        already set to the current color (0xFFFF).
+    """,
+    proposal = 'https://github.com/googlefonts/fontbakery/issues/3886'
+)
+def com_google_fonts_check_color_cpal_brightness(ttFont):
+    """Color layers should have a minimum brightness"""
+    SUGGESTED_FIX = ("To fix this, please either set the color definitions of all layers "
+                    "in question to current color (0xFFFF), "
+                    "or alter the brightness of these layers significantly.")
+    minimum_brightness = 256 * .1
+    FOREGROUND_COLOR = 0xFFFF
+    # Generic color brightness formula
+    def color_brightness(hex):
+        return (hex[0] * 299 + hex[1] * 587 + hex[2] * 114) / 1000
+    dark_glyphs = []
+    if 'COLR' in ttFont.keys() and ttFont['COLR'].version == 0:
+        for key in ttFont['COLR'].ColorLayers:
+            for layer in ttFont['COLR'].ColorLayers[key]:
+                # 0xFFFF is the foreground color, ignore
+                if layer.colorID != FOREGROUND_COLOR:
+                    layer_brightness = color_brightness(ttFont["CPAL"].palettes[0][layer.colorID])
+                    if layer_brightness < minimum_brightness or layer_brightness > 256 - minimum_brightness:
+                        if key not in dark_glyphs:
+                            dark_glyphs.append(key)
+    if dark_glyphs:
+        yield WARN,\
+            Message('glyphs-too-dark-or-too-bright',
+                    f"The following glyphs have layers that are too bright or too dark: {dark_glyphs}. " + SUGGESTED_FIX)
+
+    else:
+        yield PASS, "Looks good!"
+
 @check(
     id = 'com.google.fonts/check/description/noto_has_article',
     conditions = ['is_noto'],

data/test/color_fonts/AmiriQuranColored_too_dark.ttf

@@ -4249,6 +4249,19 @@ def test_check_colorfont_tables():
     assert_PASS(check(ttFont),
                 f'with a good font without SVG or COLR tables.')
 
+def test_check_color_cpal_brightness():
+    """Color layers should have a minimum brightness"""
+    check = CheckTester(googlefonts_profile,
+                        "com.google.fonts/check/color_cpal_brightness")
+
+    ttFont = TTFont(TEST_FILE("color_fonts/AmiriQuranColored_too_dark.ttf"))
+    assert_results_contain(check(ttFont),
+                           WARN, 'glyphs-too-dark-or-too-bright',
+                           'with a colrv0 font with doo dark layers')
+
+    ttFont = TTFont(TEST_FILE("color_fonts/AmiriQuranColored.ttf"))
+    assert_PASS(check(ttFont),
+                f'with a colrv0 font with good layer colors')
 
 def test_check_noto_has_article():
     """Noto fonts must have an ARTICLE.en_us.html file"""