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

(PR #3908)

Author: yanone

CHANGELOG.md

@@ -8,6 +8,7 @@ A more detailed list of changes is available in the corresponding milestones for
   - **[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/slant_direction]:** Check slant direction of outline to match values of slnt axis extrema. (PR #3910)
+  - **[com.google.fonts/check/color_cpal_brightness]:** Warn if COLRv0 layers are colored too dark or too bright instead of foreground color. (PR #3908)
 
 ### 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

@@ -201,6 +201,7 @@
     '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 = \
@@ -6248,6 +6249,61 @@ 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/pull/3908'
+)
+def com_google_fonts_check_color_cpal_brightness(config, ttFont):
+    """Color layers should have a minimum brightness"""
+    from fontbakery.utils import pretty_print_list
+
+    def color_brightness(hex_value):
+        '''Generic color brightness formula'''
+        return (hex_value[0] * 299 + hex_value[1] * 587 + hex_value[2] * 114) / 1000
+
+    minimum_brightness = 256 * .1
+    FOREGROUND_COLOR = 0xFFFF
+    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:
+                    hex_value = ttFont["CPAL"].palettes[0][layer.colorID]
+                    layer_brightness = color_brightness(hex_value)
+                    if (layer_brightness < minimum_brightness
+                        or layer_brightness > 256 - minimum_brightness):
+                        if key not in dark_glyphs:
+                            dark_glyphs.append(key)
+    if dark_glyphs:
+        dark_glyphs = pretty_print_list(config, sorted(dark_glyphs))
+        yield WARN,\
+              Message('glyphs-too-dark-or-too-bright',
+                      f"The following glyphs have layers that are too bright or"
+                      f" too dark: {dark_glyphs}.\n"
+                      f"\n"
+                      f" To fix this, please either set the color definitions of all"
+                      f" layers in question to current color (0xFFFF), or alter"
+                      f" the brightness of these layers significantly.")
+    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

@@ -4218,10 +4218,8 @@ def test_check_STAT(fps, new_stat, result):
 @patch("freetype.Face", side_effect=ImportError)
 def test_check_override_freetype_rasterizer(mock_import_error):
     """Check that overridden test yields FAIL rather than SKIP."""
-    check = CheckTester(
-        googlefonts_profile,
-        f"com.adobe.fonts/check/freetype_rasterizer{OVERRIDE_SUFFIX}",
-    )
+    check = CheckTester(googlefonts_profile,
+                        f"com.adobe.fonts/check/freetype_rasterizer{OVERRIDE_SUFFIX}")
 
     font = TEST_FILE("cabin/Cabin-Regular.ttf")
     msg = assert_results_contain(check(font), FAIL, "freetype-not-installed")
@@ -4263,15 +4261,30 @@ def test_check_colorfont_tables():
                 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")
+
+    font = TEST_FILE("color_fonts/AmiriQuranColored_too_dark.ttf")
+    assert_results_contain(check(font),
+                           WARN, 'glyphs-too-dark-or-too-bright',
+                           'with a colrv0 font with doo dark layers')
+
+    font = TEST_FILE("color_fonts/AmiriQuranColored.ttf")
+    assert_PASS(check(font),
+                'with a colrv0 font with good layer colors')
+
+
 def test_check_noto_has_article():
     """Noto fonts must have an ARTICLE.en_us.html file"""
     check = CheckTester(googlefonts_profile,
                         "com.google.fonts/check/description/noto_has_article")
 
-    ttFont = TTFont(TEST_FILE("notosanskhudawadi/NotoSansKhudawadi-Regular.ttf"))
-    assert_PASS(check(ttFont), "with a good font")
+    font = TEST_FILE("notosanskhudawadi/NotoSansKhudawadi-Regular.ttf")
+    assert_PASS(check(font), "with a good font")
 
-    ttFont = TTFont(TEST_FILE("noto_sans_tamil_supplement/NotoSansTamilSupplement-Regular.ttf"))
-    assert_results_contain(check(ttFont),
+    font = TEST_FILE("noto_sans_tamil_supplement/NotoSansTamilSupplement-Regular.ttf")
+    assert_results_contain(check(font),
                            FAIL, 'missing-article',
                            "with a bad font")