misc: add rgb-luminance related utils (#77644)

publik_django_templatetags/publik/templatetags/publik.py

@@ -196,3 +196,19 @@ def duration(value, arg='short'):
         except (TypeError, ValueError):
             return ''
     return utils.seconds2humanduration(int(value.total_seconds()), short=bool(arg != 'long'))
+
+
+@register.filter
+def luminance(rgb):
+    red = int(rgb[1:3], base=16)
+    green = int(rgb[3:5], base=16)
+    blue = int(rgb[5:7], base=16)
+    # Y’ from ITU-R BT.709 conversion:
+    return red * 0.2126 + green * 0.7152 + blue * 0.0722
+
+
+@register.filter
+def get_foreground_color(background_rgb):
+    # where background_rgb is a #xxyyzz hex RBG color value.
+    # the 128 luminance threshold is picked to match w.c.s. WF status display logic:
+    return 'white' if luminance(background_rgb) < 128 else 'black'

tests/test_publik.py

@@ -1,4 +1,5 @@
 from django.template import Context, Template
+from pytest import raises
 
 
 def test_get():
@@ -359,3 +360,27 @@ def test_duration():
     context = Context({'value': 'xx'})
     assert Template('{{ value|duration }}').render(context) == ''
     assert Template('{{ value|duration:"long" }}').render(context) == ''
+
+
+def test_luminance():
+    context = Context({'value': '#8c22ec'})
+    assert Template('{{ value|luminance }}').render(context) == '71.12'
+
+    context = Context({'value': '#f0d343'})
+    assert Template('{{ value|luminance }}').render(context) == '206.7686'
+
+    context = Context({'value': '#8c22'})
+    with raises(ValueError):
+        Template('{{ value|luminance }}').render(context)
+
+
+def test_get_foreground_color():
+    context = Context({'value': '#8c22ec'})
+    assert Template('{{ value|get_foreground_color }}').render(context) == 'white'
+
+    context = Context({'value': '#f0d343'})
+    assert Template('{{ value|get_foreground_color }}').render(context) == 'black'
+
+    context = Context({'value': '#8c22'})
+    with raises(ValueError):
+        Template('{{ value|get_foreground_color }}').render(context)