new check: com.google.fonts/check/varfont/slnt_range

The variable font 'slnt' (Slant) axis coordinate
specifies positive values in its range?
(issue fonttools#2572)

CHANGELOG.md

@@ -4,9 +4,11 @@ A more detailed list of changes is available in the corresponding milestones for
 
 ## 0.7.11 (2019-Aug-16)
 ### Note-worthy code changes
-  - ...
+  - Add a few more keywords to log-messages (issue #2558)
+
 ### New checks
   - **[com.google.fonts/check/metadata/undeclared_fonts]:** "Ensure METADATA.pb lists all font binaries" (issue #2575)
+  - **[com.google.fonts/check/varfont/slnt_range:]** "The variable font 'slnt' (Slant) axis coordinate specifies positive values in its range?" (issue #2572)
 
 
 ## 0.7.10 (2019-Aug-06)

Lib/fontbakery/profiles/fvar.py

@@ -256,3 +256,38 @@ def com_google_fonts_check_wdth_valid_range(ttFont):
 
   if not Failed:
     yield PASS, ("OK")
+
+
+@check(
+  id = 'com.google.fonts/check/varfont/slnt_range',
+  rationale = """
+    The OpenType spec says at
+    https://docs.microsoft.com/en-us/typography/opentype/spec/dvaraxistag_slnt that:
+
+    [...] the scale for the Slant axis is interpreted as the angle of slant in
+    counter-clockwise degrees from upright. This means that a typical, right-leaning
+    oblique design will have a negative slant value. This matches the scale used for
+    the italicAngle field in the post table.
+  """,
+  conditions = ['is_variable_font',
+                'slnt_axis'],
+  misc_metadata = {
+    'request': 'https://github.com/googlefonts/fontbakery/issues/2572'
+  }
+)
+def com_google_fonts_check_varfont_slnt_range(ttFont, slnt_axis):
+  """ The variable font 'slnt' (Slant) axis coordinate
+      specifies positive values in its range? """
+
+  if slnt_axis.minValue < 0 and slnt_axis.maxValue >= 0:
+    yield PASS, "Looks good!"
+  else:
+    yield WARN,\
+          Message("unusual-range",
+                  f'The range of values for the "slnt" axis in'
+                  f' this font only allows positive coordinates'
+                  f' (from {slnt_axis.minValue} to {slnt_axis.maxValue}),'
+                  f' indicating that this may be a back slanted design,'
+                  f' which is rare. If that\'s not the case, then'
+                  f' the "slant" axis should be a range of'
+                  f' negative values instead.')

Lib/fontbakery/profiles/opentype.py

@@ -45,6 +45,7 @@
     'com.google.fonts/check/varfont/regular_ital_coord',
     'com.google.fonts/check/varfont/regular_opsz_coord',
     'com.google.fonts/check/varfont/bold_wght_coord',
+    'com.google.fonts/check/varfont/slnt_range',
     'com.google.fonts/check/loca/maxp_num_glyphs',
     'com.adobe.fonts/check/cff2_call_depth',
     'com.adobe.fonts/check/cff_call_depth',

Lib/fontbakery/profiles/shared_conditions.py

@@ -129,6 +129,13 @@ def is_variable_font(ttFont):
   return "fvar" in ttFont.keys()
 
 
+@condition
+def slnt_axis(ttFont):
+  for axis in ttFont["fvar"].axes:
+    if axis.axisTag == "slnt":
+      return axis
+
+
 def get_instance_axis_value(ttFont, instance_name, axis_tag):
   if not is_variable_font(ttFont):
     return None

tests/profiles/fvar_test.py

@@ -276,3 +276,26 @@ def test_check_wdth_valid_range():
   print('Test FAIL with wght=1001...')
   status, message = list(check(ttFont))[-1]
   assert status == FAIL and message.code == "out-of-range"
+
+
+def test_check_slnt_range():
+  """ The variable font 'slnt' (Slant) axis coordinate
+      specifies positive values in its range? """
+  from fontbakery.profiles.fvar import com_google_fonts_check_varfont_slnt_range as check
+  from fontbakery.profiles.shared_conditions import slnt_axis
+
+  # Our reference Inter varfont has a bad slnt range
+  ttFont = TTFont("data/test/varfont/inter/Inter[slnt,wght].ttf")
+  status, message = list(check(ttFont, slnt_axis(ttFont)))[-1]
+  assert status == WARN and message.code == "unusual-range"
+
+  # We then fix the font-bug by flipping the slnt axis range:
+  for i, axis in enumerate(ttFont["fvar"].axes):
+    if axis.axisTag == "slnt":
+      minValue, maxValue = axis.minValue, axis.maxValue
+      ttFont["fvar"].axes[i].minValue = -maxValue
+      ttFont["fvar"].axes[i].maxValue = -minValue
+
+  # And it must now PASS
+  status, message = list(check(ttFont, slnt_axis(ttFont)))[-1]
+  assert status == PASS