initial implementation of L3 sub-space slicing support

Author: chrissimpkins

src/slice/instanceworker.py

@@ -20,12 +20,7 @@
 from fontTools.misc.textTools import num2binary
 from fontTools.ttLib import sfnt
 from fontTools.ttLib.ttFont import TTFont
-from fontTools.varLib.instancer import (
-    instantiateVariableFont as partial_instantiateVariableFont,
-)
-from fontTools.varLib.mutator import (
-    instantiateVariableFont as static_instantiateVariableFont,
-)
+from fontTools.varLib.instancer import instantiateVariableFont
 from PyQt5.QtCore import QObject, QRunnable, pyqtSignal, pyqtSlot
 
 
@@ -85,24 +80,11 @@ def instantiate_ttfont(self):
 
     def instantiate_variable_font(self):
         axis_instance_data = self.axis_model.get_instance_data()
-        axis_tag_number = len(self.ttfont["fvar"].axes)
-        # this is a partial instance request if the number of axis_instance_data
-        # definitions is less than the number of axes in the font
-        is_partial_instance = axis_tag_number > len(axis_instance_data)
-        # Execute the full static or partial instantiation.
-        # The static vs. partial approach is defined by the
-        # axis values contained in the axis_instance_data dict.
-        # When an axis tag is not present, that axis remains variable
-        # in the new font and uses the variable instantiator method
-        if is_partial_instance:
-            partial_instantiateVariableFont(
-                self.ttfont, axis_instance_data, inplace=True, optimize=True
-            )
-        else:
-            static_instantiateVariableFont(self.ttfont, axis_instance_data, inplace=True)
+        instantiateVariableFont(
+            self.ttfont, axis_instance_data, inplace=True, optimize=True
+        )
         print("\nAXIS INSTANCE VALUES")
         print(f"Instantiated variable font with axis definitions:\n{axis_instance_data}")
-        print(f"Partial instance: {is_partial_instance}")
 
     def edit_name_table(self):
         # string, nameID, platformID, platEncID, langID

src/slice/models.py

@@ -281,6 +281,8 @@ def get_instance_data(self):
                 # if user did not define the axis value, then
                 # it remains a variable axis
                 pass
+            elif ":" in axis_value:
+                instance_data[axistag] = self.parse_subspace_range(axis_value, axistag)
             else:
                 # else use the numeric value set in the editor
                 try:
@@ -293,6 +295,44 @@ def get_instance_data(self):
 
         return instance_data
 
+    def parse_subspace_range(self, range_string, axistag):
+        range_list = range_string.split(":")
+        # expect 2 values: min and max range sub-space values
+        # formatted as `value1:value2`
+        if len(range_list) != 2:
+            raise ValueError(
+                f"{range_string} is not a valid axis range. "
+                f"Use the format `min_value:max_value`"
+            )
+        # remove any extraneous whitespace (e.g., "100 : 200" entry)
+        # before attempt to cast to a float
+        try:
+            range_list[0] = float(range_list[0].strip())
+            range_list[1] = float(range_list[1].strip())
+        except ValueError as e:
+            raise ValueError(f"{range_string} is not a valid axis value range. {e}")
+        # order the values in case they were entered in reverse
+        # e.g., 800:400, not 400:800
+        sorted_range_list = sorted(range_list)
+        # We only support Level 3 sub-spacing due to the support
+        # that is available in fontTools lib.  Let's check that user
+        # included the default axis value in the range request
+        self.subspace_data_validates_includes_default_value(sorted_range_list, axistag)
+        # all seems well, return data formatted as tuple
+        # this is the data format required by fonttools lib
+        return (sorted_range_list[0], sorted_range_list[1])
+
+    def subspace_data_validates_includes_default_value(self, range_list, axistag):
+        """Validates Level 3 sub-space requirement that restricted axis range
+        includes the default axis value."""
+        default = self.get_default_axis_value(axistag)
+        if default < range_list[0] or default > range_list[1]:
+            raise ValueError(
+                f"{axistag} range {range_list[0]}:{range_list[1]} does not "
+                f"include the default axis value.  This is currently a "
+                f"requirement."
+            )
+
     def instance_data_validates_missing_data(self):
         # validator that returns True if there is at least one
         # axis tag with a defined instance, and False if all