[colr_to_svg] drop unsupported PaintComposites with a warning

src/nanoemoji/colr_to_svg.py

@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from absl import logging
 from nanoemoji import colors
 from nanoemoji import color_glyph
 from nanoemoji.glyph_reuse import GlyphReuseCache
@@ -265,21 +266,29 @@ def descend(parent: etree.Element, paint: otTables.Paint):
         ]:
             descend(parent_el, child_paint)
 
-    elif ot_paint.Format == PaintComposite.format and (
-        ot_paint.CompositeMode == CompositeMode.SRC_IN
-        and ot_paint.BackdropPaint.Format == PaintSolid.format
-    ):
-        # Only simple group opacity for now
-        color = _color(
-            ttfont,
-            ot_paint.BackdropPaint.PaletteIndex,
-            ot_paint.BackdropPaint.Alpha,
+    elif ot_paint.Format == PaintComposite.format:
+        if (
+            ot_paint.CompositeMode == CompositeMode.SRC_IN
+            and ot_paint.BackdropPaint.Format == PaintSolid.format
+        ):
+            # Special-case simple PaintComposite for group opacity
+            color = _color(
+                ttfont,
+                ot_paint.BackdropPaint.PaletteIndex,
+                ot_paint.BackdropPaint.Alpha,
+            )
+            if color[:3] == (0, 0, 0):
+                g = etree.SubElement(parent_el, "g")
+                g.attrib["opacity"] = ntos(color.alpha)
+                descend(g, ot_paint.SourcePaint)
+                return
+
+        # https://github.com/googlefonts/nanoemoji/issues/409
+        logging.warning(
+            "PaintComposite => SVG not supported at the moment; "
+            "only BackdropPaint is kept."
         )
-        if color[:3] != (0, 0, 0):
-            raise NotImplementedError(color)
-        g = etree.SubElement(parent_el, "g")
-        g.attrib["opacity"] = ntos(color.alpha)
-        descend(g, ot_paint.SourcePaint)
+        descend(parent_el, ot_paint.BackdropPaint)
 
     elif ot_paint.Format == PaintColrGlyph.format:
         el = parent_el

tests/colr_to_svg_test.py

@@ -131,6 +131,42 @@ def _buildPaint(source) -> ot.Paint:
             </svg>
             """,
         ),
+        # other kinds of PaintComposite are dropped for now, only BackdropPaint is kept
+        # https://github.com/googlefonts/nanoemoji/issues/409
+        (
+            "composite",
+            {
+                "composite": {
+                    "Format": ot.PaintFormat.PaintComposite,
+                    "CompositeMode": "soft_light",
+                    "SourcePaint": {
+                        "Format": ot.PaintFormat.PaintLinearGradient,
+                        "ColorLine": {
+                            "Extend": "pad",
+                            "ColorStop": [(0.0, 1, 0.5), (0.5, 2, 0.5), (1.0, 3, 0.5)],
+                        },
+                        "x0": 47,
+                        "y0": 790,
+                        "x1": 890,
+                        "y1": -342,
+                        "x2": -1085,
+                        "y2": -53,
+                    },
+                    "BackdropPaint": (
+                        ot.PaintFormat.PaintGlyph,
+                        (ot.PaintFormat.PaintSolid, 0),
+                        "box",
+                    ),
+                },
+            },
+            {"box": _draw_box},
+            """
+            <svg xmlns="http://www.w3.org/2000/svg">
+              <defs/>
+              <path d="M10,10 L90,10 L90,90 L10,90 Z"/>
+            </svg>
+            """,
+        ),
     ],
 )
 def test_colr_v1_paint_to_svg(