Fallback to GRAY text antialiasing on transparent backgrounds.

Author: anntzer

CHANGELOG.rst

@@ -7,6 +7,7 @@ next
 - Support pdf MaxVersion up to 1.7 (if the underlying cairo supports it).
 - Support custom pdf metadata entries.
 - `set_options` can now be used as a context manager.
+- Improve rendering of antialiased non-black text on transparent background.
 
 v0.5 (2022-08-18)
 =================

ext/_mplcairo.cpp

@@ -449,6 +449,10 @@ GraphicsContextRenderer::_additional_context()
   return {this};
 }
 
+void GraphicsContextRenderer::_set_subpixel_antialiased_text_allowed(bool b) {
+  subpixel_antialiased_text_allowed_ = b;
+}
+
 void GraphicsContextRenderer::_set_path(std::optional<std::string> path) {
   path_ = path;
 }
@@ -1573,7 +1577,7 @@ void GraphicsContextRenderer::draw_text(
     auto const& font_size =
       points_to_pixels(prop.attr("get_size_in_points")().cast<double>());
     cairo_set_font_size(cr_, font_size);
-    adjust_font_options(cr_);
+    adjust_font_options(cr_, subpixel_antialiased_text_allowed_);
     auto const& gac = text_to_glyphs_and_clusters(cr_, s);
     // While the warning below perhaps belongs logically to
     // text_to_glyphs_and_clusters, we don't want to also emit the warning in
@@ -1633,7 +1637,7 @@ GraphicsContextRenderer::get_text_width_height_descent(
     auto const& font_size =
       points_to_pixels(prop.attr("get_size_in_points")().cast<double>());
     cairo_set_font_size(cr_, font_size);
-    adjust_font_options(cr_);  // Needed for correct aa.
+    adjust_font_options(cr_, subpixel_antialiased_text_allowed_);  // For correct aa.
     cairo_text_extents_t extents;
     auto const& gac = text_to_glyphs_and_clusters(cr_, s);
     cairo_glyph_extents(cr_, gac.glyphs, gac.num_glyphs, &extents);
@@ -1789,7 +1793,7 @@ void MathtextBackend::draw(
 {
   if (!std::isfinite(x) || !std::isfinite(y)) {
     // This happens e.g. with empty strings, and would put cr in an invalid
-    // state. even though nothing is written.
+    // state, even though nothing is written.
     return;
   }
   [[maybe_unused]] auto const& ac = gcr._additional_context();
@@ -1805,7 +1809,7 @@ void MathtextBackend::draw(
     auto const& mtx = cairo_matrix_t{
       size * glyph.extend, 0, -size * glyph.slant * glyph.extend, size, 0, 0};
     cairo_set_font_matrix(cr, &mtx);
-    adjust_font_options(cr);
+    adjust_font_options(cr, gcr.subpixel_antialiased_text_allowed_);
     auto ft_face =
       static_cast<FT_Face>(
         cairo_font_face_get_user_data(face, &detail::FT_KEY));
@@ -2155,6 +2159,8 @@ Only intended for debugging purposes.
       [](GraphicsContextRenderer& gcr) -> bool {
         return has_vector_surface(gcr.cr_);
       })
+    .def("_set_subpixel_antialiased_text_allowed",
+         &GraphicsContextRenderer::_set_subpixel_antialiased_text_allowed)
     .def("_set_path", &GraphicsContextRenderer::_set_path)
     .def("_set_metadata", &GraphicsContextRenderer::_set_metadata)
     .def("_set_size", &GraphicsContextRenderer::_set_size)

ext/_mplcairo.h

@@ -24,6 +24,7 @@ py::object renderer_base(std::string meth_name);
 class GraphicsContextRenderer {
   public:
   cairo_t* const cr_;
+  bool subpixel_antialiased_text_allowed_ = true;
 
   private:
   std::optional<std::string> path_ = {};
@@ -70,6 +71,7 @@ class GraphicsContextRenderer {
 
   AdditionalContext _additional_context();
 
+  void _set_subpixel_antialiased_text_allowed(bool b);
   void _set_path(std::optional<std::string> path);
   void _set_metadata(std::optional<py::dict> metadata);
   void _set_size(double width, double height, double dpi);

ext/_util.cpp

@@ -867,7 +867,7 @@ long get_hinting_flag()
     .attr("get_hinting_flag")().cast<long>();
 }
 
-void adjust_font_options(cairo_t* cr)
+void adjust_font_options(cairo_t* cr, bool subpixel_antialiased_text_allowed)
 {
   auto const& font_face = cairo_get_font_face(cr);
   auto const& options = cairo_font_options_create();
@@ -877,7 +877,9 @@ void adjust_font_options(cairo_t* cr)
     auto aa = rc_param("text.antialiased");  // Normally *exactly* a bool.
     cairo_font_options_set_antialias(
       options,
-      aa.ptr() == Py_True ? CAIRO_ANTIALIAS_SUBPIXEL
+      aa.ptr() == Py_True
+      ? (subpixel_antialiased_text_allowed
+         ? CAIRO_ANTIALIAS_SUBPIXEL : CAIRO_ANTIALIAS_GRAY)
       : aa.ptr() == Py_False ? CAIRO_ANTIALIAS_NONE
       : aa.cast<cairo_antialias_t>());
   }

ext/_util.h

@@ -186,7 +186,7 @@ cairo_font_face_t* font_face_from_path(std::string path);
 cairo_font_face_t* font_face_from_path(py::object path);
 std::vector<cairo_font_face_t*> font_faces_from_prop(py::object prop);
 long get_hinting_flag();
-void adjust_font_options(cairo_t* cr);
+void adjust_font_options(cairo_t* cr, bool subpixel_antialiased_text_allowed);
 void warn_on_missing_glyph(std::string s);
 GlyphsAndClusters text_to_glyphs_and_clusters(cairo_t* cr, std::string s);
 

src/mplcairo/base.py

@@ -232,11 +232,23 @@ def get_renderer(self, cleared=False):
 
     renderer = property(get_renderer)  # NOTE: Needed for FigureCanvasAgg.
 
-    def draw(self):
+    def _draw_without_supercall(self):
         renderer = self.get_renderer()
         renderer.clear()
         with _LOCK:
+            # Subpixel (LCD) antialiasing of non-black text looks bad on
+            # transparent backgrounds, so interpret True as GRAY antialiasing
+            # in that case (it is still possible to force SUBPIXEL antialiasing
+            # by actually using that antialias_t value).  For black text we
+            # could actually keep it, but get_text_width_height_descent()
+            # doesn't know the text color.
+            if mpl.colors.to_rgba(self.figure.patch.get_facecolor())[3] == 0:
+                renderer._set_subpixel_antialiased_text_allowed(False)
             self.figure.draw(renderer)
+            renderer._set_subpixel_antialiased_text_allowed(True)
+
+    def draw(self):
+        self._draw_without_supercall()
         super().draw()
 
     def buffer_rgba(self):  # NOTE: Needed for tests.
@@ -348,11 +360,9 @@ def _print_ps_impl(self, is_eps, path_or_stream, *,
     print_eps = partialmethod(_print_ps_impl, True)
 
     def _get_fresh_straight_rgba8888(self):
-        renderer = self.get_renderer()
-        renderer.clear()
-        with _LOCK:
-            self.figure.draw(renderer)
-        return _mplcairo.cairo_to_straight_rgba8888(renderer._get_buffer())
+        self._draw_without_supercall()
+        return _mplcairo.cairo_to_straight_rgba8888(
+            self.get_renderer()._get_buffer())
 
     def print_rgba(self, path_or_stream, *,
                    dryrun=False, metadata=None, **kwargs):