common/font: avoid rendering semi-transparent background twice

Fill with the background color first only if the background color is
opaque. This is necessary for subpixel rendering to work properly (it
does not work on top of transparency).

However, if the background color is not opaque, leave the buffer
unfilled (completely transparent) since the background is already
rendered by the scene element underneath. In this case we have to
disable subpixel rendering.

v2: use 0.999 alpha cutoff and fix CodeStyleCheck

Fixes: (the rest of) #1684

diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index 14fdb88e9eff367e371e40158c60df691dc4f4db..72368147addab7dbea8b92915f224ce07cbf0096 100755 (executable)
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl
@@ -5655,15 +5655,8 @@ sub process {
                            $var !~ /^(?:Clear|Set|TestClear|TestSet|)Page[A-Z]/ &&
 
 #labwc-custom check to ignore some pango/libxml2/etc CamelCase variants
-                           $var !~ /^(?:PangoLayout|PangoFontDescription)/ &&
-                           $var !~ /^(?:PangoTabArray|PangoRectangle)/ &&
-                           $var !~ /^(?:PangoWeight|_PangoFontDescription)/ &&
-                           $var !~ /^(?:xmlNode|xmlIsBlankNode|xmlAttr)/ &&
-                           $var !~ /^(?:xmlGetProp|xmlChar|xmlDoc)/ &&
-                           $var !~ /^(?:xmlReadFile|xmlDocGetRootElement)/ &&
-                           $var !~ /^(?:xmlFreeDoc|xmlCleanupParser)/ &&
-                           $var !~ /^(?:xmlParseMemory)/ &&
-                           $var !~ /^(?:xmlFree)/ &&
+                           $var !~ /^(?:_?Pango\w+)/ &&
+                           $var !~ /^(?:xml\w+)/ &&
                            $var !~ /^(?:GString|GError)/ &&
                            $var !~ /^(?:RsvgRectangle|RsvgHandle)/ &&
                            $var !~ /^(?:XKB_KEY_XF86Switch_VT_1)/ &&

diff --git a/src/common/font.c b/src/common/font.c
index 88af348941d69e03bcd3ba7fc2862ef2a27244b1..c9e6166c6a905ea6d4484022055e3149a8e0e25a 100644 (file)
--- a/src/common/font.c
+++ b/src/common/font.c
@@ -115,13 +115,23 @@ font_buffer_create(struct lab_data_buffer **buffer, int max_width,
        cairo_surface_t *surf = cairo_get_target(cairo);
 
        /*
-        * Fill background color first - necessary for subpixel
-        * rendering, which does not work properly on transparency
+        * Fill with the background color first IF the background color
+        * is opaque. This is necessary for subpixel rendering to work
+        * properly (it does not work on top of transparency).
+        *
+        * However, if the background color is not opaque, leave the
+        * buffer unfilled (completely transparent) since the background
+        * is already rendered by the scene element underneath. In this
+        * case we have to disable subpixel rendering.
+        *
+        * Note: the 0.999 cutoff was chosen to be greater than 254/255
+        * (about 0.996) but leave some margin for rounding errors.
         */
-       set_cairo_color(cairo, bg_color);
-       cairo_rectangle(cairo, 0, 0, (*buffer)->unscaled_width,
-               (*buffer)->unscaled_height);
-       cairo_fill(cairo);
+       bool opaque_bg = (bg_color[3] > 0.999f);
+       if (opaque_bg) {
+               set_cairo_color(cairo, bg_color);
+               cairo_paint(cairo);
+       }
 
        set_cairo_color(cairo, color);
        cairo_move_to(cairo, 0, 0);
@@ -131,6 +141,15 @@ font_buffer_create(struct lab_data_buffer **buffer, int max_width,
        pango_layout_set_text(layout, text, -1);
        pango_layout_set_ellipsize(layout, PANGO_ELLIPSIZE_END);
 
+       if (!opaque_bg) {
+               /* disable subpixel rendering */
+               cairo_font_options_t *opts = cairo_font_options_create();
+               cairo_font_options_set_antialias(opts, CAIRO_ANTIALIAS_GRAY);
+               PangoContext *ctx = pango_layout_get_context(layout);
+               pango_cairo_context_set_font_options(ctx, opts);
+               cairo_font_options_destroy(opts);
+       }
+
        PangoFontDescription *desc = font_to_pango_desc(font);
        pango_layout_set_font_description(layout, desc);
        pango_font_description_free(desc);