[ps] Only use fallback images if the gradient has non-constant alpha.

If the gradient has constant alpha, then we can express it as a flattened
linear gradient. Otherwise, should the opacity vary across the gradient we
need to fallback.

src/cairo-ps-surface.c

@@ -1437,11 +1437,25 @@ static cairo_bool_t
 _gradient_pattern_supported (cairo_ps_surface_t    *surface,
 			     cairo_pattern_t *pattern)
 {
+    cairo_gradient_pattern_t *gradient = (cairo_gradient_pattern_t *) pattern;
+    uint16_t alpha;
     cairo_extend_t extend;
+    int i;
 
     if (surface->ps_level == CAIRO_PS_LEVEL_2)
 	return FALSE;
 
+    if (gradient->n_stops == 0)
+	return TRUE;
+
+    /* Alpha gradients are only supported (by flattening the alpha)
+     * if there is no variation in the alpha across the gradient. */
+    alpha = gradient->stops[0].color.alpha_short;
+    for (i = 0; i < gradient->n_stops; i++) {
+	if (gradient->stops[i].color.alpha_short != alpha)
+	    return FALSE;
+    }
+
     surface->ps_level_used = CAIRO_PS_LEVEL_3;
     extend = cairo_pattern_get_extend (pattern);
 

test/.gitignore

@@ -91,6 +91,7 @@ get-xrender-format
 glitz-surface-source
 glyph-cache-pressure
 gradient-alpha
+gradient-constant-alpha
 gradient-zero-stops
 group-paint
 imagediff

test/Makefile.am

@@ -455,7 +455,6 @@ REFERENCE_IMAGES = \
 	gradient-alpha-rgb24-ref.png	\
 	gradient-constant-alpha-pdf-argb32-ref.png	\
 	gradient-constant-alpha-pdf-rgb24-ref.png	\
-	gradient-constant-alpha-ps-argb32-ref.png	\
 	gradient-constant-alpha-ps-rgb24-ref.png	\
 	gradient-constant-alpha-ref.png	\
 	gradient-constant-alpha-rgb24-ref.png	\