Workaround for blending bug in new Agg.

mdboom · mdboom · commit 7766ebe37911 · 2014-11-11T08:38:24.000-05:00
diff --git a/src/_backend_agg.h b/src/_backend_agg.h
@@ -38,7 +38,49 @@
 #include "path_converters.h"
 #include "array.h"
 
-typedef agg::pixfmt_rgba32_plain pixfmt;
+/**********************************************************************
+ WORKAROUND: This class is to workaround a bug in Agg SVN where the
+ blending of RGBA32 pixels does not preserve enough precision
+*/
+
+template<class ColorT, class Order>
+struct fixed_blender_rgba_plain : agg::conv_rgba_plain<ColorT, Order>
+{
+    typedef ColorT color_type;
+    typedef Order order_type;
+    typedef typename color_type::value_type value_type;
+    typedef typename color_type::calc_type calc_type;
+    typedef typename color_type::long_type long_type;
+    enum base_scale_e { base_shift = color_type::base_shift };
+
+    //--------------------------------------------------------------------
+    static AGG_INLINE void blend_pix(value_type* p,
+                                     value_type cr, value_type cg, value_type cb, value_type alpha, agg::cover_type cover)
+    {
+        blend_pix(p, cr, cg, cb, color_type::mult_cover(alpha, cover));
+    }
+
+    //--------------------------------------------------------------------
+    static AGG_INLINE void blend_pix(value_type* p,
+                                     value_type cr, value_type cg, value_type cb, value_type alpha)
+    {
+        if(alpha == 0) return;
+        calc_type a = p[Order::A];
+        calc_type r = p[Order::R] * a;
+        calc_type g = p[Order::G] * a;
+        calc_type b = p[Order::B] * a;
+        a = ((alpha + a) << base_shift) - alpha * a;
+        p[Order::A] = (value_type)(a >> base_shift);
+        p[Order::R] = (value_type)((((cr << base_shift) - r) * alpha + (r << base_shift)) / a);
+        p[Order::G] = (value_type)((((cg << base_shift) - g) * alpha + (g << base_shift)) / a);
+        p[Order::B] = (value_type)((((cb << base_shift) - b) * alpha + (b << base_shift)) / a);
+    }
+};
+
+/**********************************************************************/
+
+typedef fixed_blender_rgba_plain<agg::rgba8, agg::order_rgba> fixed_blender_rgba32_plain;
+typedef agg::pixfmt_alpha_blend_rgba<fixed_blender_rgba32_plain, agg::rendering_buffer> pixfmt;
 typedef agg::renderer_base<pixfmt> renderer_base;
 typedef agg::renderer_scanline_aa_solid<renderer_base> renderer_aa;
 typedef agg::renderer_scanline_bin_solid<renderer_base> renderer_bin;
@@ -831,7 +873,7 @@ inline void RendererAgg::draw_image(GCAgg &gc,
         inv_mtx.invert();
 
         typedef agg::span_allocator<agg::rgba8> color_span_alloc_type;
-        typedef agg::image_accessor_clip<agg::pixfmt_rgba32_plain> image_accessor_type;
+        typedef agg::image_accessor_clip<pixfmt> image_accessor_type;
         typedef agg::span_interpolator_linear<> interpolator_type;
         typedef agg::span_image_filter_rgba_nn<image_accessor_type, interpolator_type>
         image_span_gen_type;
