gsk: Handle straight-alpha dmabufs

This omission was noticed by Benjamin Otte. Add a premultiply
uniform to the external shader, and add a separate premultiply
shader for the non-external case.

gsk/gl/gskglcommandqueue.c

@@ -1605,7 +1605,6 @@ gsk_gl_command_queue_do_upload_texture_chunk (GskGLCommandQueue *self,
       start_time = GDK_PROFILER_CURRENT_TIME;
     }
 
-
   glPixelStorei (GL_UNPACK_ALIGNMENT, gdk_memory_format_alignment (data_format));
 
   /* GL_UNPACK_ROW_LENGTH is available on desktop GL, OpenGL ES >= 3.0, or if

gsk/gl/gskgldriver.c

@@ -807,6 +807,8 @@ gsk_gl_driver_import_dmabuf_texture (GskGLDriver      *self,
   GskGLRenderTarget *render_target;
   guint prev_fbo;
   gboolean external;
+  GdkMemoryFormat format;
+  gboolean premultiply;
 
   gdk_gl_context_make_current (context);
 
@@ -822,6 +824,8 @@ gsk_gl_driver_import_dmabuf_texture (GskGLDriver      *self,
     }
 
   dmabuf = gdk_dmabuf_texture_get_dmabuf (texture);
+  format = gdk_texture_get_format (GDK_TEXTURE (texture));
+  premultiply = gdk_memory_format_alpha (format) == GDK_MEMORY_ALPHA_STRAIGHT;
 
   texture_id = gdk_gl_context_import_dmabuf (context,
                                              width, height,
@@ -830,12 +834,15 @@ gsk_gl_driver_import_dmabuf_texture (GskGLDriver      *self,
   if (texture_id == 0)
     return 0;
 
-  if (!external)
+  if (!external && !premultiply)
     return texture_id;
 
   gsk_gl_driver_autorelease_texture (self, texture_id);
 
+  if (external)
     program = self->external;
+  else
+    program = self->premultiply;
 
   if (!gsk_gl_driver_create_render_target (self, width, height, GL_RGBA8, &render_target))
     return texture_id;
@@ -849,10 +856,21 @@ gsk_gl_driver_import_dmabuf_texture (GskGLDriver      *self,
       set_viewport_for_size (self, program, width, height);
       reset_modelview (self, program);
 
+      if (external)
+        {
           gsk_gl_program_set_uniform_texture (program,
                                               UNIFORM_EXTERNAL_SOURCE, 0,
                                               GL_TEXTURE_EXTERNAL_OES, GL_TEXTURE0, texture_id);
 
+          gsk_gl_program_set_uniform1i (program, UNIFORM_PREMULTIPLY, 0, premultiply);
+        }
+      else
+        {
+          gsk_gl_program_set_uniform_texture (program,
+                                              UNIFORM_SHARED_SOURCE, 0,
+                                              GL_TEXTURE_2D, GL_TEXTURE0, texture_id);
+        }
+
       draw_rect (self->command_queue, 0, 0, width, height);
 
       gsk_gl_command_queue_end_draw (self->command_queue);

gsk/gl/gskglprograms.defs

@@ -96,4 +96,9 @@ GSK_GL_DEFINE_PROGRAM (unblurred_outset_shadow,
 
 GSK_GL_DEFINE_PROGRAM_NO_CLIP (external,
                                GSK_GL_SHADER_SINGLE (GSK_GL_SHADER_RESOURCE ("external.glsl")),
-                               GSK_GL_ADD_UNIFORM (1, EXTERNAL_SOURCE, u_external_source))
+                               GSK_GL_ADD_UNIFORM (1, EXTERNAL_SOURCE, u_external_source)
+                               GSK_GL_ADD_UNIFORM (2, PREMULTIPLY, u_premultiply))
+
+GSK_GL_DEFINE_PROGRAM_NO_CLIP (premultiply,
+                               GSK_GL_SHADER_SINGLE (GSK_GL_SHADER_RESOURCE ("premultiply.glsl")),
+                               GSK_GL_NO_UNIFORMS)

gsk/gl/resources/external.glsl

@@ -17,6 +17,8 @@ uniform samplerExternalOES u_external_source;
 uniform sampler2D u_external_source;
 #endif
 
+uniform int u_premultiply;
+
 void main() {
 /* Open-code this here, since GskTexture() expects a sampler2D */
 #if defined(GSK_GLES) || defined(GSK_LEGACY)
@@ -24,5 +26,9 @@ void main() {
 #else
   vec4 color = texture(u_external_source, vUv);
 #endif
+
+  if (u_premultiply == 1)
+    color.rgb *= color.a;
+
   gskSetOutputColor(color);
 }

gsk/gl/resources/premultiply.glsl

@@ -0,0 +1,19 @@
+// VERTEX_SHADER:
+// premultiply.glsl
+
+void main() {
+  gl_Position = u_projection * u_modelview * vec4(aPosition, 0.0, 1.0);
+
+  vUv = vec2(aUv.x, aUv.y);
+}
+
+// FRAGMENT_SHADER:
+// premultiply.glsl
+
+void main() {
+  vec4 color = GskTexture(u_source, vUv);
+
+  color.rgb *= color.a;
+
+  gskSetOutputColor(color);
+}

gsk/meson.build

@@ -21,6 +21,7 @@ gsk_private_gl_shaders = [
   'gl/resources/filled_border.glsl',
   'gl/resources/mask.glsl',
   'gl/resources/external.glsl',
+  'gl/resources/premultiply.glsl',
 ]
 
 gsk_public_sources = files([