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This is something that wasn't clear to me, and it
is important to know to understand how we are handling
(un)premultiplication here.

gdk/gdkgltexture.c

@@ -171,13 +171,21 @@ gdk_gl_texture_do_download (GdkGLTexture *self,
   Download *download = download_;
   GLenum gl_internal_format, gl_format, gl_type;
   int major, minor;
+  unsigned int internal_texture_format, dummy;
 
   expected_stride = texture->width * gdk_memory_format_bytes_per_pixel (download->format);
   gdk_gl_context_get_version (context, &major, &minor);
 
+  gdk_memory_format_gl_format (gdk_texture_get_format (texture),
+                               FALSE,
+                               major, minor,
+                               &internal_texture_format,
+                               &dummy, &dummy);
+
   if (download->stride == expected_stride &&
       !gdk_gl_context_get_use_es (context) &&
-      gdk_memory_format_gl_format (download->format, TRUE, major, minor, &gl_internal_format, &gl_format, &gl_type))
+      gdk_memory_format_gl_format (download->format, FALSE, major, minor, &gl_internal_format, &gl_format, &gl_type) &&
+      gl_internal_format == internal_texture_format)
     {
       glGetTexImage (GL_TEXTURE_2D,
                      0,

gdk/gdkmemoryformat.c

@@ -600,6 +600,11 @@ gdk_memory_convert (guchar              *dest_data,
   for (y = 0; y < height; y++)
     {
       src_desc->to_float (tmp, src_data, width);
+
+      /* Note: Converting from a premultiplied format to an opaque format is defined
+       * to yield the same result as compositing over black, which works out to
+       * using the premultiplied values, and just dropping alpha.
+       */
       if (src_desc->alpha == GDK_MEMORY_ALPHA_PREMULTIPLIED && dest_desc->alpha == GDK_MEMORY_ALPHA_STRAIGHT)
         unpremultiply (tmp, width);
       else if (src_desc->alpha == GDK_MEMORY_ALPHA_STRAIGHT && dest_desc->alpha != GDK_MEMORY_ALPHA_STRAIGHT)

gsk/gl/gskglcommandqueue.c

@@ -1447,6 +1447,7 @@ gsk_gl_command_queue_create_framebuffer (GskGLCommandQueue *self)
   return fbo_id;
 }
 
+
 static GdkMemoryFormat
 memory_format_gl_format (GdkMemoryFormat  data_format,
                          gboolean         use_es,
@@ -1611,7 +1612,7 @@ gsk_gl_command_queue_upload_texture_chunks (GskGLCommandQueue    *self,
   use_es = gdk_gl_context_get_use_es (self->context);
   gdk_gl_context_get_version (self->context, &major, &minor);
   data_format = gdk_texture_get_format (chunks[0].texture);
-  memory_format_gl_format (data_format,
+  data_format = memory_format_gl_format (data_format,
                                          use_es,
                                          major,
                                          minor,