debug: Adds a COGL_DEBUG=disable-fast-read-pixel option

COGL_DEBUG=disable-fast-read-pixel can be used to disable the
optimization for reading a single pixel colour back by looking at the
geometry in the journal and not involving the GPU. With this disabled we
will always flush the journal, rendering to the framebuffer and then use
glReadPixels to get the result.

clutter/cogl/cogl/cogl-debug-options.h

@@ -168,4 +168,9 @@ OPT (DISABLE_BLENDING,
      "disable-program-caches",
      "Disable program caches",
      "Disable fallback caches for arbfp and glsl programs")
-
+OPT (DISABLE_FAST_READ_PIXEL,
+     "Root Cause",
+     "disable-fast-read-pixel",
+     "Disable read pixel optimization",
+     "Disable optimization for reading 1px for simple "
+     "scenes of opaque rectangles")

clutter/cogl/cogl/cogl-debug.c

@@ -75,7 +75,8 @@ static const GDebugKey cogl_behavioural_debug_keys[] = {
   { "disable-npot-textures", COGL_DEBUG_DISABLE_NPOT_TEXTURES},
   { "wireframe", COGL_DEBUG_WIREFRAME},
   { "disable-software-clip", COGL_DEBUG_DISABLE_SOFTWARE_CLIP},
-  { "disable-program-caches", COGL_DEBUG_DISABLE_PROGRAM_CACHES}
+  { "disable-program-caches", COGL_DEBUG_DISABLE_PROGRAM_CACHES},
+  { "disable-fast-read-pixel", COGL_DEBUG_DISABLE_FAST_READ_PIXEL}
 };
 static const int n_cogl_behavioural_debug_keys =
   G_N_ELEMENTS (cogl_behavioural_debug_keys);

clutter/cogl/cogl/cogl-debug.h

@@ -31,36 +31,37 @@
 G_BEGIN_DECLS
 
 typedef enum {
-  COGL_DEBUG_SLICING          = 1 << 1,
-  COGL_DEBUG_OFFSCREEN        = 1 << 2,
-  COGL_DEBUG_DRAW             = 1 << 3,
-  COGL_DEBUG_PANGO            = 1 << 4,
-  COGL_DEBUG_RECTANGLES       = 1 << 5,
-  COGL_DEBUG_HANDLE           = 1 << 6,
-  COGL_DEBUG_BLEND_STRINGS    = 1 << 7,
-  COGL_DEBUG_DISABLE_BATCHING = 1 << 8,
-  COGL_DEBUG_DISABLE_VBOS     = 1 << 9,
-  COGL_DEBUG_DISABLE_PBOS     = 1 << 10,
-  COGL_DEBUG_JOURNAL          = 1 << 11,
-  COGL_DEBUG_BATCHING         = 1 << 12,
-  COGL_DEBUG_DISABLE_SOFTWARE_TRANSFORM = 1 << 13,
-  COGL_DEBUG_MATRICES         = 1 << 14,
-  COGL_DEBUG_ATLAS            = 1 << 15,
-  COGL_DEBUG_DUMP_ATLAS_IMAGE = 1 << 16,
-  COGL_DEBUG_DISABLE_ATLAS    = 1 << 17,
-  COGL_DEBUG_OPENGL           = 1 << 18,
-  COGL_DEBUG_DISABLE_TEXTURING = 1 << 19,
-  COGL_DEBUG_DISABLE_ARBFP    = 1 << 20,
-  COGL_DEBUG_DISABLE_FIXED    = 1 << 21,
-  COGL_DEBUG_DISABLE_GLSL     = 1 << 22,
-  COGL_DEBUG_SHOW_SOURCE      = 1 << 23,
-  COGL_DEBUG_DISABLE_BLENDING = 1 << 24,
-  COGL_DEBUG_TEXTURE_PIXMAP   = 1 << 25,
-  COGL_DEBUG_BITMAP           = 1 << 26,
-  COGL_DEBUG_DISABLE_NPOT_TEXTURES = 1 << 27,
-  COGL_DEBUG_WIREFRAME        = 1 << 28,
-  COGL_DEBUG_DISABLE_SOFTWARE_CLIP = 1 << 29,
-  COGL_DEBUG_DISABLE_PROGRAM_CACHES = 1 << 30
+  COGL_DEBUG_SLICING          = 1 << 0,
+  COGL_DEBUG_OFFSCREEN        = 1 << 1,
+  COGL_DEBUG_DRAW             = 1 << 2,
+  COGL_DEBUG_PANGO            = 1 << 3,
+  COGL_DEBUG_RECTANGLES       = 1 << 4,
+  COGL_DEBUG_HANDLE           = 1 << 5,
+  COGL_DEBUG_BLEND_STRINGS    = 1 << 6,
+  COGL_DEBUG_DISABLE_BATCHING = 1 << 7,
+  COGL_DEBUG_DISABLE_VBOS     = 1 << 8,
+  COGL_DEBUG_DISABLE_PBOS     = 1 << 9,
+  COGL_DEBUG_JOURNAL          = 1 << 10,
+  COGL_DEBUG_BATCHING         = 1 << 11,
+  COGL_DEBUG_DISABLE_SOFTWARE_TRANSFORM = 1 << 12,
+  COGL_DEBUG_MATRICES         = 1 << 13,
+  COGL_DEBUG_ATLAS            = 1 << 14,
+  COGL_DEBUG_DUMP_ATLAS_IMAGE = 1 << 15,
+  COGL_DEBUG_DISABLE_ATLAS    = 1 << 16,
+  COGL_DEBUG_OPENGL           = 1 << 17,
+  COGL_DEBUG_DISABLE_TEXTURING = 1 << 18,
+  COGL_DEBUG_DISABLE_ARBFP    = 1 << 19,
+  COGL_DEBUG_DISABLE_FIXED    = 1 << 20,
+  COGL_DEBUG_DISABLE_GLSL     = 1 << 21,
+  COGL_DEBUG_SHOW_SOURCE      = 1 << 22,
+  COGL_DEBUG_DISABLE_BLENDING = 1 << 23,
+  COGL_DEBUG_TEXTURE_PIXMAP   = 1 << 24,
+  COGL_DEBUG_BITMAP           = 1 << 25,
+  COGL_DEBUG_DISABLE_NPOT_TEXTURES = 1 << 26,
+  COGL_DEBUG_WIREFRAME        = 1 << 27,
+  COGL_DEBUG_DISABLE_SOFTWARE_CLIP = 1 << 28,
+  COGL_DEBUG_DISABLE_PROGRAM_CACHES = 1 << 29,
+  COGL_DEBUG_DISABLE_FAST_READ_PIXEL = 1 << 30
 } CoglDebugFlags;
 
 #ifdef COGL_ENABLE_DEBUG

clutter/cogl/cogl/cogl-framebuffer.c

@@ -1237,6 +1237,9 @@ _cogl_framebuffer_try_fast_read_pixel (CoglFramebuffer *framebuffer,
 {
   gboolean found_intersection;
 
+  if (G_UNLIKELY (cogl_debug_flags & COGL_DEBUG_DISABLE_FAST_READ_PIXEL))
+    return FALSE;
+
   if (source != COGL_READ_PIXELS_COLOR_BUFFER)
     return FALSE;
 

clutter/cogl/cogl/cogl-journal.c

@@ -1689,6 +1689,9 @@ try_checking_point_hits_entry_after_clipping (CoglJournalEntry *entry,
       ClipBounds clip_bounds;
       float poly[16];
 
+      if (!can_software_clip)
+        return FALSE;
+
       if (!can_software_clip_entry (entry, NULL,
                                     entry->clip_stack, &clip_bounds))
         return FALSE;