[Mesa-dev] [PATCH] mesa: Skip clearing color buffers when color writes are disabled.

[Kenneth Graunke]

WebGL Aquarium in Chrome 24 actually hits this.

v2: Move to core Mesa (wisely suggested by Ian); only consider
    components which actually exist.

Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
Cc: Ian Romanick <idr at freedesktop.org>
Cc: Paul Berry <stereotype441 at gmail.com>
Cc: Anuj Phogat <anuj.phogat at gmail.com>
---
 src/mesa/main/clear.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

Good call, Ian!  I was a bit sleepy last night when I wrote that code :)

Here's an updated patch which moves it to core Mesa.  It also only checks
the writemask of enabled components.

I didn't see any existing code to handle this.  There's similar code which
disables depth clears if Depth.Mask == 0, but I didn't see any for color.

diff --git a/src/mesa/main/clear.c b/src/mesa/main/clear.c
index 077c5fc..c9ca681 100644
--- a/src/mesa/main/clear.c
+++ b/src/mesa/main/clear.c
@@ -106,6 +106,23 @@ _mesa_ClearColorIuiEXT(GLuint r, GLuint g, GLuint b, GLuint a)
 }
 
 
+static bool
+color_buffer_writes_enabled(const struct gl_context *ctx, unsigned idx)
+{
+   struct gl_renderbuffer *rb = ctx->DrawBuffer->_ColorDrawBuffers[idx];
+   GLuint c;
+   GLubyte colorMask = 0;
+
+   if (rb) {
+      for (c = 0; c < _mesa_format_num_components(rb->Format); c++) {
+         colorMask |= ctx->Color.ColorMask[idx][c];
+      }
+   }
+
+   return colorMask != 0;
+}
+
+
 /**
  * Clear buffers.
  * 
@@ -181,7 +198,7 @@ _mesa_Clear( GLbitfield mask )
          for (i = 0; i < ctx->DrawBuffer->_NumColorDrawBuffers; i++) {
             GLint buf = ctx->DrawBuffer->_ColorDrawBufferIndexes[i];
 
-            if (buf >= 0) {
+            if (buf >= 0 && color_buffer_writes_enabled(ctx, i)) {
                bufferMask |= 1 << buf;
             }
          }
-- 
1.9.0