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

[Brian Paul]

On 03/21/2014 05:22 PM, Kenneth Graunke wrote:
> 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)
>   }
>
>

Could you add a comment to the function to explain it?

What if the buffer is a GL_ALPHA only buffer (I think that's possible)? 
  Then _mesa_format_num_components() would be one and we should check 
the colormask's A component, right?



> +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;
>               }
>            }
>