Implement support for mixed sampled render targets

include/gpu/GrTypes.h

Index: include/gpu/GrTypes.h
diff --git a/include/gpu/GrTypes.h b/include/gpu/GrTypes.h
index 3f02c62bfd07aab21c647c673beef0850a782c32..9b2134c4669a1693c603525a723c04f8a2c1c424 100644
--- a/include/gpu/GrTypes.h
+++ b/include/gpu/GrTypes.h
@@ -423,6 +423,18 @@ enum GrSurfaceOrigin {
 };
 
 /**
+ * Sample config clarifies the meaning of samples count.
+ * In default kUnified_GrSampleConfig sample count refers to MSAA sample count.
+ * If NV_framebuffer_mixed_samples is available and sample config is chosen to be
+ * kStencil_GrSampleConfig, sample count refers to the stencil sample count only,
+ * and the color sample count is assumed to be zero.
+ */
+enum GrSampleConfig {
+    kUnified_GrSampleConfig,
+    kStencil_GrSampleConfig,
+};

[Chris Dalton, 2015/03/12 07:43:35]

So I've been thinking:

rt.numSamples(kUnified_GrSampleConfig)  isn't clear
rt.isMultisampled(kStencil_GrSampleConfig)  is a confusing way to ask if it's
multisampled anywhere

What if we take this enum out of GrTypes.h, and just put it inside of
GrRenderTarget? Maybe make two enums, one for numSamples and one for
isMultisampled. Something where you can ask about whether different stages of
the pipeline have multisample. e.g:

numSamples(StencilAndColor)
numSamples(Stencil)

isMultisampled(Anythere)
isMultisampled(Everywhere)
isMultisampled(Color)
isMultisampled(Stencil)
isMultisampled(Raster)

Also, we would have to replace 'GrSampleConfig fSampleConfig' from
GrBackendRenderTargetDesc with a bool.

This is mostly just a brain dump so I'm curious to hear what you think.

+
+/**
  * Describes a surface to be created.
  */
 struct GrSurfaceDesc {
@@ -557,6 +569,14 @@ struct GrBackendRenderTargetDesc {
      * OpenGL: FBO ID
      */
     GrBackendObject                 fRenderTargetHandle;
+    /**
+     * Clarifies the meaning of fSampleCnt.  By default the value refers to
+     * MSAA sample count.  If sample config is kStencil_GrSampleConfig, then
+     * fSampleCnt refers to stencil samples only, and color sample count is
+     * assumed to be zero.  kStencil_GrSampleConfig can be chosen only if
+     * NV_framebuffer_mixed_samples is available.
+     */

[Chris Dalton, 2015/03/12 07:43:35]

Any reason why this is at the end rather than right after fSampleCnt?

Also, I may reword the comment a bit. Maybe something more along these lines:

If fSampleCnt is > 1, which buffers are multisampled (color and stencil, or just
stencil?). The default is both. Note that mixing multisampled and aliased
buffers is only possible if NV_framebuffer_mixed_samples is available.

[vbuzinov, 2015/03/13 13:37:22]

Done.

+    GrSampleConfig                  fSampleConfig;
 };
 
 /**