Implement support for mixed sampled render targets

include/gpu/GrRenderTarget.h

Index: include/gpu/GrRenderTarget.h
diff --git a/include/gpu/GrRenderTarget.h b/include/gpu/GrRenderTarget.h
index 40e006aa8aec8edb70b28cec5a3274f5b7f0e110..1245bebb955b340d241e8a32c11b056b62a71ead 100644
--- a/include/gpu/GrRenderTarget.h
+++ b/include/gpu/GrRenderTarget.h
@@ -31,14 +31,52 @@ public:
 
     // GrRenderTarget
     /**
-     * @return true if the surface is multisampled, false otherwise
+     * Pipeline stage serves as a query parameter to render target's isMultisampled
+     * and numSamples methods.  It signifies whether the caller is interested in
+     * multisampling in color buffer or stencil buffer specifically, both, or any of
+     * the two.  Used by the NV_framebuffer_mixed_samples implementation.

[Chris Dalton, 2015/03/20 07:01:45]

This comment should probably talk about the pipeline stage instead of the
buffer. I might reword it something like this:

"On some hardware it is possible for a render target to have multisampling only
in certain stages of the graphics pipeline. This enum allows the caller to query
about specific stages in isMultisampled() and numSamples()."

      */
-    bool isMultisampled() const { return 0 != fDesc.fSampleCnt; }
+    enum PipelineStage {
+        kColorBuffer_PipelineStage,
+        kStencilBuffer_PipelineStage,

[Chris Dalton, 2015/03/20 07:01:45]

These equalities seem like they need a comment.

 // We only support two sample configs at the moment, unified and stencil. So we
can query multisample info for everywhere/anywhere by checking the correct
buffer.

+        kEverywhere_PipelineStage = kColorBuffer_PipelineStage,
+        kAnywhere_PipelineStage = kStencilBuffer_PipelineStage
+    };
+
+    /**
+     * @param  pipelineStage  specifies the pipeline stage for which
+     *                        multisampling is queried
+     * @return true if the surface is multisampled in all of the buffers
+     *         specified by the sample config, false if any are non-MSAA

[Chris Dalton, 2015/03/20 07:01:45]

"true if the surface is multisampled in the specified pipeline stage(s), false
otherwise"

+     */
+    bool isMultisampled(PipelineStage pipelineStage =
+                                      kEverywhere_PipelineStage) const {
+        return 0 != numSamples(pipelineStage);
+    }
 
     /**
-     * @return the number of samples-per-pixel or zero if non-MSAA.
+     * @param  pipelineStage  specifies the pipeline stage for which
+     *                        multisampling is queried
+     * @return the number of samples-per-pixel or zero if any of the specified
+     *         buffers is non-MSAA.

[Chris Dalton, 2015/03/20 07:01:45]

"the number of samples-per-pixel, or zero if the render target is not
multisampled in the specified pipeline stage(s)"

      */
-    int numSamples() const { return fDesc.fSampleCnt; }
+    int numSamples(PipelineStage pipelineStage =
+                                 kEverywhere_PipelineStage) const {

[Chris Dalton, 2015/03/20 07:01:45]

Now I'm nit-picking, but I'm wondering if this function could be more clear if
it were implemented in this way?

  return this->isMultisampled(pipelineStage) ? fDesc.fSampleCnt : 0;

Then isMultisampled would obviously be the one to check fSampleConfig instead:

 if (kStencil_GrSampleConfig == fSampleConfig &&
     kColorBuffer_PipelineStage == pipelineStage) {
     return false;
 }
 return 0 != fDesc.fSampleCnt;

Thoughs? This way seems more natural to me at the moment...

+        if (kStencil_GrSampleConfig == fSampleConfig &&
+            kColorBuffer_PipelineStage == pipelineStage) {
+            return 0;
+        }
+        return fDesc.fSampleCnt;
+    }
+
+    /**
+     * @return sample config: kUnified_GrSampleConfig if both color and stencil
+               buffers are MSAA, or kStencil_GrSampleConfig if only stencil
+               buffer is MSAA.
+     */
+    GrSampleConfig sampleConfig() const {
+        return fSampleConfig;
+    }
 
     /**
      * Call to indicate the multisample contents were modified such that the
@@ -94,8 +132,10 @@ public:
     const GrRenderTargetPriv renderTargetPriv() const;
 
 protected:
-    GrRenderTarget(GrGpu* gpu, LifeCycle lifeCycle, const GrSurfaceDesc& desc)
+    GrRenderTarget(GrGpu* gpu, LifeCycle lifeCycle, const GrSurfaceDesc& desc,
+                   GrSampleConfig sampleConfig)
         : INHERITED(gpu, lifeCycle, desc)
+        , fSampleConfig(sampleConfig)
         , fStencilBuffer(NULL) {
         fResolveRect.setLargestInverted();
     }
@@ -110,6 +150,8 @@ private:
 
     friend class GrRenderTargetPriv;
 
+    GrSampleConfig    fSampleConfig;
+
     GrStencilBuffer*  fStencilBuffer;
 
     SkIRect           fResolveRect;