Redefine the meaning of sample counts in GPU backend.

Old: 0  -> nonMSAA
     1+ -> MSAA

New:
     0  -> error/unsupported
     1  -> nonMSAA
     2+ -> MSAA

We still allow 0 to mean nonMSAA in three sets of public APIs for backwards compatibility:

1) SkSurface factories
2) GrBackendRenderTarget constructors
3) GrCaps::getSampleCnt()'s requestedCount parameter

However, we immediately clamp to 1 and treat 0 as invalid/non-renderable internally.

This also changes the behavior when using a large sample count. We now fail in that case rather than using the largest sample available sample count. GrCaps::getSampleCount() will return 0 in this case.


Bug: skia:
Change-Id: Ida22c6b22c1365e563c9046b611e88bf5eb3ff33
Reviewed-on: https://skia-review.googlesource.com/101560
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>

1 files changed, 9 insertions, 10 deletions

diff --git a/include/gpu/GrTypes.h b/include/gpu/GrTypes.h
index 3081543aac..ab21fba732 100644
--- a/include/gpu/GrTypes.h
+++ b/include/gpu/GrTypes.h
@@ -402,13 +402,12 @@ struct GrMipLevel {
  */
 struct GrSurfaceDesc {
     GrSurfaceDesc()
-        : fFlags(kNone_GrSurfaceFlags)
-        , fOrigin(kTopLeft_GrSurfaceOrigin)
-        , fWidth(0)
-        , fHeight(0)
-        , fConfig(kUnknown_GrPixelConfig)
-        , fSampleCnt(0) {
-    }
+            : fFlags(kNone_GrSurfaceFlags)
+            , fOrigin(kTopLeft_GrSurfaceOrigin)
+            , fWidth(0)
+            , fHeight(0)
+            , fConfig(kUnknown_GrPixelConfig)
+            , fSampleCnt(1) {}
 
     GrSurfaceFlags         fFlags;  //!< bitfield of TextureFlags
     GrSurfaceOrigin        fOrigin; //!< origin of the texture
@@ -422,11 +421,11 @@ struct GrSurfaceDesc {
     GrPixelConfig          fConfig;
 
     /**
-     * The number of samples per pixel or 0 to disable full scene AA. This only
+     * The number of samples per pixel. Zero is treated equivalently to 1. This only
      * applies if the kRenderTarget_GrSurfaceFlag is set. The actual number
      * of samples may not exactly match the request. The request will be rounded
-     * up to the next supported sample count, or down if it is larger than the
-     * max supported count.
+     * up to the next supported sample count. A value larger than the largest
+     * supported sample count will fail.
      */
     int                    fSampleCnt;
 };