Add "sample locations" feature to GrProcessor

src/gpu/gl/GrGLGpu.cpp

Index: src/gpu/gl/GrGLGpu.cpp
diff --git a/src/gpu/gl/GrGLGpu.cpp b/src/gpu/gl/GrGLGpu.cpp
index 73d219d03323dcc863786736df427dd4ad7e0348..bfed57e88191c1d9aff6d19decba8d09b0e00bad 100644
--- a/src/gpu/gl/GrGLGpu.cpp
+++ b/src/gpu/gl/GrGLGpu.cpp
@@ -3783,6 +3783,33 @@ bool GrGLGpu::copySurfaceAsBlitFramebuffer(GrSurface* dst,
     return true;
 }
 
+void GrGLGpu::onGetMultisampleSpecs(GrRenderTarget* rt,
+                                    const GrStencilSettings& stencil,
+                                    int* effectiveSampleCnt,
+                                    SamplePattern* pattern) {
+    SkASSERT(!rt->hasMixedSamples() || rt->renderTargetPriv().getStencilAttachment() ||
+             stencil.isDisabled());
+
+    this->flushStencil(stencil);
+    this->flushHWAAState(rt, true, !stencil.isDisabled());
+    this->flushRenderTarget(static_cast<GrGLRenderTarget*>(rt), &SkIRect::EmptyIRect());
+
+    if (0 != this->caps()->maxRasterSamples()) {
+        GR_GL_GetIntegerv(this->glInterface(), GR_GL_EFFECTIVE_RASTER_SAMPLES, effectiveSampleCnt);
+    } else {
+        GR_GL_GetIntegerv(this->glInterface(), GR_GL_SAMPLES, effectiveSampleCnt);
+    }
+    if (this->caps()->sampleLocationsSupport()) {
+        pattern->reset(*effectiveSampleCnt);
+        for (int i = 0; i < *effectiveSampleCnt; ++i) {
+            GrGLfloat pos[2];
+            GL_CALL(GetMultisamplefv(GR_GL_SAMPLE_POSITION, i, pos));
+            // OpenGL pixel space and Skia device space have inverted y directions.
+            (*pattern)[i].set(pos[0] - 0.5f, 0.5f - pos[1]);

[Mark Kilgard, 2016/02/22 18:41:04]

so do samples in Skia are in a [-0.5, 0.5) range where OpenGL uses [0,1] so
(0.5,0.5) is at the pixel center?

This is a bit surprising

The comment probably should note not just the inverted y direction but also the
2D shift by half-pixel for OpenGL pixel space vs. Skia device space

or is this to make interpolation in shaders easier by having pixel center at
(0,0)?

[Chris Dalton, 2016/02/22 19:10:43]

The 0.5, 0.5 offset was added by me just because it makes sense for
interpolation in shaders.

You bring up a good point. As far as the skia API is concerned, maybe it would
make more sense for 0,0 to be the top left corner? Brian? Josh? Thoughts? The
builders can easily account for this elsewhere if needed.

[bsalomon, 2016/02/22 20:28:26]

Skia's device coordinates have integers at the borders between pixels, so I
think it makes sense for 0.5, 0.5 in sample coordinates to be at the pixel
center.

[Chris Dalton, 2016/02/22 21:19:11]

Agreed. Now that I think about it I also prefer 0.5, 0.5 for the center.

[Chris Dalton, 2016/02/22 22:17:55]

Done.

+        }
+    }
+}
+
 void GrGLGpu::xferBarrier(GrRenderTarget* rt, GrXferBarrierType type) {
     SkASSERT(type);
     switch (type) {