gpu, cmaa: don't blend the leftmost and bottom-most edges.

gpu/command_buffer/service/gles2_cmd_apply_framebuffer_attachment_cmaa_intel.cc

Index: gpu/command_buffer/service/gles2_cmd_apply_framebuffer_attachment_cmaa_intel.cc
diff --git a/gpu/command_buffer/service/gles2_cmd_apply_framebuffer_attachment_cmaa_intel.cc b/gpu/command_buffer/service/gles2_cmd_apply_framebuffer_attachment_cmaa_intel.cc
index 851c2bf79db7f7b11c0b02758831c51fc58dd71a..25f3eda2bd49171d5f69aa40655db81f95a10c92 100644
--- a/gpu/command_buffer/service/gles2_cmd_apply_framebuffer_attachment_cmaa_intel.cc
+++ b/gpu/command_buffer/service/gles2_cmd_apply_framebuffer_attachment_cmaa_intel.cc
@@ -1370,6 +1370,7 @@ const char ApplyFramebufferAttachmentCMAAINTELResourceManager::cmaa_frag_s2_[] =
     void CombineEdges() {
       ivec3 screenPosIBase = ivec3(ivec2(gl_FragCoord.xy) * 2, 0);
       vec3 screenPosBase = vec3(screenPosIBase);
+      uvec2 notBottomLeft = uvec2(screenPosIBase.xy != ivec2(0, 0));

[piman, 2016/06/28 21:08:09]

"screenPosIBase.xy != ivec2(0, 0)" is a boolean scalar. So notBottomLeft.x ==
notBottomLeft.y. What is the intention in setting this to a uvec2? Or did you
mean to use equal(screenPosIBase.xy, ivec2(0, 0))?

[dshwang, 2016/06/29 10:35:56]

sorry for obfuscated code, because I want to avoid to use "if statement", which
is not good for shader.

Let me explain what this code intends.

Following two elements stores one pixel below the current pixel.
packedEdgesArray[(1) * 3 + (0)] = sampA.w;
packedEdgesArray[(2) * 3 + (0)] = sampA.z;

so if current pixel is (_, 0), the shader must avoid storing the value. The
intuitive code would be

if (screenPosIBase.y == 0) {
packedEdgesArray[(1) * 3 + (0)] = 0;
packedEdgesArray[(2) * 3 + (0)] = 0;
} else {
packedEdgesArray[(1) * 3 + (0)] = sampA.w;
packedEdgesArray[(2) * 3 + (0)] = sampA.z;
}

However, in general, multiplying is fater than "if statement" in shader. So I
replace "if" to multiplying.

If you want more intuitive code, let me use "if statement". WDYT?

[dshwang, 2016/06/29 13:54:36]

I actually wanted component wise comparison; notEqual(screenPosIBase.xy,
ivec2(0, 0))
I updated this CL to use notEqual for component wise comparison, instead of
scalar boolean. Adrian, Thank you for advicing me how to fix it.

       uint packedEdgesArray[3 * 3];
 
       // use only if it has the 'prev frame' flag:[sample * 255.0 - 127.5]
@@ -1387,12 +1388,15 @@ const char ApplyFramebufferAttachmentCMAAINTELResourceManager::cmaa_frag_s2_[] =
                            ivec2(1, 1)).r * 255.0 - 127.5);
 
       packedEdgesArray[(0) * 3 + (0)] = 0u;
-      packedEdgesArray[(1) * 3 + (0)] = sampA.w;
-      packedEdgesArray[(2) * 3 + (0)] = sampA.z;
+      // The bottom-most edge of the texture is not edge.
+      // Note: texelFetch() on out of range gives an undefined value.
+      packedEdgesArray[(1) * 3 + (0)] = sampA.w * notBottomLeft.y;
+      packedEdgesArray[(2) * 3 + (0)] = sampA.z * notBottomLeft.y;
       packedEdgesArray[(1) * 3 + (1)] = sampA.x;
       packedEdgesArray[(2) * 3 + (1)] = sampA.y;
-      packedEdgesArray[(0) * 3 + (1)] = sampB.w;
-      packedEdgesArray[(0) * 3 + (2)] = sampB.x;
+      // The left-most edge of the texture is not edge.
+      packedEdgesArray[(0) * 3 + (1)] = sampB.w * notBottomLeft.x;
+      packedEdgesArray[(0) * 3 + (2)] = sampB.x * notBottomLeft.x;
       packedEdgesArray[(1) * 3 + (2)] = sampB.y;
       packedEdgesArray[(2) * 3 + (2)] = sampC;