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

gpu/command_buffer/service/gles2_cmd_apply_framebuffer_attachment_cmaa_intel.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "gpu/command_buffer/service/gles2_cmd_apply_framebuffer_attachment_cmaa_intel.h"
 
 #include "base/logging.h"
 #include "gpu/command_buffer/service/framebuffer_manager.h"
 #include "gpu/command_buffer/service/gles2_cmd_decoder.h"
 #include "ui/gl/gl_context.h"
@@ skipping 1352 matching lines @@
                      vec4(float(0x80u | pe) / 255.0, 0, 0, 0));
         }
       }
     }
     \n#endif\n  // DETECT_EDGES2
 
     \n#ifdef COMBINE_EDGES\n
     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]
       //-> if it has the last bit flag (128), it's going to stay above 0
       uvec4 sampA = uvec4(
           textureGatherOffset(g_src0TextureFlt,
                               screenPosBase.xy * g_OneOverScreenSize,
                               ivec2(1, 0)) * 255.0 - 127.5);
       uvec4 sampB = uvec4(
           textureGatherOffset(g_src0TextureFlt,
                               screenPosBase.xy * g_OneOverScreenSize,
                               ivec2(0, 1)) * 255.0 - 127.5);
       uint sampC = uint(
           texelFetchOffset(g_src0TextureFlt, screenPosIBase.xy, 0,
                            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;
 
       uvec4 pixelsC = uvec4(packedEdgesArray[(1 + 0) * 3 + (1 + 0)],
                             packedEdgesArray[(1 + 1) * 3 + (1 + 0)],
                             packedEdgesArray[(1 + 0) * 3 + (1 + 1)],
                             packedEdgesArray[(1 + 1) * 3 + (1 + 1)]);
       uvec4 pixelsL = uvec4(packedEdgesArray[(0 + 0) * 3 + (1 + 0)],
                             packedEdgesArray[(0 + 1) * 3 + (1 + 0)],
                             packedEdgesArray[(0 + 0) * 3 + (1 + 1)],
@@ skipping 429 matching lines @@
       \n#ifdef OUT_FBO\n
         outColor = pixel;
       \n#else\n
         imageStore(outTexture, screenPosI, pixel);
       \n#endif\n
       }
     );
 /* clang-format on */
 
 }  // namespace gpu