gpu, cmaa: optimize COMBINE_EDGES path to reduce fragment shader tasks

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 381 matching lines @@
 
     glActiveTexture(GL_TEXTURE1);
     glBindTexture(GL_TEXTURE_2D, mini4_edge_texture_);
     glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
     glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
 
     glDrawArrays(GL_TRIANGLES, 0, 3);
   }
 
   //  - Combine RightBottom (.xy) edges from previous pass into
-  //    RightBottomLeftTop (.xyzw) edges and output it into the mask (have to
-  //    fill in the whole buffer including empty ones for the line length
-  //    detection to work correctly).

[dshwang, 2016/07/06 10:27:30]

It's wrong, because all edge information is generated by DETECT_EDGES1 path. All
pixel which DETECT_EDGES1 didn't mark don't have any information of edges.

+  //    RightBottomLeftTop (.xyzw) edges and output it into the mask.
   //  - On all pixels with any edge, input buffer into a temporary color buffer
   //    needed for correct blending in the next pass (other pixels not needed
   //    so not copied to avoid bandwidth use).
   //  - On all pixels with 2 or more edges output positive depth mask for the
   //    next pass.
 
   // Inputs
   //  g_src0TextureFlt                    edge_texture_b               tex1 //ps
   // Outputs
   //  image2D g_resultTextureSlot2        edge_texture_a               image2
   //  gl_FragDepth                        mini4_edge_texture_          fbo.depth
   {
     // Combine edges: each pixel will now contain info on all (top, right,
-    // bottom, left) edges; also create depth mask as above depth and mark
-    // potential Z sAND also copy source color data but only on edge pixels
+    // bottom, left) edges; also mark depth 1 value on all pixels with any edge
+    // and also copy source color data but only on edge pixels
     glUseProgram(edges_combine_shader_);
-    glUniform1f(0, 1.0f);
+    glUniform1f(0, 0.0f);
     glUniform2f(1, 1.0f / width_, 1.0f / height_);
     glDepthMask(GL_TRUE);
-    glDepthFunc(GL_ALWAYS);
+    glDepthFunc(GL_LESS);
     glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE);
 
     if (!is_gles31_compatible_) {
       glUniform1i(edges_combine_shader_result_texture_slot2_, 2);
     }
     glBindImageTextureEXT(2, edge_texture_a, 0, GL_FALSE, 0, GL_WRITE_ONLY,
                           edge_format);
 
     glActiveTexture(GL_TEXTURE1);
     glBindTexture(GL_TEXTURE_2D, edge_texture_b);
@@ skipping 1448 matching lines @@
       \n#ifdef OUT_FBO\n
         outColor = pixel;
       \n#else\n
         imageStore(outTexture, screenPosI, pixel);
       \n#endif\n
       }
     );
 /* clang-format on */
 
 }  // namespace gpu