[Command Buffer] emulate SRGB color format for BlitFramebuffer in OpenGL

gpu/command_buffer/service/gles2_cmd_srgb_converter.cc

Index: gpu/command_buffer/service/gles2_cmd_srgb_converter.cc
diff --git a/gpu/command_buffer/service/gles2_cmd_srgb_converter.cc b/gpu/command_buffer/service/gles2_cmd_srgb_converter.cc
new file mode 100644
index 0000000000000000000000000000000000000000..2deee0d2e594fca34364eecdd4bda1ca3f285470
--- /dev/null
+++ b/gpu/command_buffer/service/gles2_cmd_srgb_converter.cc
@@ -0,0 +1,521 @@
+// Copyright (c) 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_srgb_converter.h"
+
+#include "gpu/command_buffer/service/texture_manager.h"
+#include "ui/gl/gl_version_info.h"
+
+namespace {
+
+void CompileShader(GLuint shader, const char* shader_source) {
+  glShaderSource(shader, 1, &shader_source, 0);
+  glCompileShader(shader);
+#ifndef NDEBUG
+  GLint compile_status;
+  glGetShaderiv(shader, GL_COMPILE_STATUS, &compile_status);
+  if (GL_TRUE != compile_status)
+    DLOG(ERROR) << "CopyTexImage: shader compilation failure.";
+#endif
+}
+
+}  // anonymous namespace
+
+namespace gpu {
+namespace gles2 {
+
+SRGBConverter::SRGBConverter(
+    const gles2::FeatureInfo* feature_info)
+    : feature_info_(feature_info) {
+}
+
+SRGBConverter::~SRGBConverter() {}
+
+
+
+void SRGBConverter::InitializeSRGBConverterProgram() {
+  if (srgb_converter_program_) {
+    return;
+  }
+
+  srgb_converter_program_ = glCreateProgram();
+
+  // Compile the vertex shader
+  const char* vs_source =
+      "#version 150\n"
+      "out vec2 v_texcoord;\n"
+      "\n"
+      "void main()\n"
+      "{\n"
+      "    const vec2 quad_positions[6] = vec2[6]\n"
+      "    (\n"
+      "        vec2(0.0f, 0.0f),\n"
+      "        vec2(0.0f, 1.0f),\n"
+      "        vec2(1.0f, 0.0f),\n"
+      "\n"
+      "        vec2(0.0f, 1.0f),\n"
+      "        vec2(1.0f, 0.0f),\n"
+      "        vec2(1.0f, 1.0f)\n"
+      "    );\n"
+      "\n"
+      "    vec2 xy = vec2((quad_positions[gl_VertexID] * 2.0) - 1.0);\n"
+      "    gl_Position = vec4(xy, 0.0, 1.0);\n"
+      "    v_texcoord = quad_positions[gl_VertexID];\n"
+      "}\n";
+  GLuint vs = glCreateShader(GL_VERTEX_SHADER);
+  CompileShader(vs, vs_source);
+  glAttachShader(srgb_converter_program_, vs);
+  glDeleteShader(vs);
+
+  // Compile the fragment shader
+
+  // Sampling texels from a srgb texture to a linear image, it will convert
+  // the srgb color space to linear color space automatically as a part of
+  // filtering. See the section <sRGB Texture Color Conversion> in GLES and
+  // OpenGL spec. So in decoder, we don't need to decode again.
+  // Drawing to a srgb image, it will convert linear to srgb automatically.
+  // See the section <sRGB Conversion> in GLES and OpenGL spec.
+  // So we just use a simple fragment shader. We don't need to use an equation
+  // in shader to do explicit srgb conversion to/from linear.
+  const char* fs_source =
+      "#version 150\n"
+      "uniform sampler2D u_source_texture;\n"
+      "in vec2 v_texcoord;\n"
+      "out vec4 output_color;\n"
+      "\n"
+      "void main()\n"
+      "{\n"
+      "    vec4 c = texture(u_source_texture, v_texcoord);\n"
+      "    output_color = c;\n"
+      "}\n";
+
+  GLuint fs = glCreateShader(GL_FRAGMENT_SHADER);
+  CompileShader(fs, fs_source);
+  glAttachShader(srgb_converter_program_, fs);
+  glDeleteShader(fs);
+
+  glLinkProgram(srgb_converter_program_);
+#ifndef NDEBUG
+  GLint linked = 0;
+  glGetProgramiv(srgb_converter_program_, GL_LINK_STATUS, &linked);
+  if (!linked) {
+    DLOG(ERROR) << "BlitFramebuffer: program link failure.";
+  }
+#endif
+
+  GLuint texture_uniform =
+      glGetUniformLocation(srgb_converter_program_, "u_source_texture");
+  glUseProgram(srgb_converter_program_);
+  glUniform1i(texture_uniform, 0);
+}
+
+void SRGBConverter::InitializeSRGBDecoder(
+    const gles2::GLES2Decoder* decoder) {
+  if (srgb_decoder_initialized_) {
+    return;
+  }
+
+  InitializeSRGBConverterProgram();
+
+  glGenTextures(srgb_decoder_textures_.size(), srgb_decoder_textures_.data());
+  glActiveTexture(GL_TEXTURE0);
+  for (auto srgb_decoder_texture : srgb_decoder_textures_) {
+    glBindTexture(GL_TEXTURE_2D, srgb_decoder_texture);
+
+    // Use linear, non-mipmapped sampling with the srgb decoder texture
+    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
+    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
+    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
+    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
+  }
+
+  glGenFramebuffersEXT(1, &srgb_decoder_fbo_);
+  glGenVertexArraysOES(1, &srgb_decoder_vao_);
+
+  decoder->RestoreTextureUnitBindings(0);
+  decoder->RestoreActiveTexture();
+  decoder->RestoreProgramBindings();
+
+  srgb_decoder_initialized_ = true;
+}
+
+void SRGBConverter::InitializeSRGBEncoder(
+    const gles2::GLES2Decoder* decoder) {
+  if (srgb_encoder_initialized_) {
+    return;
+  }
+
+  InitializeSRGBConverterProgram();
+
+  glGenTextures(srgb_encoder_textures_.size(), srgb_encoder_textures_.data());
+  glActiveTexture(GL_TEXTURE0);
+  for (auto srgb_encoder_texture : srgb_encoder_textures_) {
+    glBindTexture(GL_TEXTURE_2D, srgb_encoder_texture);
+
+    // Use linear, non-mipmapped sampling with the srgb encoder texture
+    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
+    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
+    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
+    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
+  }
+
+  glGenFramebuffersEXT(1, &srgb_encoder_fbo_);
+  glGenVertexArraysOES(1, &srgb_encoder_vao_);
+
+  decoder->RestoreTextureUnitBindings(0);
+  decoder->RestoreActiveTexture();
+  decoder->RestoreProgramBindings();
+
+  srgb_encoder_initialized_ = true;
+}
+
+void SRGBConverter::Destroy() {
+  if (srgb_decoder_initialized_) {
+    glDeleteTextures(srgb_decoder_textures_.size(),
+                     srgb_decoder_textures_.data());
+    srgb_decoder_textures_.fill(0);
+
+    glDeleteFramebuffersEXT(1, &srgb_decoder_fbo_);
+    srgb_decoder_fbo_ = 0;
+
+    glDeleteVertexArraysOES(1, &srgb_decoder_vao_);
+    srgb_decoder_vao_ = 0;
+
+    srgb_decoder_initialized_ = false;
+  }
+
+  if (srgb_encoder_initialized_) {
+    glDeleteTextures(srgb_encoder_textures_.size(),
+                     srgb_encoder_textures_.data());
+    srgb_encoder_textures_.fill(0);
+
+    glDeleteFramebuffersEXT(1, &srgb_encoder_fbo_);
+    srgb_encoder_fbo_ = 0;
+
+    glDeleteVertexArraysOES(1, &srgb_encoder_vao_);
+    srgb_encoder_vao_ = 0;
+
+    srgb_encoder_initialized_ = false;
+  }
+
+  glDeleteProgram(srgb_converter_program_);
+  srgb_converter_program_ = 0;
+}
+
+void SRGBConverter::SRGBToLinear(
+    const gles2::GLES2Decoder* decoder,
+    GLint srcX0,
+    GLint srcY0,
+    GLint srcX1,
+    GLint srcY1,
+    GLint dstX0,
+    GLint dstY0,
+    GLint dstX1,
+    GLint dstY1,
+    GLbitfield mask,
+    GLenum filter,
+    const gfx::Size& framebuffer_size,
+    GLuint src_framebuffer,
+    GLenum src_framebuffer_internal_format,
+    GLuint dst_framebuffer) {
+  // This function blits srgb image in src fb to linear image in dst fb.
+  // The steps are:
+  // 1) Copy and crop pixels from source srgb image to the 1st texture(srgb).
+  // 2) Sampling from the 1st texture and drawing to the 2nd texture(linear).
+  //    During this step, color space is converted from srgb to linear.
+  // 3) Finally, blit pixels from the 2nd texture to the target, which is
+  //    also a linear image.
+  DCHECK(srgb_decoder_initialized_);
+
+  // Copy the image from read buffer to the decoder's 1st texture(srgb).
+  // TODO(yunchao) If the read buffer is a fbo texture, we can sample
+  // directly from that texture. In this way, we can save gpu memory.
+  glBindFramebufferEXT(GL_FRAMEBUFFER, src_framebuffer);
+  glActiveTexture(GL_TEXTURE0);
+  glBindTexture(GL_TEXTURE_2D, srgb_decoder_textures_[0]);
+
+  // We should not copy pixels outside of the read framebuffer. If we read
+  // these pixels, they would become in-bound during BlitFramebuffer. However,
+  // Out-of-bounds pixels will be initialized to 0 in CopyTexSubImage. But they
+  // should read as if the GL_CLAMP_TO_EDGE texture mapping mode were applied
+  // during BlitFramebuffer when the filter is GL_LINEAR.
+  GLuint x = srcX1 > srcX0 ? srcX0 : srcX1;
+  GLuint y = srcY1 > srcY0 ? srcY0 : srcY1;
+  GLuint width = srcX1 > srcX0 ? srcX1 - srcX0 : srcX0 - srcX1;
+  GLuint height = srcY1 > srcY0 ? srcY1 - srcY0 : srcY0 - srcY1;
+  gfx::Rect c(0, 0, framebuffer_size.width(), framebuffer_size.height());
+  c.Intersect(gfx::Rect(x, y, width, height));
+  GLuint xoffset = c.x() - x;
+  GLuint yoffset = c.y() - y;
+  glCopyTexImage2D(GL_TEXTURE_2D, 0, src_framebuffer_internal_format,

[yunchao, 2016/09/14 16:11:33]

If the read buffer is a multisampled renderbuffer with srgb color format, then
this code would copy from a msaa renderbuffer to a texture. It seems to be wrong
(any gles spec about this?). 
Do you have any suggestion to handle this corner case, @piman. 

[piman, 2016/09/15 03:34:31]

I'm pretty sure that's the intended behavior - it would do the MSAA resolve,
just like glBlitFramebuffer would.

[yunchao, 2016/09/16 06:20:52]

Done.

+                   c.x(), c.y(), c.width(), c.height(), 0);
+
+  // Make a temporary linear texture as the decoder's 2nd texture, where we
+  // render the converted (srgb to linear) result to.
+  glBindBuffer(GL_PIXEL_UNPACK_BUFFER, 0);
+
+  glBindTexture(GL_TEXTURE_2D, srgb_decoder_textures_[1]);
+  glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA,
+               c.width(), c.height(),
+               0, GL_RGBA, GL_UNSIGNED_BYTE, nullptr);
+  glBindFramebufferEXT(GL_FRAMEBUFFER, srgb_decoder_fbo_);
+  glFramebufferTexture2DEXT(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0,
+                            GL_TEXTURE_2D, srgb_decoder_textures_[1], 0);
+
+  // Sampling from the decoder's first texture(srgb) and drawing to the
+  // decoder's 2nd texture(linear),
+  glUseProgram(srgb_converter_program_);
+  glViewport(0, 0, width, height);
+  glDisable(GL_SCISSOR_TEST);
+  glDisable(GL_DEPTH_TEST);
+  glDisable(GL_STENCIL_TEST);
+  glDisable(GL_CULL_FACE);
+  glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
+  glDepthMask(GL_FALSE);
+  glDisable(GL_BLEND);
+  glDisable(GL_DITHER);
+
+  glBindTexture(GL_TEXTURE_2D, srgb_decoder_textures_[0]);
+  glBindVertexArrayOES(srgb_decoder_vao_);
+
+  glDrawArrays(GL_TRIANGLES, 0, 6);
+
+  // Finally, bind the decoder framebuffer as read framebuffer,
+  // blit the converted texture in decoder fbo to the destination texture
+  // in destination framebuffer.
+  glBindFramebufferEXT(GL_READ_FRAMEBUFFER, srgb_decoder_fbo_);
+  glBindFramebufferEXT(GL_DRAW_FRAMEBUFFER, dst_framebuffer);
+  // Note that the source region has been changed in decoder framebuffer.
+  // The xoffset/yoffset can make bliting clamp to the correct edge if
+  // CLAMP_TO_EDGE is necessary.
+  glBlitFramebuffer(srcX0 < srcX1 ? 0 - xoffset : width - xoffset,
+                    srcY0 < srcY1 ? 0 - yoffset : height - yoffset,
+                    srcX0 < srcX1 ? width - xoffset : 0 - xoffset,
+                    srcY0 < srcY1 ? height - yoffset : 0 - yoffset,
+                    dstX0, dstY0, dstX1, dstY1, mask, filter);
+
+  // Restore state
+  decoder->RestoreAllAttributes();
+  decoder->RestoreTextureUnitBindings(0);
+  decoder->RestoreActiveTexture();
+  decoder->RestoreProgramBindings();
+  decoder->RestoreBufferBindings();
+  decoder->RestoreFramebufferBindings();
+  decoder->RestoreGlobalState();
+}
+
+void SRGBConverter::LinearToSRGB(
+    const gles2::GLES2Decoder* decoder,
+    GLint srcX0,
+    GLint srcY0,
+    GLint srcX1,
+    GLint srcY1,
+    GLint dstX0,
+    GLint dstY0,
+    GLint dstX1,
+    GLint dstY1,
+    GLbitfield mask,
+    GLenum filter,
+    GLuint src_framebuffer,
+    GLenum src_framebuffer_internal_format,
+    GLenum src_framebuffer_format,
+    GLenum src_framebuffer_type,
+    GLuint dst_framebuffer) {
+  // This function blits linear image in src fb to srgb image in dst fb.
+  // The steps are:
+  // 1) BlitFramebuffer from source linear image to a temp linear texture.
+  // 2) Sampling from the temp texture and drawing to the target srgb
+  //    image. During this step, color space is converted from linear to srgb.
+
+  DCHECK(srgb_encoder_initialized_);
+
+  // Create a temp linear texture as draw buffer. Blit framebuffer from
+  // source linear image to the temp linear texture. Filtering is done
+  // during bliting. Note that the src and dst coordinates may be reversed.
+  glActiveTexture(GL_TEXTURE0);
+  glBindTexture(GL_TEXTURE_2D, srgb_encoder_textures_[0]);
+
+  GLuint width = dstX1 > dstX0 ? dstX1 - dstX0 : dstX0 - dstX1;
+  GLuint height = dstY1 > dstY0 ? dstY1 - dstY0 : dstY0 - dstY1;
+  glBindBuffer(GL_PIXEL_UNPACK_BUFFER, 0);
+  glTexImage2D(GL_TEXTURE_2D, 0, src_framebuffer_internal_format,
+               width, height,
+               0, src_framebuffer_format, src_framebuffer_type, nullptr);
+
+  glBindFramebufferEXT(GL_DRAW_FRAMEBUFFER, srgb_encoder_fbo_);
+  glFramebufferTexture2DEXT(GL_DRAW_FRAMEBUFFER, GL_COLOR_ATTACHMENT0,
+                            GL_TEXTURE_2D, srgb_encoder_textures_[0], 0);
+  glBindFramebufferEXT(GL_READ_FRAMEBUFFER, src_framebuffer);
+  glBlitFramebuffer(srcX0, srcY0, srcX1, srcY1,
+                    dstX0 < dstX1 ? 0 : width,
+                    dstY0 < dstY1 ? 0 : height,
+                    dstX0 < dstX1 ? width : 0,
+                    dstY0 < dstY1 ? height : 0,
+                    mask, filter);
+
+  // Sampling from the linear texture and drawing to the target srgb image.
+  // During this step, color space is converted from linear to srgb. We should
+  // set appropriate viewport to draw to the correct location in target FB.
+  GLuint xstart = dstX0 < dstX1 ? dstX0 : dstX1;
+  GLuint ystart = dstY0 < dstY1 ? dstY0 : dstY1;
+  glBindFramebufferEXT(GL_DRAW_FRAMEBUFFER, dst_framebuffer);
+  glUseProgram(srgb_converter_program_);
+  glViewport(xstart, ystart, width, height);
+  glDisable(GL_SCISSOR_TEST);
+  glDisable(GL_DEPTH_TEST);
+  glDisable(GL_STENCIL_TEST);
+  glDisable(GL_CULL_FACE);
+  glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
+  glDepthMask(GL_FALSE);
+  glDisable(GL_BLEND);
+  glDisable(GL_DITHER);
+
+  glBindTexture(GL_TEXTURE_2D, srgb_encoder_textures_[0]);
+  glBindVertexArrayOES(srgb_encoder_vao_);
+
+  glDrawArrays(GL_TRIANGLES, 0, 6);
+
+  // Restore state
+  decoder->RestoreAllAttributes();
+  decoder->RestoreTextureUnitBindings(0);
+  decoder->RestoreActiveTexture();
+  decoder->RestoreProgramBindings();
+  decoder->RestoreBufferBindings();
+  decoder->RestoreFramebufferBindings();
+  decoder->RestoreGlobalState();
+}
+
+void SRGBConverter::SRGBToSRGB(
+    const gles2::GLES2Decoder* decoder,
+    GLint srcX0,
+    GLint srcY0,
+    GLint srcX1,
+    GLint srcY1,
+    GLint dstX0,
+    GLint dstY0,
+    GLint dstX1,
+    GLint dstY1,
+    GLbitfield mask,
+    GLenum filter,
+    const gfx::Size& framebuffer_size,
+    GLuint src_framebuffer,
+    GLenum src_framebuffer_internal_format,
+    GLuint dst_framebuffer) {
+  // This function blits srgb image in src fb to srgb image in dst fb.
+  // It needs to use decoder's resource, as well as encoder's resources,
+  // for instance, decoder's and encoder's fbos, programs and textuers.
+  // The steps are:
+  // 1) Copy and crop pixels from source srgb image to the 1st texture(srgb).
+  // 2) Sampling from the 1st texture and drawing to the 2nd texture(linear).
+  //    During this step, color space is converted from srgb to linear.
+  // 3) Blit pixels from the 2nd texture to the 3rd texture(linear).
+  // 4) Sampling from the 3rd texture and drawing to the dst image(srgb).
+  //    During this step, color space is converted from linear to srgb.

[piman, 2016/09/15 03:34:31]

The 2 other functions could be considered a simplification of this one.
SRGBToLinear skips step 4 (with 3 directly blitting to the destination FBO), and
LinearToSRGB skips steps 1 and 2 (with 3 directly blitting from the source FBO).

Could we unify? This would significantly reduce code duplication, both here and
on the caller side - you could have a single SRGBConverter::Blit(bool
needs_srgb_decode, bool needs_srgb_encode, ...)

Relatedly, the conceptual separation of "srgb decoder" vs "srgb encoder" is a
bit artificial at this point - they use the same shader, and many of the
resources could be shared (e.g. vao, temporary textures, temporary FBO).
Especially the textures, since we reallocate the images every time anyway
(glCopyTexImage2D/glTexImage2D), there's not a lot of gains to be had by keeping
them separate for the encoder step and the decoder step. Note, you only really
need 2 textures overall, since step 3 could reuse texture 1 for the destination 

[yunchao, 2016/09/16 06:20:52]

Done. Very good idea to remove the code duplication.  
I have thought about this. But I didn't reuse the texture 1. You know, it is
necessary to get parameters need_encode and need_decode from the caller. So, I
think keep the logic "srgb encoder" and "srgb decoder" would make the code
clear: 
srgb -- (decode) -- linear -- (blit) -- linear -- (encode) -- srgb. 
Moreover, during the blit, we need two fbos, I named them decoder_fbo and
encoder_fbo. If encoder_fbo's image (texture 3) re-use the one by decoder_fbo's
image (texture 1). It is seems to be in a mess when read the code.  After
combine all srgb converters into one function, I think the code is already
complicated now. 

WDYT, @piman. If you think it is better to reuse the texture 1, I will update
the code accordingly. 

[yunchao, 2016/09/16 14:07:56]

It is not necessary to associate texture and vao with decoder_fbo or encoder_fbo
respectively. we can still have decoder_fbo and encoder_fbo, but use unified
textures/vao/program, and share these resources between decoder_fbo and
encoder_fbo. 
Code has been updated accordingly. I think it is OK now.  

+  DCHECK(srgb_decoder_initialized_ && srgb_encoder_initialized_);
+
+  // Copy the image from read buffer to the decoder's 1st texture(srgb).
+  // TODO(yunchao) If the read buffer is a fbo texture, we can sample
+  // directly from that texture. In this way, we can save gpu memory.
+  glBindFramebufferEXT(GL_FRAMEBUFFER, src_framebuffer);
+  glActiveTexture(GL_TEXTURE0);
+  glBindTexture(GL_TEXTURE_2D, srgb_decoder_textures_[0]);
+
+  // We should not copy pixels outside of the read framebuffer. If we read
+  // these pixels, they would become in-bound during BlitFramebuffer. However,
+  // Out-of-bounds pixels will be initialized to 0 in CopyTexSubImage. But they
+  // should read as if the GL_CLAMP_TO_EDGE texture mapping mode were applied
+  // during BlitFramebuffer when the filter is GL_LINEAR.
+  GLuint x = srcX1 > srcX0 ? srcX0 : srcX1;
+  GLuint y = srcY1 > srcY0 ? srcY0 : srcY1;
+  GLuint width_read = srcX1 > srcX0 ? srcX1 - srcX0 : srcX0 - srcX1;
+  GLuint height_read = srcY1 > srcY0 ? srcY1 - srcY0 : srcY0 - srcY1;
+  gfx::Rect c(0, 0, framebuffer_size.width(), framebuffer_size.height());
+  c.Intersect(gfx::Rect(x, y, width_read, height_read));
+  GLuint xoffset = c.x() - x;
+  GLuint yoffset = c.y() - y;
+  glCopyTexImage2D(GL_TEXTURE_2D, 0, src_framebuffer_internal_format,
+                   c.x(), c.y(), c.width(), c.height(), 0);
+
+  // Make a temporary linear texture as the decoder's 2nd texture, where we
+  // render the converted (srgb to linear) result to.
+  glBindBuffer(GL_PIXEL_UNPACK_BUFFER, 0);
+
+  glBindTexture(GL_TEXTURE_2D, srgb_decoder_textures_[1]);
+  glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA,
+               c.width(), c.height(),
+               0, GL_RGBA, GL_UNSIGNED_BYTE, nullptr);
+  glBindFramebufferEXT(GL_FRAMEBUFFER, srgb_decoder_fbo_);
+  glFramebufferTexture2DEXT(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0,
+                            GL_TEXTURE_2D, srgb_decoder_textures_[1], 0);
+
+  // Sampling from the decoder's first texture(srgb) and drawing to the
+  // decoder's 2nd texture(linear),
+  glUseProgram(srgb_converter_program_);
+  glViewport(0, 0, width_read, height_read);
+  glDisable(GL_SCISSOR_TEST);
+  glDisable(GL_DEPTH_TEST);
+  glDisable(GL_STENCIL_TEST);
+  glDisable(GL_CULL_FACE);
+  glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
+  glDepthMask(GL_FALSE);
+  glDisable(GL_BLEND);
+  glDisable(GL_DITHER);
+
+  glBindTexture(GL_TEXTURE_2D, srgb_decoder_textures_[0]);
+  glBindVertexArrayOES(srgb_decoder_vao_);
+
+  glDrawArrays(GL_TRIANGLES, 0, 6);
+
+  // Create the 3rd texture(linear) as encoder's draw buffer. Blit framebuffer
+  // from the 2nd texture(linear) to the 3rd texture. Filtering is done
+  // during bliting. Note that the src and dst coordinates may be reversed.
+  glBindTexture(GL_TEXTURE_2D, srgb_encoder_textures_[0]);
+
+  GLuint width_draw = dstX1 > dstX0 ? dstX1 - dstX0 : dstX0 - dstX1;
+  GLuint height_draw = dstY1 > dstY0 ? dstY1 - dstY0 : dstY0 - dstY1;
+  glBindBuffer(GL_PIXEL_UNPACK_BUFFER, 0);
+  glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA,
+               width_draw, height_draw,
+               0, GL_RGBA, GL_UNSIGNED_BYTE, nullptr);
+
+  glBindFramebufferEXT(GL_DRAW_FRAMEBUFFER, srgb_encoder_fbo_);
+  glFramebufferTexture2DEXT(GL_DRAW_FRAMEBUFFER, GL_COLOR_ATTACHMENT0,
+                            GL_TEXTURE_2D, srgb_encoder_textures_[0], 0);
+  glBindFramebufferEXT(GL_READ_FRAMEBUFFER, srgb_decoder_fbo_);
+  glBlitFramebuffer(srcX0 < srcX1 ? 0 - xoffset : width_read - xoffset,
+                    srcY0 < srcY1 ? 0 - yoffset : height_read - yoffset,
+                    srcX0 < srcX1 ? width_read - xoffset : 0 - xoffset,
+                    srcY0 < srcY1 ? height_read - yoffset : 0 - yoffset,
+                    dstX0 < dstX1 ? 0 : width_draw,
+                    dstY0 < dstY1 ? 0 : height_draw,
+                    dstX0 < dstX1 ? width_draw : 0,
+                    dstY0 < dstY1 ? height_draw : 0,
+                    mask, filter);
+
+  // Sampling from the 3rd texture(linear) and drawing to the target srgb image.
+  // During this step, color space is converted from linear to srgb. We should
+  // set appropriate viewport to draw to the correct location in target FB.
+  GLuint xstart = dstX0 < dstX1 ? dstX0 : dstX1;
+  GLuint ystart = dstY0 < dstY1 ? dstY0 : dstY1;
+  glBindFramebufferEXT(GL_DRAW_FRAMEBUFFER, dst_framebuffer);
+  glUseProgram(srgb_converter_program_);
+  glViewport(xstart, ystart, width_draw, height_draw);
+
+  glBindTexture(GL_TEXTURE_2D, srgb_encoder_textures_[0]);
+  glBindVertexArrayOES(srgb_encoder_vao_);
+
+  glDrawArrays(GL_TRIANGLES, 0, 6);
+
+  // Restore state
+  decoder->RestoreAllAttributes();
+  decoder->RestoreTextureUnitBindings(0);
+  decoder->RestoreActiveTexture();
+  decoder->RestoreProgramBindings();
+  decoder->RestoreBufferBindings();
+  decoder->RestoreFramebufferBindings();
+  decoder->RestoreGlobalState();
+}
+
+}  // namespace gles2.
+}  // namespace gpu