如何将抗锯齿图像渲染到纹理（然后写入 PNG）？

Question

我想使用“渲染到纹理”范例来编写我的 OpenGL 3D 渲染的 .png 屏幕截图。我让它在没有多重采样的情况下工作，但我正在努力获得抗锯齿图像。

首先，这可能吗？

第二，API调用的正确组合是什么？

（元第三个问题，我怎样才能更好地调试这个？glCheckFramebufferStatus 显然不够）。

这是我正在使用的：

  // https://stackoverflow.com/questions/7402504/multisampled-render-to-texture-in-ios
  GLuint texture;
  glGenTextures(1, &texture);
  glBindTexture(GL_TEXTURE_2D, texture);
  glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA4, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);

  GLuint resolved_framebuffer, resolvedColorRenderbuffer;
  glGenFramebuffers(1, &resolved_framebuffer);
  glBindFramebuffer(GL_FRAMEBUFFER, resolved_framebuffer);
  glGenRenderbuffers(1, &resolvedColorRenderbuffer);
  glBindRenderbuffer(GL_RENDERBUFFER, resolvedColorRenderbuffer);
  glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, texture, 0);

  assert(glCheckFramebufferStatus(GL_FRAMEBUFFER) == GL_FRAMEBUFFER_COMPLETE);

  GLuint framebuffer;
  glGenFramebuffers(1, &framebuffer);
  glBindFramebuffer(GL_FRAMEBUFFER, framebuffer);

  GLuint colorRenderbuffer, depthRenderbuffer;
  glGenRenderbuffers(1, &colorRenderbuffer);
  glBindRenderbuffer(GL_RENDERBUFFER, colorRenderbuffer);
  glRenderbufferStorageMultisample(GL_RENDERBUFFER, 4, GL_RGBA8, width, height);
  glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, colorRenderbuffer);

  glGenRenderbuffers(1, &depthRenderbuffer);
  glBindRenderbuffer(GL_RENDERBUFFER, depthRenderbuffer);
  glRenderbufferStorageMultisample(GL_RENDERBUFFER, 4, GL_DEPTH_COMPONENT16, width, height);
  glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, depthRenderbuffer);


  assert(glCheckFramebufferStatus(GL_FRAMEBUFFER) == GL_FRAMEBUFFER_COMPLETE);

  glBindFramebuffer( GL_FRAMEBUFFER, framebuffer );
  glClearColor(background_color(0), background_color(1), background_color(2), 0.f);
  glClear(GL_COLOR_BUFFER_BIT);// | GL_DEPTH_BUFFER_BIT);

  draw_scene();

  glBindFramebuffer( GL_READ_FRAMEBUFFER, framebuffer );
  glBindFramebuffer( GL_DRAW_FRAMEBUFFER, resolved_framebuffer );
  // https://forum.juce.com/t/ios-8-getting-the-demo-building/13570/20
  glBlitFramebuffer(0, 0, width, height, 0, 0, width, height, GL_COLOR_BUFFER_BIT, GL_NEAREST);
  // ^--- this is throwing `GL_INVALID_OPERATION`


  GLubyte* pixels = (GLubyte*)calloc(width*height*4,sizeof(GLubyte));
  glReadPixels(0, 0,width, height,GL_RGBA, GL_UNSIGNED_BYTE, pixels);

  writePNG(pixels);

目前我得到一个空白图像，glBlitFramebuffer 正在抛出GL_INVALID_OPERATION。显然是this error can correspond to many things，我不确定哪个在申请。根据glCheckFramebufferStatus，我的缓冲区看起来“不错”。

之前有人以类似的形式提出过这个问题：

• Cannot render to texture with multisampling
• Multisampled render to texture in ios

但是没有一个答案可以导致一个完整的工作示例。我很想找到/创建一个 minimal 的例子。

Answer 1

https://learnopengl.com/Advanced-OpenGL/Anti-Aliasing 的教程基本上有我需要的东西。有效的解决方案是：

  unsigned int framebuffer;
  glGenFramebuffers(1, &framebuffer);
  glBindFramebuffer(GL_FRAMEBUFFER, framebuffer);
  // create a multisampled color attachment texture
  unsigned int textureColorBufferMultiSampled;
  glGenTextures(1, &textureColorBufferMultiSampled);
  glBindTexture(GL_TEXTURE_2D_MULTISAMPLE, textureColorBufferMultiSampled);
  glTexImage2DMultisample(GL_TEXTURE_2D_MULTISAMPLE, 4, GL_RGBA, width, height, GL_TRUE);
  glBindTexture(GL_TEXTURE_2D_MULTISAMPLE, 0);
  glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D_MULTISAMPLE, textureColorBufferMultiSampled, 0);
  // create a (also multisampled) renderbuffer object for depth and stencil attachments
  unsigned int rbo;
  glGenRenderbuffers(1, &rbo);
  glBindRenderbuffer(GL_RENDERBUFFER, rbo);
  glRenderbufferStorageMultisample(GL_RENDERBUFFER, 4, GL_DEPTH24_STENCIL8, width, height);
  glBindRenderbuffer(GL_RENDERBUFFER, 0);
  glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_STENCIL_ATTACHMENT, GL_RENDERBUFFER, rbo);
  assert(glCheckFramebufferStatus(GL_FRAMEBUFFER) == GL_FRAMEBUFFER_COMPLETE);
  glBindFramebuffer(GL_FRAMEBUFFER, 0);

  // configure second post-processing framebuffer
  unsigned int intermediateFBO;
  glGenFramebuffers(1, &intermediateFBO);
  glBindFramebuffer(GL_FRAMEBUFFER, intermediateFBO);
  // create a color attachment texture
  unsigned int screenTexture;
  glGenTextures(1, &screenTexture);
  glBindTexture(GL_TEXTURE_2D, screenTexture);
  glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);
  glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
  glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
  glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, screenTexture, 0);    // we only need a color buffer
  assert(glCheckFramebufferStatus(GL_FRAMEBUFFER) == GL_FRAMEBUFFER_COMPLETE);
  glBindFramebuffer(GL_FRAMEBUFFER, 0);

  glBindFramebuffer(GL_FRAMEBUFFER, framebuffer);

  draw_scene();

  glBindFramebuffer(GL_READ_FRAMEBUFFER, framebuffer);
  glBindFramebuffer(GL_DRAW_FRAMEBUFFER, intermediateFBO);
  glBlitFramebuffer(0, 0, width, height, 0, 0, width, height, GL_COLOR_BUFFER_BIT, GL_NEAREST);

  glBindFramebuffer(GL_FRAMEBUFFER, intermediateFBO);
  GLubyte* pixels = (GLubyte*)calloc(width*height*4,sizeof(GLubyte));
  glReadPixels(0, 0,width, height,GL_RGBA, GL_UNSIGNED_BYTE, pixels);

  writePNG(pixels);

看来原问题的代码至少有两个错误：

1. framebuffer 应该附加一个多重采样纹理，而不是渲染缓冲区（glFramebufferTexture2D(... GL_TEXTURE_2D_MULTISAMPLE 而不是glRenderbufferStorageMultisample）
2. 必须在 blitting 之后和 glReadPixels 之前调用 glBindFramebuffer(GL_FRAMEBUFFER, intermediateFBO);