将 gl_PrimitiveID 输出到自定义帧缓冲区对象 (FBO) 时出现问题

Question

我有一个非常基本的片段着色器，我想将“gl_PrimitiveID”输出到我定义的片段缓冲区对象 (FBO)。下面是我的片段着色器：

#version 150

uniform vec4 colorConst;

out vec4 fragColor;
out uvec4 triID;

void main(void)
{ 
   fragColor = colorConst;
   triID.r = uint(gl_PrimitiveID);
}

我这样设置我的 FBO：

  GLuint renderbufId0;
  GLuint renderbufId1;
  GLuint depthbufId;
  GLuint framebufId;


  // generate render and frame buffer objects
  glGenRenderbuffers( 1, &renderbufId0 );
  glGenRenderbuffers( 1, &renderbufId1 );
  glGenRenderbuffers( 1, &depthbufId   );
  glGenFramebuffers ( 1, &framebufId   );


  // setup first renderbuffer (fragColor)
  glBindRenderbuffer(GL_RENDERBUFFER, renderbufId0);
  glRenderbufferStorage(GL_RENDERBUFFER, GL_RGBA,    gViewWidth, gViewHeight);


  // setup second renderbuffer (triID)
  glBindRenderbuffer(GL_RENDERBUFFER, renderbufId1);
  glRenderbufferStorage(GL_RENDERBUFFER, GL_RGB32UI, gViewWidth, gViewHeight);


  // setup depth buffer
  glBindRenderbuffer(GL_RENDERBUFFER, depthbufId);
  glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT32, gViewWidth, gViewHeight);


  // setup framebuffer
  glBindFramebuffer(GL_FRAMEBUFFER, framebufId);  
  glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, renderbufId0);
  glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT1, GL_RENDERBUFFER, renderbufId1);
  glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT,  GL_RENDERBUFFER, depthbufId  );


  // check if everything went well
  GLenum stat = glCheckFramebufferStatus(GL_FRAMEBUFFER);  
  if(stat != GL_FRAMEBUFFER_COMPLETE) { exit(0); }


  // setup color attachments
  const GLenum att[] = {GL_COLOR_ATTACHMENT0, GL_COLOR_ATTACHMENT1};
  glDrawBuffers(2, att);


  // render mesh
  RenderMyMesh()


  // copy second color attachment (triID) to local buffer
  glReadBuffer(GL_COLOR_ATTACHMENT1);
  glReadPixels(0, 0, gViewWidth, gViewHeight, GL_RED, GL_UNSIGNED_INT, data);

出于某种原因，glReadPixels 给了我一个“GL_INVALID_OPERATION”错误？但是，如果我将 renderbufId1 的内部格式从“GL_RGB32UI”更改为“GL_RGB”，并且在 glReadPixels 中使用“GL_FLOAT”而不是“GL_UNSIGNED_INT”，那么一切正常。有谁知道我为什么会收到“GL_INVALID_OPERATION”错误以及如何解决它？

是否有另一种输出“gl_PrimitiveID”的方法？

PS：我想像这样输出'gl_PrimitiveID'的原因在这里解释：Picking triangles in OpenGL core profile when using glDrawElements

Answer 1

glReadPixels(0, 0, gViewWidth, gViewHeight, GL_RED, GL_UNSIGNED_INT, data);

如on the OpenGL Wiki 所述，传输真整数数据时需要使用GL_RED_INTEGER。否则，OpenGL 会尝试对其使用浮点转换。

顺便说一句，请确保您使用glBindFragDataLocation 来设置片段着色器输出的缓冲区。或者，如果您使用 GLSL 3.30 或更高版本，您也可以set it up explicitly in the shader。