How to render specific FBO Texture attachment (assume attachment1) with 2 attached

Question

1. Create FBO & its textures like normal.

   // FBO & Texture
   glGenFramebuffers(1, &mFbo);
   glBindFramebuffer(GL_FRAMEBUFFER, mFbo);
   glGenTextures(2, mTexture);
   
   for(int i=0; i<2; i++)
   {
       // Init texture
       glBindTexture(GL_TEXTURE_2D, mTexture[i]);
       glTexImage2D( GL_TEXTURE_2D, 0, GL_RGBA, SCREEN_W, SCREEN_H, \
           0, GL_RGBA, GL_UNSIGNED_BYTE, 0 );
       glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE );
       glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE );
       glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR );
       glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR );
       glBindTexture( GL_TEXTURE_2D, 0 );
   
       // attach to FBO
       glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0+i, GL_TEXTURE_2D, \
           mTexture[i], 0);
   }
   glBindFramebuffer( GL_FRAMEBUFFER, 0 );
   glUseProgram(0);
   

2. Bind FBO and render it inside draw loop.

   // Preparing FBO texture.
   glBindFramebuffer(GL_FRAMEBUFFER, mFbo);
   glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
   
   GLuint at0[1] =  {GL_COLOR_ATTACHMENT0};
   GLuint at1[1] = {GL_COLOR_ATTACHMENT0+1};
   
   glDrawBuffers(1, at0);
   // Filled texture attachment0 less white (transparency = 0.5f)
   GUtils::VideoDriver::GetInstance()->DrawFillRect(glm::vec2(0,0), \
           SCREEN_W, SCREEN_H, glm::vec4(glm::vec3(1.0f/(1)), 0.5f));
   
   glDrawBuffers(1, at1);
   // Filled texture attachment0+1 white.
   GUtils::VideoDriver::GetInstance()->DrawFillRect(glm::vec2(0,0), \
           SCREEN_W, SCREEN_H, glm::vec4(glm::vec3(1.0f/(1)), 1.0f));
   
   glBindFramebuffer(GL_FRAMEBUFFER, 0);
   

After all things I did above:

• texture attachment0 (mTexture[0]) will be filled in less white.
• texture attachment1 (mTexture[1]) will be filled in white.

Right?

I cannot understand when render FBO texture:

glBindTexture(GL_TEXTURE_2D, mTexture[index]);
glDrawElements( GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0 );
glBindTexture(GL_TEXTURE_2D, 0);

1. mTexture[0] , screen = white. (It must be less White)
2. mTexture[1] , screen = black. (It must be White)

Firstly, Did i do a right way to render attached FBO texture or am I misunderstood fbo attachment?

Lastly, How can I fix this?

Answer 1

To use Multiple Render Targets you need to use pixel shaders to tell the GPU where to send each color values.

layout(location = 0) out vec4 my_first_FBO_color_buffer;
layout(location = 1) out vec4 my_second_FBO_color_buffer;

void main() {
   my_first_FBO_color_buffer = vec4(0, 1, 0, 1);
   my_second_FBO_color_buffer = vec4(1, 0, 1, 1);
}

See What is the recommended way to output values to FBO targets? (OpenGL 3.3 + GLSL 330)