OpenGL FBO with MRT writing to back buffer - c++

Question

I have a confusing situation in OpenGL 3.3 on the Mac. I have created an FBO with five attachment points sized at 512x512 apiece. I constructed a shader that writes to gl_FragData[0-4] for diffuse, normal, position, specular and emissive for my geometry. When I render the scene the back buffer AND the render targets are being updated even though I’ve only bound the FBO!
Here’s some code:
void OpenGLESDriver::setFrameBufferAttachments( u32 nAttachments, const u32* aAttachments ){
pushText( "setFrameBufferAttachments" );
#if USE_MRT
GLint max;
glGetIntegerv( GL_MAX_DRAW_BUFFERS, &max );
GLenum aBuffers[max];
if( nAttachments > max ){
nAttachments = max;
}
for( u32 i=0; i<nAttachments; ++i ){
aBuffers[i] = GL_COLOR_ATTACHMENT0+aAttachments[i];
}
for( u32 i=nAttachments; i<max; ++i ){
aBuffers[i] = GL_NONE;
}
glDrawBuffers( max, aBuffers );
glAssert();
#else
glDrawBuffer( GL_COLOR_ATTACHMENT0+aAttachments[0] );
glAssert();
#endif
popText();
}
And the FBO binder:
bool OpenGLESDriver::setFrameBuffer( const FrameBuffer::handle& hFrameBuffer ){
if( hFrameBuffer ){
pushText( "setFrameBuffer" );
glBindFramebuffer( GL_FRAMEBUFFER, hFrameBuffer->toFBO() );
glAssert();
if( !hFrameBuffer->toColorTargets().empty() ){
u32 nAttachments = hFrameBuffer->toColorTargets().size();
u32 aAttachments[nAttachments];
for( u32 i=0; i<nAttachments; ++i ){
aAttachments[i] = i;
}
setFrameBufferAttachments( nAttachments, aAttachments );
}else{
setFrameBufferAttachments( 0, 0 );
}
int w = hFrameBuffer->toDepthTexture()->toWidth();
int h = hFrameBuffer->toDepthTexture()->toHeight();
glViewport( 0, 0, w, h );
glAssert();
//clear out all texture stages because we don't want a left over
//frame buffer texture being bound to the shader.
for( u32 i=0; i<Material::kMaxSamplers; ++i ){
setTextureStage( i, 0 );
}
popText();
return true;
}
return false;
}
I create the FBO with:
FrameBuffer::handle OpenGLESDriver::createFrameBuffer( const FrameBuffer::ColorTargets& vColorTargets, const DepthTarget::handle& hDT ){
//--------------------------------------------------------------------
// Save off default FBO.
//--------------------------------------------------------------------
if( s_iFBOMaster < 0 ){
glGetIntegerv( GL_FRAMEBUFFER_BINDING, &s_iFBOMaster );
glAssert();
}
//--------------------------------------------------------------------
// Generate frame buffer object.
//--------------------------------------------------------------------
GLuint fbo;
glGenFramebuffers( 1, &fbo );
glAssert();
glBindFramebuffer( GL_FRAMEBUFFER, fbo );
glAssert();
//--------------------------------------------------------------------
// Attach color RBO.
//--------------------------------------------------------------------
FrameBuffer::ColorTargets::const_iterator itCT = vColorTargets.getIterator();
u32 mrtIndex = 0;
while( itCT ){
const ColorTarget::handle& hCT = itCT++;
if( !hCT ){
continue;
}
if( hCT->toTexID() ){
glFramebufferTexture2D(
GL_FRAMEBUFFER,
GL_COLOR_ATTACHMENT0+mrtIndex,
GL_TEXTURE_2D,
hCT->toTexID(),
0 );
}else if( hCT->toRBO() ){
glFramebufferRenderbuffer(
GL_FRAMEBUFFER,
GL_COLOR_ATTACHMENT0+mrtIndex,
GL_RENDERBUFFER,
hCT->toRBO() );
}else{
DEBUG_ASSERT_ALWAYS( "No color texture or RBO to attach!" );
}
glAssert();
++mrtIndex;
if( !checkFBStatus() ){
e_log( "GL", "Couldn't create color attachment!" );
hCT.as<ColorTarget>()->toFlags()->bFailed = true;
}
}
//--------------------------------------------------------------------
// Attach depth RBO.
//--------------------------------------------------------------------
if( hDT ){
if( hDT->toTexID() ){
glFramebufferTexture2D(
GL_FRAMEBUFFER,
GL_DEPTH_ATTACHMENT,
GL_TEXTURE_2D,
hDT->toTexID(),
0 );
}else if( hDT->toRBO() ){
glFramebufferRenderbuffer(
GL_FRAMEBUFFER,
GL_DEPTH_ATTACHMENT,
GL_RENDERBUFFER,
hDT->toRBO() );
}else{
DEBUG_ASSERT_ALWAYS( "No depth texture or RBO to attach!" );
}
glAssert();
if( !checkFBStatus() ){
e_log( "GL", "Couldn't create depth attachment!" );
hDT.as<DepthTarget>()->toFlags()->bFailed = true;
}
}
//--------------------------------------------------------------------
// New handle.
//--------------------------------------------------------------------
glBindFramebuffer( GL_FRAMEBUFFER, 0 );
glAssert();
FrameBuffer::handle hFrameBuffer = e_new( FrameBuffer );
hFrameBuffer->setColorTargets( vColorTargets );
hFrameBuffer->setDepthTarget( hDT );
hFrameBuffer->setFBO( u32( fbo ));
return hFrameBuffer;
}
And I go back to the back buffer with:
void OpenGLESDriver::setDefaultTarget(){
pushText( "setDefaultTarget" );
glBindFramebuffer( GL_FRAMEBUFFER, 0 );//s_iFBOMaster );
glAssert();
glViewport( 0, 0, IEngine::cxView(), IEngine::cyView() );
glAssert();
popText();
}
So the final rendering code looks like:
pushText( "Render MRT pass" );
if( setFrameBuffer( m_tPostFx.buffers[0] )){
setColorMask( true, true, true, true );
clearZ();
enableZBuffer( false );
setColor( color );
clearMRT( m_tPostFx.clearMRTShader );
enableZBuffer( true );
drawMRTPass();
}
popText();
And for some reason the back buffer is being rendered to as well as the FBO. I must be missing something but haven’t got a clue what. Can anyone see what I’m doing wrong?

Answer 1

After some poking around I finally found the answer. My renderer uses a vector of RenderNode objects which I fill up during rendering. I wasn't clearing that vector after I finished my post-effect. For some reason that unfilled vector was being rendered with the next FBO target, which drew all the first lot of geometry again in the second FBO. By clearing the vector at the beginning of the render pass and at the end I got rid of the problem. I'm still trying to track down who was committing all the render nodes again. Hopefully the code I pasted will help anyone who's looking to do FBOs because it does work after all. :)