How to use Multisampling with OpenGL FBOs
c++, fbo, multisampling, opengl
Solution
You need to allocate a multisampled depth buffer for this to work correctly and give it the same number of samples as your color buffer. In other words, you should be calling `glRenderbufferStorageMultisample (...)` instead of `glRenderbufferStorage (...)`.
Your FBO should be failing a completeness check the way it is allocated right now. A call to `glCheckFramebufferStatus (...)` ought to return `GL_FRAMEBUFFER_INCOMPLETE_MULTISAMPLE` because your depth buffer has exactly 1 sample and your color buffer attachment has 4.
Since you are also using a multisampled texture attachment in this FBO, you should be aware of differences between sampling a single-sampled texture vs. multisampled in GLSL shaders.
Multisampled textures have a special sampler uniform type (e.g. `sampler2DMS`) and you have to explicitly fetch each sample in the texture by its integer (non-normalized) texel coordinate and sample index using `texelFetch (...)`. This also means that they cannot be filtered or mip-mapped.
You probably do not want a multisampled texture in this case, you probably want to use `glBlitFramebuffer (...)` to do the MSAA resolve into a single-sampled FBO. If you do this instead you can read the anti-aliased results in your shaders rather than having to fetch each sample and implement the anti-aliasing yourself.
Problem
I'm trying to enable mutlisampling and alpha-to-coverage for an FBO. Using the default framebuffer, all I have to do is call `glEnable(GL_MULTISAMPLE)` and `glEnable(GL_SAMPLE_ALPHA_TO_COVERAGE)`. However, I am unable to achieve the same effect using my own FBO. My goal: Draw the scene to an FBO the same way it would be drawn to the default framebuffer with the above properties. From there I want to be able to use the image as a texture for future passes through a shader. This works: Code for making an FBO without multisampling/alpha-to-coverage, 1 color attachment, 1 depth attachment: ``` // Generate the color attachment glGenTextures(1,&defaultColorAttachment0); glActiveTexture(GL_TEXTURE0); glBindTexture(GL_TEXTURE_2D,defaultColorAttachment0); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR); glTexImage2D(GL_TEXTURE_2D,0,GL_RGBA,screenWidth,screenHeight,0,GL_RGBA,GL_UNSIGNED_BYTE,NULL); // Bind the texture to the FBO glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, defaultColorAttachment0,0); // Generate the depth attachment glGenRenderbuffers(1,&defaultDepthBuffer); glBindRenderbuffer(GL_RENDERBUFFER, defaultDepthBuffer); glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT, screenWidth, screenHeight); glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, defaultDepthBuffer); ``` This doesn't work. Code trying to make a multisampled FBO: ``` // Generate the color attachment glGenTextures(1,&defaultColorAttachment0); glBindTexture(GL_TEXTURE_2D_MULTISAMPLE, defaultColorAttachment0); glTexImage2DMultisample(GL_TEXTURE_2D_MULTISAMPLE, 4, GL_RGBA, screenWidth, screenHeight, GL_FALSE); glFramebufferTexture(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, defaultColorAttachment0,0); // Generate the depth attachment glGenRenderbuffers(1,&defaultDepthBuffer); glBindRenderbuffer(GL_RENDERBUFFER, defaultDepthBuffer); glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT, screenWidth, screenHeight); glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, defaultDepthBuffer); ``` I have tried looking through the OpenGL wiki on this, although the it's incomplete (various unfinished headings make it look unprofessional). `glGetError` never complains. I've tried messing around with this, but I either get a black screen or a screen full of garbage pixels. Main Question: What things do I need to consider/change and where (FBO creation, textures, shaders) in order to get multisampling and alpha-to-coverage to work with an FBO?