How to render anti-aliased image to a texture (and then write to PNG)? - opengl

Question

I'd like to use the "render to texture" paradigm to write a .png screenshot of my OpenGL 3D rendering. I have it working without multi-sampling, but I'm struggling to get an anti-aliased image.
First of all, is this possible?
Second, what is the right combination of API calls?
(meta third question, how can I better debug this? glCheckFramebufferStatus is clearly not enough).
Here's what I'm working with:
// 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);
Currently I'm getting a blank image and glBlitFramebuffer is throwing GL_INVALID_OPERATION. Apparently this error can correspond to many things, and I'm not sure which is applying. My buffers seem "good" according to glCheckFramebufferStatus.
This question has been asked in similar forms before:
Cannot render to texture with multisampling
Multisampled render to texture in ios
But none of the answers have lead to a complete working example. I would love to find/create a minimal example of this.

Answer 1

The tutorial at https://learnopengl.com/Advanced-OpenGL/Anti-Aliasing basically had what I needed. The working solution is:
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);
It seems there're at least two errors in the code in the original question:
The framebuffer should have a multisample texture attached, not a renderbuffer (glFramebufferTexture2D(... GL_TEXTURE_2D_MULTISAMPLE instead of glRenderbufferStorageMultisample)
Must call glBindFramebuffer(GL_FRAMEBUFFER, intermediateFBO); after blitting and before glReadPixels