How to draw from a Multisampled buffer to default frame buffer - opengl

Question

I am trying to render from a multisampled framebuffer to a default frame buffer.
first i enable multisampling with these commands.
glfwWindowHint(GLFW_SAMPLES, 4);
glEnable(GL_MULTISAMPLE);
Than i create a multisampled buffer
unsigned int fboMsaaId, rboDepthId;
unsigned int MsaaTexture;
// Generate texture
glGenTextures(1, &MsaaTexture);
glTexImage2DMultisample(GL_TEXTURE_2D_MULTISAMPLE, 4, GL_RGBA8, 800, 600, false);
// create a multisample buffer
glGenFramebuffers(1, &fboMsaaId);
glGenRenderbuffers(1, &rboDepthId);
glBindFramebuffer(GL_FRAMEBUFFER, fboMsaaId);
glBindRenderbuffer(GL_RENDERBUFFER, rboDepthId);
glRenderbufferStorageMultisample(GL_RENDERBUFFER, 4, GL_DEPTH24_STENCIL8, 800, 600);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D_MULTISAMPLE, MsaaTexture, 0);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_STENCIL_ATTACHMENT, GL_RENDERBUFFER,rboDepthId);
glBindFramebuffer(GL_FRAMEBUFFER, 0);
In the render loop.
while (!glfwWindowShouldClose(window))
{
// input
// -----
processInput(window);
// render
// ------
glBindFramebuffer(GL_FRAMEBUFFER, fboMsaaId);
glClearColor(0.2f, 0.3f, 0.3f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT);
// draw our triangle
glUseProgram(shaderProgram);
glBindVertexArray(VAO);
glDrawArrays(GL_TRIANGLES, 0, 3);
glBindFramebuffer(GL_FRAMEBUFFER, 0);
glClearColor(0.2f, 0.3f, 0.3f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT);
glBindFramebuffer(GL_DRAW_FRAMEBUFFER, 0);
glBindFramebuffer(GL_READ_FRAMEBUFFER, fboMsaaId);
glDrawBuffer(GL_BACK);
glBlitFramebuffer(0, 0, 800, 600, 0, 0, 800, 600, GL_COLOR_BUFFER_BIT, GL_LINEAR);
glfwSwapBuffers(window);
glfwPollEvents();
}
If i render into the default framebuffer i am able to see the drawn object but when i blit from multisampled frame buffer to default frame buffer nothing is drawn on the screen.

Answer 1

What you want to do is make it so that the default framebuffer is not multisampled. That is, you want to blit from a multisampled framebuffer to a non-multisampled framebuffer.
Yes, it is legal to blit between two multisampled framebuffers, but this is only legal if they have the same sample count. And while you did request a 4-sampled default framebuffer, the default framebuffer is not entirely under your control. Implementations get to play fast-and-loose as to how many samples a default framebuffer gets.
So it's best to avoid this possibility altogether and use a non-multisampled default framebuffer.