Aliasing artifacts during transparent MSAA FBO resolve - c++

Question

I have the following rendering flow:
Clear MSAA x8 FBO color attachment (RGBA) with {0.0f,0.0f,0.0f,0.0f}
Issue single draw call (draw rectangular shape).
Bind Resolve FBO (just single RGBA color attachment)
Blit MSAA FBO into Resolve FBO.
What I noticed that when I clear MSAA FBO to all zeros, AA still has some aliasing artifacts along the diagonal edges.
But if I clear with {0.0f,0.0f,0.0f,1.0f} ,anti-aliasing looks ok.
To verify that the problem is with alpha channel,I attached color texture to resolve FBO that has GL_RGB format instead of GL_RGBA (same as resolving into screen framebuffer which doesn't show this issue),and in this case, the problem doesn't appear.
So my question is, why does it happen when MSAA RT cleared with zero alpha?
And how can I solve it,except clearing alpha to one? Why I need it cleared to zero? Because this is intermediate pass,results of which are used later in alpha compositing.
Here is some of the important parts of the code. I omit pure OpenGL init logic for textures and FBOs setup as it's abstracted behind C++ API,which is well tested in production. Both FBO use texture attachments of RGBA format.
Render loop:
const float clearColorZero[4] = { 0.0f,0.0f,0.0f,0.0f };
glClearNamedFramebufferfv(msaaFBO, GL_COLOR, 0, clearColorZero);
glBindFramebuffer(GL_FRAMEBUFFER, msaaFBO);
glViewport(0, 0,width,height);
//Bind shader program and mesh:
BindProgram(mainProg);
BindMesh(mesh);
glProgramUniformMatrix4fv(mainProg, 0, 1, GL_FALSE,glm::value_ptr(matrixMVP));
Draw(mesh);//calls glDrawArrays
//Resolve MSAA
glBlitNamedFramebuffer(
msaaFBO,
resolveFBO,
0, 0, width, height,
0, 0, width, height,
GL_COLOR_BUFFER_BIT,GL_NEAREST);
Simple fragment shader:
#version 450 core
out vec4 o_color;
void main()
{
o_color = vec4(1.0,228.0 /255.0,123.0/255.0,1.0);
}
Blit results with MSAA FBO alpha cleared to zero:
Blit results with MSAA FBO alpha cleared to one:
PS: Same happens if I resolve MSAA manually inside shader:
#version 450 core
#define NUM_MSAA_SAMPLES 8
layout(binding = 0) uniform sampler2DMS colorMap;
out vec4 o_color;
void main()
{
vec4 color = texelFetch(colorMap,ivec2(gl_FragCoord.xy), 0);
for(int i = 1; i < NUM_MSAA_SAMPLES; ++i)
{
vec4 samplePoint = texelFetch(colorMap, ivec2(gl_FragCoord.xy), i);
color += samplePoint;
}
color /= float(NUM_MSAA_SAMPLES);
o_color= color;
}
Here is magnified version of the screenshots that shows:
MSAA FBO cleared to zero:
MSAA FBO cleared to one:
I also can't see any changes even if I disable blending with glDisable(G_BLEND)