Reallocating FBO with different format doesn't work properly

@cuinjune wrote:

I don't understand why reallocating FBO doesn't work properly.
Please look at my example code below,
in setup() I allocated an FBO with GL_RGB format and then cleared it with red color.
And then in draw(), I drew a white circle onto the FBO.
And if I press any key, it will re-allocate FBO with GL_RGBA format and then clear it with 0,0,0,0 which should make the FBO transparent.
But what I see is a black background and not transparent.
Is this a natural behaviour?

//--------------------------------------------------------------
void ofApp::setup(){

    fbo.allocate(400, 400, GL_RGB);
    fbo.begin();
    ofClear(ofColor(255,0,0));
    fbo.end();
}

//--------------------------------------------------------------
void ofApp::draw(){

    fbo.begin();
    ofSetColor(255, 255, 255);
    ofSetCircleResolution(50);
    ofDrawCircle(200, 200, 200);
    fbo.end();

    fbo.draw(0, 0);
}

//--------------------------------------------------------------
void ofApp::keyPressed(int key){

    fbo.allocate(400, 400, GL_RGBA);
    fbo.begin();
    ofClear(ofColor(0,0,0,0));
    fbo.end();
}

Before key press

Screen Shot 2017-04-20 at 1.49.02 PM.png2042x1576 76.1 KB

After key press

Screen Shot 2017-04-20 at 1.49.13 PM.png2050x1586 84 KB

Also, the opposite way doesn't work properly either (GL_RGBA -> GL_RGB)
In this case, if I reallocate FBO with GL_RGB, and then if I clear color with 0,0,0,0 the background becomes transparent which it should not be.

I think it means once FBO is allocated with a specific format, the format never changes even if it is reallocated. I wonder if it is a natural behaviour or a bug.

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