gl_FragCoord is upside down inside FBO

FBOs are rendered upside down compared to not using an FBO, which means anything based gl_FragCoord (which is not affected by any model or view matrix transformations) gets rendered upside down.

so if you do this:

#version 120
uniform float height;
void main() {
    float y = gl_FragCoord.y / height;
    gl_FragColor = vec4(vec3(y), 1.);
}

#include "ofMain.h"
class ofApp : public ofBaseApp {
public:
    ofShader shader;
    void setup() {
        shader.setup("", "shader.fs");
    }
    void draw() {
        shader.begin();
        shader.setUniform1f("height", ofGetHeight());
        ofDrawRectangle(0, 0, ofGetWidth(), ofGetHeight());
        shader.end();
    }
};
int main() {
    ofSetupOpenGL(1280, 720, OF_WINDOW);
    ofRunApp(new ofApp());
}

you get black at top and white at bottom.

and then you stick everything inside an FBO:

...
    void draw() {
        ofFbo fbo;
        fbo.allocate(ofGetWidth(), ofGetHeight());
        fbo.begin();
        shader.begin();
        shader.setUniform1f("height", ofGetHeight());
        ofDrawRectangle(0, 0, ofGetWidth(), ofGetHeight());
        shader.end();
        fbo.end();
        fbo.draw(0, 0);
    }
};
...

you get white at top and black at bottom.

not sure if there's anything we can do about this, but i wanted to at least have an issue for it in case someone else runs into it.

Start with the shader.fs example and the ofApp::draw path, focusing on the ofShader and ofFbo calls shown in the report. Reproduce the comparison between direct drawing and FBO drawing, then trace the relevant FBO and coordinate-handling entry points; done means gl_FragCoord-based output has consistent orientation in both cases.