void ofBackgroundGradient(const ofColor& start, const ofColor& end, ofGradientMode mode) {
    float w = ofGetWidth(), h = ofGetHeight();
    ofMesh mesh;
    mesh.setMode(OF_PRIMITIVE_TRIANGLE_FAN);
    if(mode == OF_GRADIENT_CIRCULAR) {
        // this could be optimized by building a single mesh once, then copying
        // it and just adding the colors whenever the function is called.
        ofVec2f center(w / 2, h / 2);
        mesh.addVertex(center);
        mesh.addColor(start);
        int n = 32; // circular gradient resolution
        float angleBisector = TWO_PI / (n * 2);
        float smallRadius = ofDist(0, 0, w / 2, h / 2);
        float bigRadius = smallRadius / cos(angleBisector);
        for(int i = 0; i <= n; i++) {
            float theta = i * TWO_PI / n;
            mesh.addVertex(center + ofVec2f(sin(theta), cos(theta)) * bigRadius);
            mesh.addColor(end);
        }
    }
    // ...
    glDepthMask(false);
    mesh.draw();
    glDepthMask(true);
}