ofxMask

ofxMask mask;

// creating masker texture
mask.beginMask();
// draw masker here in gray scale
ofSetColor(255);
ofRect(100, 200, 100, 50);
ofSetColor(128);    // more black, more transparent
ofEllipse(250, 100, 150, 150);
mask.endMask();

// creating maskee texture
mask.begin();
// draw maskee here
sometexture.draw();
mask.end();

// draw result
mask.draw();