Triangular Cropping of an Image?

So I’m looking to grab the screen, crop it to a triangle and then draw it, like the above.
I’ve found a few ways to do it on desktop, but not with OpenGL ES.
Any pointers would be great!
Thanks

`
void ofApp::setup(){
ofEnableAlphaBlending();
img.load("images/bikers.jpg");
fbo.allocate(img.getWidth(), img.getHeight(),GL_RGBA);
fbo.begin();
{
    ofClear(0,0,0,0);
    ofSetColor(255);
    ofDrawTriangle(img.getWidth()/2, 0, 0, img.getHeight(), img.getWidth(), img.getHeight());
}
fbo.end();
img.getTexture().setAlphaMask(fbo.getTexture());
}
void ofApp::update(){}

//--------------------------------------------------------------
void ofApp::draw()
{
    fbo.begin();
{
    ofSetColor(0);
}
fbo.end();
ofSetColor(255);
img.draw(0, 0);
}

`