By default, in openFrameworks (and in most computer programs dealing with 2D graphics), the center of the screen coordinate system is placed in the top-left corner of the screen. The horizontal axis is directed to the right and the vertical axis is directed downwards. The measurement unit is the pixel.

For a screen with size 1280 × 720 pixels, the top-left corner has the coordinates (0, 0) and the bottom-right corner has the coordinates (1280-1, 720-1) = (1279, 719), as shown in the following screenshot:

This shows the default coordinate system

We are planning to implement generative graphics, which is situated around the screen center. To make the code easier, it's a good approach to move the coordinate system origin to the screen center by adding the following lines to the ofApp::draw() function:

ofPushMatrix();
ofTranslate( ofGetWidth() / 2, ofGetHeight() / 2 );
//----
//...       We will place our drawing code here
//----
ofPopMatrix();

The ofPushMatrix() function...