By default, the screen is cleared each time before
testApp:draw() is called, so you need to draw all the contents of the screen inside
testApp::draw() again and again. It is appropriate in most cases, but sometimes we want the screen to accumulate our drawings. In openFrameworks, you can do this by disabling screen clearing using the
ofSetBackgroundAuto( false ) function. All successive drawings will accumulate on the screen. (In this case you should call
ofBackground() rarely, only for clearing the current screen).
This method is very simple to use, but is not flexible enough for serious projects. Also, currently it has some issues: