public ScreenshotTest setFramesToTakeScreenshotsOn(Integer... frames){
        framesToTakeScreenshotsOn.clear();
        framesToTakeScreenshotsOn.addAll(Arrays.asList(frames));
        return this;
    }
Sets the frames to take screenshots on. Frames are at a hard coded 60 FPS (from JME's perspective, clock time may vary).
jme3-screenshot-tests/src/main/java/org/jmonkeyengine/screenshottests/testframework/ScreenshotTest.java