QPainter has many composition modes but none called additive. I'm interested because additive blending is used all the time in games for lighting / particles whatever. The overlay mode is the only one that had something like the effect of lighting.
EDIT: I figured it out, heres how you can efficiently make different coloured lights in Qt.
In constructor or where ever, not in paint event:
light = QPixmap("light.png");
QPainter pix(light);
pix.setCompositionMode(QPainter::CompositionMode_Overlay);
pix.fillRect(light.rect(), QColor(255, 0, 0, 255)); // colorize the light in any color
Paint Event:
// Do drawing, e.g. a background
p.drawPixmap(0, 0, QPixmap("background.png"));
// draw the lighting
p.setCompositionMode(QPainter::CompositionMode_Plus);
p.drawPixmap(100, 100, light);
You can reuse the same pixmap as much as you like and draw it with different opacity or size etc.
