Comment by pan69

It's certainly the most rudimentary. Small optimisation on the inner-loop would be to pre-calculate the scanline offset before going into the pixel loop:

    int s = y*screenRect.w;
    
    for (int x = 0; x < screenRect.w; x++) {
       pixels[s + x] = argb(255, frame>>3, y+frame, x+frame);
    }