g0 = av_rescale_q(s->pts, s->time_base, av_make_q(1, 128));
for (x = 0; x < s->w; x += dx) {
g = (av_rescale(x, 6 * 256, s->w) + g0) % (6 * 256);
g0 = av_rescale_q(s->pts, s->time_base, av_make_q(1, 128));
for (x = 0; x < s->w; x += dx) {
g = (av_rescale(x, 6 * 256, s->w) + g0) % (6 * 256);