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++frame;
for (int y=0; y<240; y+=PARALLEL_LINES) {
    //Calculate lines.
    pretty_effect_calc_lines(lines[calc_line], y, frame, PARALLEL_LINES);
    // wait for the last frame to finish. Don't need this unless transactions are > 7
    if(-1!=sending_line)
        draw::wait_all_async(lcd);
    //Swap sending_line and calc_line
    sending_line=calc_line;
    calc_line=(calc_line==1)?0:1;
    //Send the lines we currently calculated. This call allows for max throughput
    // draw::bitmap_async works better the larger the transfer size. Here ours is pretty big
    const lines_bmp_type& sending_bmp = line_bmps[sending_line];
    rect16 src_bounds = sending_bmp.bounds();
    draw::bitmap_async(lcd,(srect16)src_bounds.offset(0,y),sending_bmp,src_bounds);
    //The line set is queued up for sending now; the actual sending happens in the
    //background. We can go on to calculate the next line set as long as we do not
    //touch lines[sending_line] or the bitmap for it; 
    // the SPI sending process is still reading from that.
}