I'm trying to copy behaviour shown here: http://jvspac.kirurg.org/?page=mame with SDLMAME, on directfb.

Screen is a fixed frequency 640x480 VGA monitor (Nanao MS29-34). SDLMAME is happy enough splatting video up at this resolution, but when combined with a scanline effect looks horrendous - because it's applying the scanline effect BEFORE the image is prescaled with either yuv2x2 or yv12x2 (yuv2 and yv12 modes mess with colours too much).

I've tried fiddling with all the settings as much as possible but I get nowhere. Verbose output is showing me it's rendering a 320x240 overlay, I'm getting full screen video but it's clear that the artwork is being applied at the 320x240 stage.

I could be getting things entirely backwards - if the YUV modes are pure hardware, is the prescaling being done in hardware?