[quote=101230]i’ve been experimenting with the “wii” scanlines overlay preset in retroarch, and it seems to line-up with each pixel, across different emulators (MD/SNES/PSX/FBA-libretro). is that normal? i don’t even have integer scaling on!
i realise that i haven’t got the “true” aspect ratios, as per this thread, but i liked the idea of filling the whole screen (vertically) if possible.
one thing i did find is that i would get strange moire effects unless i turned the overlay opacity down to .35, but i wonder if that is just my TV, or a scaling issue? it’s really annoying in piFBA and Mame4All-Pi as they don’t have retrorch’s configuration, just a single scanlines on/off setting 
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That’s a scaling problem. I’ve explained how to get perfect scaling while filling the vertical area in my two posts “how to get scanlines” and “how to get perfect video scaling.” This is accomplished by cropping the overscan and letterboxing that is normally visible at 4x vertical scale by setting the vertical scale to 5x scale.
The wii scanline overlay is 640×480, so if Retroarch is set to render at a higher resolution than this, the overlay will be scaled with bilinear scaling, which blurs the image and causes artifacts. You need to use an overlay that matches the resolution you’re using.
Also make sure that your tv is not stretching the picture by any amount. Using your tv’s zoom feature will scale the content.