I got a suggestion about scanlines overlay. I don't know if that is possible, but this is just an idea for consideration.
As output resolutions are always fixed in standards like: 800x600, 1024x768, 1240x1024, 1920x1080 etc.
maybe it is possible to create embeded and fixed (not scalable) scanlines overlays for every resolution.
With only intensity value for set. That overlay should be layered after all input scaling and before output
Something like Indivision overlay with current parameters informations that appears in the top-corner.
so the scanlines would't be affected when input image is scaled and moved, so it would avoid distortions.
Also there would be possible then to add some different pattern variations.