um, i guess that is a "no" 😉 although i didn't mean empty lines but duplicated them. Also i made a wrong calculation. An additional line would be needed every 5th line (not 50). But since this was a rather naiv thought as it seems, we can leave that standing.

Actually my idea was to get to 4:3 with unlocked, natural-number xy-scaling as close as possible, add the few, missing lines as long as their number is large enough to be added in a symmetrical or forget about them if the diffrence to 4:3 is less than...maybe 5%.

That'd be kinda hard to program and I don't think it's worth it. ~5% ar error shouldn't be noticeable and there's always aspect= true to get correct ar 😀 .
But really...empty lines, not duplicated (since that would distort the picture). I can clearly see dark scanlines on my crt monitor when using some low-res modes and I rather like the effect. I'll see what comes out..I might call it the crt scaler 😉

Well scanlines would be marvelous. I am sort of a scanlines-fetichist. 😉 Do not make them black (100% intensity), give them something arround 30% of intensity. Well..or...make them the way YOU like them. Since you are the person capable of coding. 😉

However i do not think that it will be solvable in a good looking way. To get the usual scanline-effect you would need to add them every 2nd line, which would leave you with an ordinary 2xscaling situation. Wouldn't it?

Adding them every for example every 3rd,4th,5th,6th,7th or 8th line will look very strange i guess.

You're right...I just simulated the effect with a picture editor...it doesn't look that good. Dosbox already has a scanline-effect scaler (tv2x), you might wanna try that 😀

hehe, i just got a brillant idea how to get the right aspect ratio: first do the closest full-number-scale-approximation to 4:3 and then simply turn your monitor-display left/right or tilt it up/down for the fine-tuning. 😉 works only with new TFT with large viewing-angle though. 🤣

But wait a minute: Is the Dosbox-aspect-correction done by CPU? Wouldn't it be possible to make the grafic-cards calculate it by putting the output on a mesh-plane as texture and then tilt that in a fake-3d-window? Or would that take even more CPU-usage and isn't worth the effort (maybe because the result is not good)?

Depends on the output mode. Surface mode is unaccelerated so CPU has to do all the work. Only Direct3D and Opengl work by drawing to the texture and then applying the texture to a quad (overlay & ddraw work with surfaces). I'm not sure I completely understand what you described...but even if you turn the quad in the z-direction, the pixels on the texture have to be approximated somehow and that means distortion. But if you're looking to add some weird 3d effects to dosbox, feel free to check my d3d code 😁

Never mind. Regarding to your description i was planning to do exactly what is already done. =) So turning your monitor left/right or tilting it up/down is the only distortion-free method to apply aspect-correction...erm..no, i am not 100% serious about using this method. Just a thought. 😉

And you could be pretty sure, that if _I_ would start to code, the output would BE some WEIRD 3d-effect due to lacking programming-skills. 😁