Crazy idea for emulators...

Wow! Fantastic job getting it to work in a emulator!

Hmmm... I see what you mean. Would just rendenting them "flat", or just scaling it up/down (drawing it as a square, and scaling it accordingly to Y-position) fix this?

No doubt about it. Maybe a XML kind of file, specifying how which sprite is handled? This way it turns into an expandlable system: just check CRC, and if there is a "rom_name.improved.xml" or something present, read from it and apply the changes?

One thing that can be done to not have so much empty space, is to simple have a background image - took the liberty of editing your image to show it

I like your background. Makes it much nicer. I guess this too can be added as a configurable parameter. In addition, it is necessary to be able to indicate which part of the screen actually needs to be transformed. For example, sky jaguar uses a different section of the screen than knightmare does.

That sounds roughly like how I fixed it. Let me elaborate. You can see in the image that the knight is drawn correctly. Before fixing it, the horns, blue part and black part of the knight (sprites 0, 1, 2) were drawn at wrong x positions relative to each other. This is because each of the three sprites has a different y position, leading the (naive) transformation to put them at different depths, and applying a different scaling (wider/narrower) to them.

For the cannon balls and birds I didn't yet fix it. The problem is perhaps best visible if you look at the shadows of the birds. If yo look carefully you see that the shadows are not exactly drawn under the birds. Reason is that each bird is drawn at a higher y position than its shadow. Thus, the naive transformation applies a different scaling to these positions. The effect is that the scaling applied to the shadows is "wider" than the scaling applied to the birds.

The hack that I applied to fix the appearance of the knight was to draw sprites 1 and 2 (the horns and black part) at the same depth as sprite 0 (the blue part). The same trick would fix the other problems (e.g. draw shadow of bird at same depth as bird) but I didn't yet figure out the sprite numbers used.

Another thing that I didn't do yet is sprite zooming. Sprites further away should be smaller.

Also, I used a very crude transformation algorithm. We obviously need some kind of anti aliasing to make it look good.

A few considerations:

As for fixing the sprite scaling, the XML could have sprite groups, groups of sprites rendered together? I don't know how knightmare does it, but I guess games if a game does sprite cycling, this would break :/

Hmmm... didn't think of that, but it actually it make a LOT of sense! It could actually keep the side panel on screen! Nice!

Are you rendering that image manually like that? If you're using OpenGL, wouldn't it be easier to just create a texture with the image information and apply it to a "distorted" quad? That would give you automatically a "blurry" effect

That's the general idea, yes. I don't think the ones in the front would be larger, but actually, the ones in the back/top of the screen, smaller
It would be just a simple linear transform = y [0..191(maybe less? 191 would actually be bigger?)], to scale [0..1] - and maybe this could also be set by the config file... this is already giving me headaches hehehe

Yeah, the idea is to have a custom background - I just draw that thing in a couple of seconds as an example
Another idea would be (not so sure how that would work out) to just get the last 8x8 block on each side, and repeat it until it covers the screen. But this would be a LOT of work, and I'm not sure how good the result would be :/

Knightmare in 3D may look similar to this
https://www.youtube.com/watch?v=lX4b5C2YrY4