Hi-res display capture: we're getting there!

< Display capture: oh, the fun!melonDS 1.1 is out! >

Hi-res display capture: we're getting there!
Nov 16th 2025, by Arisotura

Sneak peek of the blackmagic3 branch:

(click them for full-res versions)

Those are both dual-screen 3D scenes, but notice how both screens are nice and smooth and hi-res.

Now, how far along are we actually with this?

As I said in the previous post, this is an improved version of the old renderer, which was based on a simple but limited approach. At the time, it was easy enough to hack that on top of the existing 2D engine. But now, we're reaching the limits of what is possible with this approach. So, consider this a first step. The second step will be to build a proper OpenGL-powered 2D engine, which will open up more crazy possibilities as far as graphical enhancements go.

I don't know if this first step will make it in melonDS 1.1, or if it will be for 1.2. Turns out, this is already a big undertaking.

I added code to keep track of which VRAM blocks are used for display captures. It's not quite finished, it's missing some details, like freeing capture buffers that are no longer in use, or syncing them with emulated VRAM if the CPU tries to access VRAM.

It also needs extensive testing and optimization. For this first iteration, for once, I tried to actually build something that works, rather than spend too much time trying to imagine the perfect design. So, basically, it works, but it's inefficient... Of course, the sheer complexity of VRAM mapping on the DS doesn't help at all. Do you remember? You can make the VRAM banks overlap!

So, yeah. Even if we end up making a new renderer, all this effort won't go to waste: we will have the required apparatus for hi-res display capture.

So far, this renderer does its thing. It detects when a display capture is used, and replaces it with an adequate hi-res version. For the typical use cases, like dual-screen 3D or motion blur, it does the job quite well.

However, I made a demo of "render-to-rotscale-BG": like my render-to-texture demo in the previous post, but instead of rendering the captured texture on the faces of a bigger cube, it is simply rendered on a rotating 128x128 BG layer. Nothing very fancy, but those demos serve to test the various possibilities display capture offers, and some games also do similar things.

Anyway, this render-to-rotscale demo looks like crap when upscaling is used. It's because the renderer's shader works with the assumption that display capture buffers will be drawn normally and not transformed. The shader goes from original-resolution coordinates and interpolates between them in order to sample higher-resolution images. In the case of a rotated/scaled BG layer, the interpolation would need to take the BG layer's transform matrix into account.

I decided to postpone this to the second step. Just think of the possibilites the improved renderer would offer: hi-res rotation/scale, antialiasing, filtering on layers and sprites, ...

And the render-to-texture demo won't even work for now. This one is tricky, it will require some communication between the 2D and 3D renderers. It might also require reworking the way texturing is done: for example my old OpenGL renderer just streams raw VRAM to the GPU and lets the shader do the decoding. It's lazy, but it was a simple way to get texturing working. But again, a proper texture cache here would open up more enhancement possibilities. Generic did use such a cache in his compute shader renderer, so it could probably serve for both renderers.

That's about it for what this renderer can do, for now. I also have a lot of cleanup and tying-loose-ends to do. I made a mess.

Stay tuned!

3 comments have been posted.

< Display capture: oh, the fun!melonDS 1.1 is out! >

Klauserus says:
Nov 17th 2025

Wow. That looks great, even if it's not quite to your satisfaction yet. Technically. I'm really looking forward to it and hope you have fun and make quick progress.

somelinuxer says:
Nov 17th 2025

Can't believe you guys shilled for blackmagic design.... smh....... I had so much respect for you

ThiccFurLizzy says:
Nov 17th 2025

Nice