Could NES Doom be possible with custom mappers?

[Blastfrog]

No, not people who make maps, I'm talking about hardware built into the cartridge for increasing the capabilities of the NES. The majority of games made for the NES used 'mappers', that did stuff like add more memory, extra audio channels (Famicom games only), the ability to access more data from the rom, etc. etc..

I've been thinking though, similar to how SNES Doom used the SuperFX2 chip, what if there were a port of Doom to the NES, that used an extra processor inside to handle all the rendering, at which point, the NES draws the cartrige's screen buffer onto the screen using a fake bitmap.


Anyway, I was thinking that the screen would be 192x128 or something like that (using double horizontal pixels), no lighting effects (like GBA Doom), and sprites always face you. Probably use the Jaguar levels as a base.

I think that it would run at 30 instead of 35 fps, just to keep compatible with 60hz NTSC (Sorry PAL people, you get to play at 25 fps :P). The screen would render in 256 colors, but draw as 2 (left and right color). There would be a primary and secondary color assigned to each palette index, to create a dithered view.

[Bucket]

Nah. Catacombs 3D, perhaps. If you scale it down a bit.

[natt]

Well, what does the PPU do when it wants to display a tile? It asks the cartridge for it; sets the appropriate CHR A? pins, pulls down CHR /RD, and waits for the result on CHR D?.

If you hook up special hardware there, and it's fast enough, you could completely forego the concept of CHR ROM or CHR RAM and simply generate tile data on the fly. I seen no reason why you couldn't generate full motion images at 256x240 and send them to the NES screen at 60fps. Your external CPU could render whatever it wants to an internal framebuffer and then translate the PPU's read requests on the fly.

Course, you still only get 4 colors at a time. And it would be the most pointless project ever; simply having the specialized hardware generate its own composite video signal would be cheaper, easier, and not subject to arbitrary color limitations. Of course, at that point, it's no longer a nes cartridge.

[Blastfrog]

natt said:

Course, you still only get 4 colors at a time.

That is, per tile, not the entire screen. And correct me if I'm wrong, but isn't there some programming trick where you can have new palettes per scanline? Though, can it be done more than once per frame? I've only ever seen it done with the top half having one palette, the bottom having another, to simulate transparent water (I know the Megadrive/Genesis Sonic games used this trick).

Still, it would probably suck anyway.

[natt]

Sodaholic said:

And correct me if I'm wrong, but isn't there some programming trick where you can have new palettes per scanline? Though, can it be done more than once per frame?

It gets better if you have custom hardware. Since the cartridge has an IRQ request line, it can synch up to the PPU and generate Hblank IRQs. The whole thing would be a mess, but you'd be able to change palettes every scanline.

You're still somewhat limited in the horizontal direction, and also, your total color palette is actually pretty small; 64 colors I think? Of which there are quite a few blacks.

[Maes]

If the hardware in the cartridge is, in fact, an embedded system running Doom (all aspects of it) and the NES is used only as a video output device, then yeah.

Perhaps a more educative exercise would be if you can get a NES to run just the Doom's game logic by using a suitable C compiler, perhaps with a 2D top-down display (like e.g. Tower Assault) or a map view. But a full 3D view (assuming a source port of the Doom's engine, not a mazewar engine) requires just too many computrons.

Oh and....

Something like that is technically possible on the NES (if you use solid polygonal walls).

[hex11]

Maybe a Dragon Warrior style Doom RPG would work better...

[wildweasel]

hex11 said:

Maybe a Dragon Warrior style Doom RPG would work better...

Or a sidescroller (see my avatar). =P

[Quasar]

There is an arcane feature in the NES PPU which allows a video overlay, and AFAIK it was never exploited by any released commercial project; however, one was in the works. IIRC it was a Terminator game, and was going to be an actual FPS in 3D. It was canceled around the same time Nintendo discontinued the NES.

Custom hardware could render arbitrary contents to this overlay AFAIK.

[Blastfrog]

Quasar said:

There is an arcane feature in the NES PPU which allows a video overlay, and AFAIK it was never exploited by any released commercial project; however, one was in the works. IIRC it was a Terminator game, and was going to be an actual FPS in 3D. It was canceled around the same time Nintendo discontinued the NES.

Custom hardware could render arbitrary contents to this overlay AFAIK.

Interesting, I had no idea that existed. Where could I learn more about it? (I mean the video overlay feature, but the more info on the game would be nice too)

Anyway, what was this video overlay feature called?