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Inunah

Posted on 08-20-24 06:06 PM, in Cropping or replacing an entire row of pixels in an NES game? (rev. 8 of 08-20-24 06:25 PM by Inunah)

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The game is Akumajou Special: Boku Dracula Kun, also known as Kid Dracula. It was released on Famicom, and has a port on the Castlevania Anniversary Collection.

What I'm wanting to do is crop or replace a row of pixels in this game, because it has massive flickering that does not reduce or stop when applying any sort of modification that stops flickering. This flickering row of pixels is not an effect of an emulator, but rather something that is visible on actual hardware and can be seen in every release of the game. Additionally, some people cannot see the flickering at all even when it is highlighted and zoomed in upon. But it is highly distracting and gives me a headache.

I have tried to isolate what is causing it, and I believe it is due to how the backgrounds of each stage are drawn, as even when the game is rendering that area as black it'll still flicker with some random color.

You can kind of see a couple out of place bits of color in this general area of the screen. That's the flickering bits, and it does not reduce by disabling the sprite limit. The flickering usually affects from 1-3 rows of pixels, and often ends up looking like something you'd get rid of by enabling overscan cropping because it's textured like the parts of the stage being drawn above and around it. Generally, it'll be two separate flickering bits on the left and right halves of the screen at the same time, and on rare occasions you can find it going across the entire screen all at once.

Here are a couple example videos you can see the flickering in, in case you want to see it in action:

(First Video: TAS Playthrough || Second Video: Original 1990 Advert)

Is there some way for me to either crop out this row of pixels, or replace them with a solid color?

Inunah

Posted on 08-21-24 12:41 AM, in Cropping or replacing an entire row of pixels in an NES game? (rev. 2 of 08-21-24 12:49 AM by Inunah)

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Posted by RT-55J
This appears to be an issue with how the raster split in the game is coded. The game is likely updating the scroll registers in an improper order, leading to garbage displaying on screen. Proper, clean raster splits are possible on the NES. The nesdev wiki has some example code here.

The first step to fixing the bug would be to look at the code being executed in the game's IRQ handler.

So this would probably be a pretty advanced fix, you think? Because like... my current ability and understanding level when it comes to NES hacking is uh... kinda lacking. I only understood up to the list of contents on that page. Rest is moon runes to me.

Apparently games using the same mapper as this game all have this problem to some degree, though I've never seen it before this game (I guess I got lucky).

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