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DS development > Scanline Rasterization or Tiled?

#68490 - acox - Wed Jan 25, 2006 1:19 am

Looking around I?ve seen references to the rasterization hardware being line-oriented, with a 256x1 z-buffer and tris filled on scan-out.

Is there any evidence that this is the way it really works?

Given ARM?s links to Imagination, offering the latter?s PowerVR technology with their cores for SOC solutions, I cant help wondering if the 2048 tri limit might have another source. I was thinking that rasterization might be tiled. It could be backed by a non-addressable, hidden framebuffer (fullscreen 256x192) (hidden just for engineering reasons so it is on a highspeed dedicated bus, connected to the rasterizer). The 2048 tri limit would then be explained as the size of the tile-sorting buffer of the rendering core.

This setup would have the same limitations people are seeing: No access to z-buffer (because it is temporary and 32x32(say), as opposed to 256x1); Framebuffer can be captured to a background but only at extra cost.

Before I go too far with speculation I?ll just ask again: Is there any evidence for the ?standard model? of DS rasterization?

Any pointers to info on Display Capture would be most appreciated.
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3D on GBA

#68493 - Joat - Wed Jan 25, 2006 1:57 am

The limit is actually 6144 verts, which points to a simple ram limitation for the hardware rasterizer.

There are two major reasons we think it's a scanline based engine (and it'd have two lines of buffer, not one), the polygon id system / alpha blending limitations really point to a just in time rasterizer with possibly 64 units (hence 6 bits of sprite id), and the fact that there is a line overflow bit, which indicates it ran out of time on a scanline (not a dead giveaway, but pretty damn close).
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Joat
http://www.bottledlight.com

#68495 - acox - Wed Jan 25, 2006 2:09 am

Thanks for the response.

That all sounds resonable. I also went and checked the specs of the PoverVR MBX and even the LITE, lowest-end version has features beyond what the DS is capable of so it at least isn't a stock one of those.

Quote:
Any pointers to info on Display Capture would be most appreciated.


While I have you, any help with the above?
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3D on GBA

#68496 - ecurtz - Wed Jan 25, 2006 2:16 am

acox wrote:
Any pointers to info on Display Capture would be most appreciated.


http://forum.gbadev.org/viewtopic.php?t=7958

The source has the information that was originally pasted here on the board in a comment.

#68499 - acox - Wed Jan 25, 2006 2:54 am

Thanks ecurtz that code looks like exactly what I am after.

Worth staying up for :).
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3D on GBA