A White Paper on Broadcoms Graphics Architecture

How I missed this before is beyond me, but here is a link to a white paper on Broadcoms 3D graphics architecture supporting OpenGL ES.

There are a few interesting things in this paper. Firstly texturing:

[The px3d] Reads 40 million unrotated texels per second. Rotations around the Y-axis and, to a larger extent. the Z-axis cause texture cache misses, with larger rotations causing more misses.

40million texels is very very few (a texel is a texture pixel). And thats a peak figure. The rotations make things worse, the worst case being rotations around Z or spinning an image. Actually, four texels are usually required to render a single pixel on the screen as hardware does a weighted average four texels to get the best colour.

The practical upshot of this is, as the paper says:

The 3D device cannot render at full screen at 60 fps when the display is
1920x1080, but can only do it at about 15 fps. Instead, the 3D device could render
to a quarter the display size (960x540), and have the 2D device scale it vertically to
960x1080 with filtering.

And thats without "excessive" use of transparency, rotations or even scaling. In other words, if we don't actually use much 3D.

The px3d quoted in the paper is used in the 7400 device but has been improved for later Broadcom chipsets. Nontheless, the barriers still exist, as this is essentially the same architecture, and even in newer BCM devices, in my opinion, expert design is required to get 60fps out of the device in full screen 3D. For the time being , as the paper says, 3D rendering should be limited to small areas of the screen. The image below illustrates that:

The blitter is much faster at filling, even greater than ten times faster when scaling is used! However with a more modest goal of 20-30fps, no transparency, only small amounts of rotation and rendering at 1/4 full HD and using the blitter to scale, full screen 3D can be achieved. I'm a purist from the graphics world, not DTV, and maintain that full screen 3D at 20fps is a painful, to be avoided experience. It never ceases to amaze me the happiness some DTV folks show when their 3d user interfaces run at 10fps and cause me a headache in minutes.