PowerVR

The PowerVR PCX and PCX2

VideoLogic Apocalypse 3Dx

The PowerVR PCX2 up-close

Everyone remembers the 3D/fx Voodoo Graphics as being the first "real" high-end 3D graphics accelerator with a blistering fill rate of 45MTexels/sec. The PowerVR PCX graphics chip, developed by VideoLogic and NEC, could have been even faster. With a Pentium II/300, the PowerVR was offering fillrates of 66MTexels/sec, which would effectively be even higher due to the deferred rendering technology.

Why it could have been a contender

The PowerVR PCX was a dedicated 3D graphics PCI card with 4MB of SDRAM. This memory was used exclusively for textures, because the PowerVR directly transferred its images to the main 2D graphics framebuffer. While cards such as the Voodoo Graphics could only deal with 640x480 full-screen 3D, the PowerVR supported windowed-3D and resolutions as high as 1024x768. The PowerVR PCX actually had the fillrate to support these high resolutions, as it was a tile based scene renderer. Unlike other graphics chips at the time, which drew all of triangles as they came in, the PowerVR only drew visible triangles. With a baseline fillrate of 66MPixels/sec, it wasn't uncommon to see real-world fillrates approaching the magic 100MPixel/sec mark.

GLQuake at 640x480 ran at 33fps while the nearest 3D-in-a-window competitor, the Voodoo Rush did around 25-26 fps. With the PowerVR PCX, Tomb Raider was playable at 1024x768 at 30 fps, something no other graphics card could dream of pulling off... and remember, the first Tomb Raider was actually a fun game.

The PowerVR PCX graphics chips were designed around "infinite planes" as opposed to triangles. This allowed for volume rendering effects and shadows that are closer to the effects we're now only seeing with DirectX8 level tech. In order to take advantage of these technologies, games had to be programmed in the native PowerSGL API. The API even had a lens flare feature.

So what happened?

Well one disadvantage to infinite planes is that it required CPU power to translate OpenGL/Direct3D triangles info infinite planes. However, the PowerVR's first chip, the PCX, was doomed from the very beginning -- it lacked bilinear filtering. Without bilinear filtering, the PCX1 offered hardware perspective correction and true color textures, but things looked blocky.

Point sampled

Bilinear Filtered

Here's where the story gets more interesting. The PCX was supposed to have bilinear filtering. The engineers knew it was an important quality feature but during chip development, the transistor block required for bilinear filtering cycle was inadvertently left out by an engineer. By the time they realized the mistake, the decision was made that it would have been too costly and too time consuming to fix it. They had to go with the flawed chip.

UPDATE 1/22/04: Now the interesting story is why the PCX went with per-pixel linear MIP mapping as opposed to including bilinear filtering. (Note that per-pixel as opposed to per-polygon MIP mapping was also a key point in PowerVR's premium design). We've heard stories of how the PCX could have had bilinear filtering and that the engineers knew that it was possible to add the feature but overlooked it.

In our first run of this article, we erroneously went with the "bilinear filtering left out", but we were corrected by one of the architects of the PowerVR, a trusted resource. It is fact that the PCX1 never had anything other than per-pixel linear MIP mapping as the design choice. Our PowerVR stories come from current and ex-VideoLogic engineers as well as board manufacturers courted by PowerVR. In our opinion, the most likely story is an overzealous individual somewhere along the chain between engineering and a 3rd-party board manufacturer with an exaggerated story. That said, the fact that the story even exists reflects the desire that many had for PowerVR's success.