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AGP Texture Download Problem Revealed
EconolineCrush writes "The latest high-end graphics cards are capable of rendering games at 1600x1200 in 32-bit color at jaw-dropping frame rates, but that might be all they're good for. For all their gaming prowess, all of these cards have horrific AGP download speeds that realize only 1/100th of their theoretical peak. This article lays it all out, testing video cards from ATI, Matrox, and NVIDIA, and clearly illustrates just how bad the problem is. While these cards have no problems rendering images to your screen, you're out of luck if you want to capture those images with any kind of reasonable frame rate via the AGP bus."
There are actually some good reasons to be able to do this apart from just taking screenshots. I did (sad but true) these tests over 4 years ago finishing grad school, and the results (read back speed is very bad) were much the same.
Two reasons for wanting to grab the framebuffer (or parts of it) are for
a) texture imposters (realtime adaptive billboarding) and
b) split world/image-space occlusion culling.
With faster readback, both these techniques would probably be used more in "normal" software (ie games).
0.02
Tales from behind the Lagom Curtain
If you're doing multi-pass rendering, it might be extremely convenient to capture the results back to main memory. Especially if the board doesn't have enough texture memory to support all of your temporary buffers.
And boards are starting to ship with 128-bit IEEE floating point buffers.
Essentially, you're right - a human can't tell the difference beyond 24-bit on a given image. But if 100 images were composited together (very likely, to support something like RenderMan-style rendering in hardware), 24 bits is nowhere near enough - you'd get all sorts of accumulation error.
Education is the silver bullet.
...that I have ever read. Either that, or I am missing something here... The idea that graphics subsytems have 'bandwidth to burn' is kind of ironic, given that every graphics chip is ultimately held back in performance by the amount of bandwidth available to it - especially when using high quality options like anti-aliasing. The main focus of the article is actually a very niche segment... the idea of transeferring back rendered images over the AGP bus for TV / film / etc. is a joke... Rendering at high quality takes a huge amount of bandwidth (ie. textures and geometry)... as someone else pointed out, transferring back high-res images would take up over 200MB - that's a quarter of your AGP bandwidth! And without taking into account contention and timing issues in uploading/downloading that would mean that you simple couldn't realise the full potential of the bandwidth without a lot of other (expensive?) hardware... The simple fact is that for production uses, you would be *far* better off taking a stream of data from the DVI connector, and storing that for later use... Screen capture for business use is a reasonable point - however when does that require 3d rendering to be taking place? There should be no contention and no reason why the AGP bus couldn't be utilised fully - although would the graphics companies make enough out of this to justify the effort? As for internet streaming - how many people have access to bandwidth fast enough for high quality, full screen video streaming? Enough said...
Our ray intersection algorithm implemented on the GPU (an "old" Radeon 8500) was able to intersect 114M rays per second. This was loads faster than the best CPU implementation, which could handle between 20 and 40 intersections.
But when we tried to implement a ray tracer based on this, and an efficient one that didn't intersect every ray with every triangle, the readback rate killed us. Our execution times slowed down to the low end of the fastest CPU implementations.
And the readback delay seems to be completely due to the drivers, which apparently still use the old PCI-bus code. If the drivers could use the full potential of the AGP bus, our ray tracer could approach twice the speed of the best CPU ray tracers.