I've done my graduate research for the past two years on topics requiring fast frame buffer readback, and here's what I've found:
For nVidia, reading back in native format (GL_BGRA_EXT) with OpenGL is very fast. I get performance in the range of 40-50 Mpix/sec, which comes out to be 160-200 MB/sec. I know people at some of the other research labs in California have been able to reproduce these numbers.
Reading back in any other format is slow, and reading back in DirectX is slow. Reading back anything but color information (e.g. Z-buffer) is really, really slow. I've talked to the driver people at nVidia, and my understanding is that they just haven't optimized these paths yet.
The driver code reads data from the card one word at a time and doesn't use any of AGP's block transfer modes simply because that would take development time away from providing features that most people are going to use.
As much as we all like to bash Microsoft, I don't think it has anything to do with them. Since nVidia writes both the Microsoft and Linux drivers, we'd probably see any improvement in readback performance in both drivers at approximately the same time, but I could be wrong...
The market drives companies like ATI and nVidia, so as soon as people start demanding fast frame buffer feedback, they'll put it in. Until then, there's still the fast OpenGL path that nVidia has put in for research purposes.
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The technique works because they unknowingly imitate moles? What happens when they find out what they're doing?
I've done my graduate research for the past two years on topics requiring fast frame buffer readback, and here's what I've found: For nVidia, reading back in native format (GL_BGRA_EXT) with OpenGL is very fast. I get performance in the range of 40-50 Mpix/sec, which comes out to be 160-200 MB/sec. I know people at some of the other research labs in California have been able to reproduce these numbers. Reading back in any other format is slow, and reading back in DirectX is slow. Reading back anything but color information (e.g. Z-buffer) is really, really slow. I've talked to the driver people at nVidia, and my understanding is that they just haven't optimized these paths yet. The driver code reads data from the card one word at a time and doesn't use any of AGP's block transfer modes simply because that would take development time away from providing features that most people are going to use. As much as we all like to bash Microsoft, I don't think it has anything to do with them. Since nVidia writes both the Microsoft and Linux drivers, we'd probably see any improvement in readback performance in both drivers at approximately the same time, but I could be wrong... The market drives companies like ATI and nVidia, so as soon as people start demanding fast frame buffer feedback, they'll put it in. Until then, there's still the fast OpenGL path that nVidia has put in for research purposes.