View Full Version : How to compute the time spent by AGP8X?

10-28-2004, 03:47 AM
I have a AGP 8X graphics card.When I use glReadPixels to read a 512X512 floating point buffer.The time cost is 22.8ms.
But according to AGP8X,the transfer speed is 2.134GB/S, so the time cost should not exceed 2ms,that is 512X512X16/2,134,000,000=0.00196ms.
why does this happen?
Any reply will be appreciate.

10-28-2004, 04:19 AM
Readback speed depends a lot on your graphics hardware.

The Geforce 6 can readback at about 1Gb/sec, previous generation geforces about 200 mb/sec.

The latest generation ATI's are about 200 mb/sec, the previous generation was about 80mb/sec.

The optimal format for readback is BGRA.
Reading floating point buffers may not be as well optimised in the driver.

You will get a little extra performance if you use the PBO or PDR extensions.

There are a lot of posts about readback performance in this forum, try using the search.

10-28-2004, 05:57 PM
Readback on AGP is OFTEN very limited, because it OFTEN uses PCI cycles, not AGP cycles. If you want fast readback, you want a PCI-Express motherboard and PCI-Express graphics card.