Screendumps - Fact or fiction?

[Brian R.]

Say you want to post images showing the differences in display quality between video cards (comparing for example a G400 and a G450 or ASUS V7700). Would a screen dunp show this difference? or would it only show the displayed image before the RAMDAC processed it? I think the image would be identical prior to the RAMDAC stage right? Therefore, a screenshot from two cards would look identical. Where's the reality here? The video buffer is going to have the same image in both cases right? Where does the image typically get degraded in a poor quality (fuzzy appearing) video card? Isn't it artifacts inserted after the signal is made analog?

(Assume the graphic image will not be lossy compressed or degraded by transfer)

[This message has been edited by Brian R. (edited 05 December 2000).]

[Surfwienix]

very interesting question !!!

could anybody answer it, please?????!!!!!

[Thundrchez]

For 2D stuff, you should get an identical image in the framebuffers.

For 3D stuff, though, you will get slightly to noticably different images among various cards. Some chipsets & drivers take more shortcuts than others. You should be able to see the differences in a screen dump.

There are some tools out there that let you do a lossless grab of a direct3d game shot. They're great for getting cool backgrounds of your favorite game!

Then there is a lot to be said about the RAMDAC quality, especially as you get into the higher resolutions. The RAMDAC is usually what contributes to "fuzziness" in the image.

[Brian R.]

Thundrchez - see http://arstechnica.infopop.net/OpenT...65&m=550096624

[This message has been edited by Brian R. (edited 06 December 2000).]

[Thundrchez]

That is an interesting discussion.

Direct3D framebuffer captures have been used to compare the pre-RAMDAC images among various cards. The tricky part is to get to the exact same point in a Direct3D program on multiple cards to do the framebuffer capture. Different chipsets and driver combinations have slightly different ideas as to how the scene should look. Then there are so many variables to tweak as well. Some video chips do all of their internal colors in 32 bit, but if you are running in 16 bit, the last stage will dither down the 32 bit color into 16 bits. Some chipsets do true tri-linear filtering, where others cheat and do tri-linear dithering. The list goes on and on. I'm sure that this makes things somewhat chaotic for PC game developers to have the game look like it is supposed to on a dozen different video cards.

I don't know of a completely scientific way to judge the RAMDAC quality of video cards. In order to keep a level playing field, the only fair way to test the ramdac quality is to use the same monitor in the same conditions for multiple video cards. Still, it is a judgement call as to how vibrant the colors look, how crisp the image is, etc. And virtually every video card these days has gamma correction. There are test patterns to look at, and there are blue filters to look through, etc, to setup the color and gamma correction and brightness and contrast to be just right. Some cards will look more natural than others, but people tend to like slightly oversaturated images.