[eyadams]

Will having two monitors hooked up to the same card make games and iMovie slower?

I thought this had been covered before, but I couldn't find the answer. I have a G5 (late 2003) with the ATI 9600 video card. I am currently running 1 VGA monitor with the Apple-provided DVI-VGA adapter. I would like to connect a second VGA monitor, using the Dr. Bott ADC-VGA adapter. But I'm worried about video performance under games and iMovie will suffer.

I'm not a big gamer, but I do occasionally play current games. I've played Doom 3 on the current set up and been satisfied. But I'm concerned that if I hook up a second monitor, the VRAM will be halved, even though ne monitor won't be in use.

For iMovie, the second monitor won't be blacked out, so I imagine the situation will be worse.

So: does anyone know for sure? Would I just better off putting a PCI video card (ATI 9200 or similar) to drive my second monitor?

[Tuoder]

I don't think you will ever run into a problem. Your VRAM isn't halved by doing this. You video card is busier, no doubt, but I don't think that you will have a problem.

[jamil5454]

Wrong. The VRAM is halved as soon as you connect a secondary display. And AFAIK, when you start a 3d game, all the VRAM is allocated back to the main display.

[Tuoder]

Originally Posted by jamil5454

Wrong. The VRAM is halved as soon as you connect a secondary display. And AFAIK, when you start a 3d game, all the VRAM is allocated back to the main display.

Okay, so you are telling me that, if I run a secondary display at 640x480, and my primary at 1600x1200, they will use the same amout of VRAM?

[jamil5454]

Originally Posted by Tuoder

Okay, so you are telling me that, if I run a secondary display at 640x480, and my primary at 1600x1200, they will use the same amout of VRAM?

They will have allocated the same amount, yes. This doesn't mean they are using the same amount.

It's better explained here:
http://en.wikipedia.org/wiki/Dual_monitor

[Tuoder]

Originally Posted by jamil5454

They will have allocated the same amount, yes. This doesn't mean they are using the same amount.

It's better explained here:
http://en.wikipedia.org/wiki/Dual_monitor

I actually read that before the last post, to fact check. It doesn't say "in half". It says that less is available to each monitor. It doesn't say that half of VRAM is allocated for each monitor, completely regardless of the architechture of the card.

[P]

The VRAM is used for several things - storing textures and backing store for the pixels on screen. The backing store is separate, the etxture memory is shared. 640*480*4 (4 bytes = 32 bits) means that a 640*480 screen at millions of colors with full alpha channel will take up 1.2 megs of RAM for the backing store.. I suspect that only 3 bytes per pixel is actually used on the board (skipping the alpha), so you'd end up with 900k of RAM for the backing store. This is what you lose by adding a second monitor at that resolution. You do the math to see what you lose by running it at a higher resolution. In addition, I know that many games disable the second monitor when running fullscreen to get that VRAM back in any case.

[rhashem]

Originally Posted by jamil5454

They will have allocated the same amount, yes. This doesn't mean they are using the same amount.

It's better explained here:
http://en.wikipedia.org/wiki/Dual_monitor

The Wikipedia article is completely wrong. They say the effective resources are halved, but that's not true. There is a performance hit, to be sure, because you now have two framebuffers that must occupy space in memory, but frame-buffers are actually not that big in comparison to the 128-512MB you find on a modern graphics card. For a 1600x1200x32bpp display, the framebuffer takes about 8mb, so driving 2 such displays uses up about 16mb on the graphics card. Of course, if you've got both displays full of windows, the window buffers will use up more memory, but it'd be the same as if you'd opened up that many windows on a single display.

The Wikipedia article seems to erroneously attribute some particular limitations of the DRI implementation in Linux to dual-monitor support in general. For example, they say you cannot get 3D acceleration simultaniously on multiple heads, which is not true in OS X, or in Linux with NVIDIA's TwinView drivers, or even in Linux with the DRI if you use the "MergedFB" option.

To put things succinctly: with any sane driver architecture (and most modern designs are sane), you'll only pay for the pixels you use. If you're playing Doom, and the second monitor is just displaying the desktop, there will be no perceptible performance hit (except the extra RAM usage of the second framebuffer, which is trivial on most cards). On the other hand, if you've got OpenGL apps rendering full-screen to both displays, then you'll get a hit --- but only because you're rendering more pixels, not because there are two monitors.

[jamil5454]

Crap. I lose.

[eyadams]

Just a quick follow up. I haven't tested everything, but I got the Dr. Bott adapter (nicely made, BTW), and everything is working great. Games do not appear to be adversely affected. At least, the few that I've tested, which correctly black out the secondary monitor.

Alls well.