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How do I use GEM in a performance?

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This FAQ applies to: Any version.

This is a constant problem, because there is no consistent way to display video on any platform. Also, you usually do not want to send the entire screen, but only the GEM window. It is also useful to be able to edit/control the Pd patch window while the patch is actually running.

On SGIs, the best way is to get a video out option. On the SGI O2, Impact, and Onyx (Mark has used all of these), there is a simple connector or breakout box to do video. If you run the video out program, then you will get a rectangle on your screen which shows what is being sent out the video connector. Make your GEM window a little larger than 640x480 and center it in the rectangle. You can now project this with a standard video projector.

On PCs it is a bit harder. Several modern video-cards have the possibility to output several screens (either 2 (or more) VGA-screens or 1 VGA-screen and 1 TV (Composite or S-HVS) or a combination with DFTs) If you have a Canopus Voodoo2 card it has a video and s-video output on it. As described in question 2.12, you can get a Voodoo to work with GEM. If any one else has a better solution, please let me know. The nVidia Riva TNTs require that you output the full screen, so this is not a very good option. You can use a video scan convertor. Some of them only display a part of the scene, which is exactly what you want.

With modern multi-headed cards it is more simple: Configure your card to display the desktop spread over your multiple screens (e.g.: from left-to-right). On windows and macOS you can do this via the display-properties dialog. On linux you will have to edit your /etc/X11/XF86Config-4 file either by hand or (if your system supports it) via an appropriate editor (yes, nowadays there are some). Now create your gem-window on the second screen: it should have the same dimensions as the 2nd screen (e.g: [dimen 800 600(). to place it at the second screen use the offset (e.g: if your primary sreen (the one you want for patch-editing) has the dimension 1024x768 use [offset 1024 0(, which will create the gem-window 1024 pixels right of the upper-left corner of the total screen (and 0 pixels below it), which is exactly the upper-left corner of the 2nd screen. You most probably want to turn off the borders with [border 0(. Note: some grafix-card have openGL-hardware-acceleration only on the 1st screen (so you should create the gem-window on the 1st screen and move your patches to the 2nd screen)

If you are using an XServer for displaying (under linux) you can also use another computer for rendering. You can specify the place where the gem-window should be created with something like [create <>:0.0(

If you are doing audio with graphics, one solution to prevent clicking is to run 2 computers and have them communicate with [netsend]/[netreceive]. We are working on making Pd/GEM multi-processor friendly, so if you have a multi-processor system, you can run everything on one machine eventually.

see also:

Why do I get clicks in the audio?
by IOhannes m zmoelnig last modified 2007-07-10 04:18 PM

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