leo-j said:
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No it is rendered once. One monitor is camera 1 and the other camera two. The scene isn't rendered twice; this is a trick that old 3D games use to optimise proformance. The only way the video card will have to reder two differenct things is if the controller and the TV are displaying two very different scenes.
The interactions with the eviroment are not something the video card has to deal with, that is a problem with updating, hence it is a CPU problem. The GPU takes the updated information from the CPU and renders the screen as discribed in the paragraph above.
The reason why there is a profromance drop when you add another Upad is when the screen is rendered to optimise proformance unneed pologons/textures are culled from the scene. The more detail you can take out the more efficantly the system will run. But when you are dealing with 3 cameras fewer details get taken out of the scene causing the proformance drop. In addition more specialized occultion(spelling) technicas need to be used which each more time with each render.