In fact, I do need exact numbers. I'm using an eye-tracker and, you're right the geometry of the virtual and real-world may differ. In my calculations, I must stay true to one of the geometries. In my situation, I choose to stay true to the virtual geometry. This means taking the point-of-regard values returned by the eye-tracker and mapping them onto the viewing frustum, in the style of ray-tracing. TO do this, I need to know the true aspect ratio and FOV.
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