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# glViewport math

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### #1Null and Void  Members   -  Reputation: 1087

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Posted 20 October 2001 - 01:43 PM

I am wondering what the math behind what glViewport does to the projection matrix is. This is not crutial, I''d just really like to know . Thanks in advance. [Resist Windows XP''s Invasive Production Activation Technology!]

### #2mittens  Members   -  Reputation: 1323

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Posted 20 October 2001 - 04:08 PM

I wouldn''t mind knowing the math of it either (in world coordinates), so if a vertex I''m going to pass is off-screen, then I don''t even have to make a call (very helpful for particle engines).... It''d be nice to know.

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### #3 Anonymous Poster_Anonymous Poster_*   Guests   -  Reputation:

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Posted 21 October 2001 - 12:25 AM

What glViewport does is convert normalised device coordinates to window coordinates. The normalised coordinates are in the range [-1..1], and the window coordinates can be anything.

If your window is 640 pixels wide and 480 pixels high, then the conversion would be:

windowx = (normalisedx + 1) * 320
windowy = (normalisedy + 1) * 240

glViewport also defines an offset x,y, so the final formula is:

windowx = (normalisedx + 1) * (width/2) + x
windowy = (normalisedy + 1) * (height/2) + y

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