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Window coordinates

Started by patrrr Aug 10, 2006 at 10:29 AM 2 replies 400+ views

Original Post

Aug 10, 2006 at 10:29 AM

Hello, I'm trying to use the upper-left as origin for window coordinates in OpenGL, I've got a function that converts between those and it works great when setting up the viewport (glViewport). But when I try to render to a texture using a FBO, I get the lower-left corner even though my viewport is in the upper-left! Don't FBOs use the current viewport? Thanks for any help.

Brother Bob

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Brother Bob

Aug 10, 2006 at 11:08 AM

FBO uses the viewport, yes, but the viewport's origin is always in the lower left corner. The best you can do is to convert from your upper-left system into OpenGL's lower-left system, but that only changes the parameters passed, it doesn't change the fact that the viewport is specified in a lower-left system.

patrrr

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patrrr

Aug 10, 2006 at 11:18 AM

So the FBO covers (0, 0, viewport.width, viewport.height) and not (viewport.x, viewport.y, viewport.width, viewport.height)?
Ie, it doesn't care about the viewport's position?

Brother Bob

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Brother Bob

Aug 10, 2006 at 11:42 AM

The coordinate system ranges from 0 in the lower/left corner, to height/width in the upper/right corner. This is the coordinate system in which you set the viewport. The viewport itself only defines the transformation from normalized device coordinates (the stage after perspective division) to device coordinates, nothing else.

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