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xqwww

Help..beginner's question for view transformation

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I saw this order in lots of book modelview ->projection->Perspective division-> viewport but why I see the code always implement this order like reversed? in resize function call. viewport->glMatrixMode (GL_PROJECTION)->glMatrixMode (GL_MODELVIEW); ??? anyone can help me figure out this question? Thanks [edited by - xqwww on June 1, 2004 4:52:53 PM]

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Guest Anonymous Poster
Hi,

where do you see it implemented reversed

When you need to go from world to screen coordinates, you do Vertex * Modelview * Projection

When you need to go from screen to world coords, you do Vertex * InvertedProjection * InvertedModelview

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I think he''s refering to program setup, where you create a view port, set the projection matix and then do everything in modelview, which is where the confusion lays.

the modelview->projection->prespective divsion->viewport is the path vertex data takes to make it to the screen.

glviewport() is used to setup the viewport at teh start of a program.
glMatrixMode(GL_PROJECTION) set OGL so that any modifications are done to the projection matrix
and
glMatrixMode(GL_MODELVIEW) is the same but for the modelview matrix and ensures your vertex data gets transformed by the correct matrices.

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