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sandbucket
08-08-2013, 12:48 PM
hi,
i am learning about frustum projections. i believe i have the basic idea understood, it takes fragments between the near plane and far plane and scales them depending on their z values and then projects them on the near plane. but what is meant by "the asymmetric frustum"? is this when a frustum is viewing something at an angle or is it simply when the line running through the frustum is parallel to the z axis just at a different x and y value? also do frustums have anything to do with the effect in almost all 3d games where you can see more of the edges of a 3d object the further away from the viewing center they are?
thanks much

carsten neumann
08-08-2013, 02:19 PM
In terms of glFrustum (http://www.opengl.org/sdk/docs/man2/xhtml/glFrustum.xml) an asymmetric frustum is one where abs(left) != abs(right) or abs(top) != abs(bottom). The result is that the line from the eye point to the center of the near plane is not orthogonal to the near plane. One use case for these frusta (sp?) is for stereoscopic projections where one uses projections that are shifted a bit to the left/right for the left/right eye respectively.

sandbucket
08-08-2013, 03:54 PM
In terms of glFrustum (http://www.opengl.org/sdk/docs/man2/xhtml/glFrustum.xml) an asymmetric frustum is one where abs(left) != abs(right) or abs(top) != abs(bottom). The result is that the line from the eye point to the center of the near plane is not orthogonal to the near plane. One use case for these frusta (sp?) is for stereoscopic projections where one uses projections that are shifted a bit to the left/right for the left/right eye respectively.

Kk, but what do you mean by abs?

carsten neumann
08-08-2013, 04:28 PM
Sorry, it's absolute value.

sandbucket
08-08-2013, 04:40 PM
Kk thanks)():$:):6

sandbucket
08-08-2013, 04:59 PM
Does the frustum give you the 3d effect of seeing the edges of a 3d object more the further off center it is?

carsten neumann
08-08-2013, 05:21 PM
Sorry, no idea what effect you are describing, I've never noticed anything happening to the edges of an object depending on where it is relative to the eye point. You see different parts of an object when it moves inside the view frustum, just like your eyes see different parts of an object when it moves inside your field of view.

GClements
08-08-2013, 07:56 PM
Does the frustum give you the 3d effect of seeing the edges of a 3d object more the further off center it is?
That's a consequence of perspective projection. With a parallel projection, moving the object in 3D simply moves its 2D projection, it doesn't change the shape of the projected image.

The term "frustum" refers to the shape of the visible region (a pyramid with the top cut off) when using a perspective projection. When using a parallel projection, the visible region is a parallelepiped (typically a cuboid).