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visible_surface_determination

# visible_surface_determination - CAP4730 Computational...

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CAP4730: Computational Structures in Computer Graphics Visible Surface Determination

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Outline The goal of visible surface determination Normals Backface Culling Depth Buffer BSP Trees Determining if something isn’t in the view frustum (research topics)
Goal of Visible Surface Determination To draw only the surfaces (triangles) that are visible, given a view point and a view direction

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Three reasons to not draw something 1. It isn’t in the view frustum 2. It is “back facing” 3. Something is in front of it (occlusion) We need to do this computation quickly. How quickly?
Surface Normal Surface Normal - vector perpendicular to the surface Three non-collinear points (that make up a triangle), also describes a plane. The normal is the vector perpendicular to this plane.

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Normals
How do we compute a normal? Q: Given a Triangle, how do we compute a normal? A: Normal = V 0 V 1 X V 0 V 2 But…. we know V 0 V 1 X V 0 V 2 != V 0 V 2 X V 0 V 1

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Vertex Order Vertex order matters . We usually agree that counterclockwise determines which “side” or a triangle is labelled the “front”. Think: Right handed coordinate system.
What do the normals tell us? Q: How can we use normals to tell us which “face” of a triangle we see?

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Examine the angle between the normal and the view direction V N Front if V . N <0
Viewing Coordinates If we are in viewing coordinates, how can we simplify our comparison? Think about the different components of the normals you want and don’t want.

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Backface Culling Before scan converting a triangle, determine if it is facing you Compute the dot product between the view vector (V) and triangle
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