CODEWITHSUNDEEP
graphics
Selin Gök
Selin Gök

Reputation: 363

In computer graphics, faces of a polygon

In computer graphics, why do we need to know that backward face and forward face of a polygon are different?

Upvotes: 0

Views: 99

Answers (1)

Nicol Bolas
Nicol Bolas

Reputation: 473966

There are several reasons why a triangle's face might be important.

Face Culling

If you draw a cube, you can only ever see at most 3 sides of it. The front three sides will block your view of the back 3 sides. And while depth testing will prevent drawing the fragments corresponding to the back sides... why bother? In order to do depth testing, you have to rasterize those triangles. That's a lot of work for triangles that won't be seen.

Therefore, we have a way to cull triangles based on their facing, before performing rasterization on them. While vertex processing will still be done on those triangles, they will be discarded before doing heavy-weight operations like rasterization.

Through face culling, you can eliminate approximately half of the triangles in a closed mesh. That's a pretty decent performance savings.

Two-Sided Rendering

A leaf is a thin object, so you might render it as one flat polygon, without face culling. However, a leaf does not look the same on both sides. The top side is usually quite a bit darker than the bottom side.

You can achieve this effect by sending two colors when rendering the leaf; one meant for the top side and one for the bottom. In your fragment shader, you can detect which side of the polygon that fragment was generated from, by looking at the built-in variable gl_FrontFacing. That boolean can be used to select which color to use.

It could even be used to select which texture to sample from, if you want to do more complex two-sided rendering.

Upvotes: 2

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