CODEWITHSUNDEEP
c++opengl
eatonphil
eatonphil

Reputation: 13712

OpenGL glDrawElements with variable number of indices

I've been using glDrawElements to draw triangles that describe objects. However, I'm trying to load some OFF objects I found online and I saw this describing the indices of the vertices:

...
4 195 209 210 196
4 196 210 211 197
3 197 211 15
3 0 212 198
4 198 212 213 199
4 199 213 214 200
...

My question is, how do I switch between drawing elements described by 3 indices of vertices and 4 (and any other number of indices). Currently I can only load in OFF files that use 3 indices to describe the vertices:

glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, IBO);
int size; glGetBufferParameteriv(GL_ELEMENT_ARRAY_BUFFER, GL_BUFFER_SIZE, &size);
glDrawElements(GL_TRIANGLES, size/sizeof(GLushort), GL_UNSIGNED_SHORT, 0);

Any tips or references/pointers to tutorials or anything related to help me generalize things?

Upvotes: 1

Views: 316

Answers (2)

Reto Koradi
Reto Koradi

Reputation: 54642

As long as the polygons are convex, triangulating them is easy. One approach is that you split each polygon into triangles, and then draw with primitive type GL_TRIANGLES. For a polygon with n vertices, you will end up with n - 2 triangles defined by the vertices with the following 0-based indices:

0 1 2
0 2 3
0 3 4
...
0 n-2 n-1

For your example, the sequence of indices would be:

195 209 210 195 210 196
196 210 211 196 211 197
197 211 15
0 212 198
198 212 213 198 213 199
199 213 214 199 214 200

The more elegant approach is to draw each polygon with primitive type GL_TRIANGLE_FAN. The order of vertices for a triangle fan is exactly the same as for a polygon, so you can simply specify the indices in order:

0 1 2 3 ... n-1

To separate the polygons, you can use primitive restart. Enable it with:

glPrimitiveRestartIndex(0xffff);
glEnable(GL_PRIMITIVE_RESTART);

and then insert the restart index between polygons. For your example:

195 209 210 196 0xffff
196 210 211 197 0xffff
197 211 15 0xffff
0 212 198 0xffff
198 212 213 199 0xffff
199 213 214 200

Upvotes: 1

didierc
didierc

Reputation: 14750

The first entry of your dataset is a quad, which can be split into 2 triangles using vertices {1,2,3} and {1,3,4} (ie {195,209,210} and {195,210,196} for the first entry). Use the same scheme for each quad you have in your dataset. If you get inverted tris, try flipping the order of the vertices ({195,210,209} instead of {195,209,210} for instance).

Upvotes: 0

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