Quick confirmation on Triangle Strips / Degenerate Triangles

Blips · 2012-09-28T07:39:32

In order to squeeze some increased performance out of iOS devices, I'm working on converting my geometry into indexed triangle strips. I found a program (htgen) that works with wavefront obj files and produces a list of indices to produce triangle strips. Brilliant I thought! But then when I tried rendering the newly generated data, the end result looked worse than this: Now, initially I knew that I had to connect strips with degenerate triangles, however I had no idea that the number of degenerate triangles between strips were not constant. After spending hours reading up on triangle strips, I finally discovered the winding of a proceeding strip can be altered if the incorrect number of degenerate triangles were inserted in between two strips. I spent a few more hours writing out some basic triangle strips over pages and pages of paper and figured out (or so I thought?) exactly how many degenerate triangles were needed depending upon the previous triangle's number. So please correct me if I'm wrong, but: 1) If I have 4 triangles, defined as the indices 0 1 2 3 4 5, OpenGL will draw 4 triangles in the following order: 0 1 2, 2 1 3, 2 3 4, 4 3 5 (which let's assume is CCW for this example). 2) When joining two strips, normally only 2 extra indices need to be inserted, generating 4 degenerate triangles. So building on the previous strip, if I were to want to join a new strip consisting of 3 new triangles defined as a strip of 6 7 8 9 10 (triangles 6 7 8, 8 7 9, 8 9 10), the final joined list of indices would look like 0 1 2 3 4 5 5 6 6 7 8 9 10 where the bolded numbers are the extra inserted vertices creating 4 new degenerate triangles. 3) If joining two strips, and the first triangle of the new strip is odd (as in, the first strip had 3 triangles, numbered 0, 1 and 2, and now the starting triangle of the 2nd strip is numbered 3) three additional indices must be inserted in between the two strips generating a total 5 degenerate triangles in order for the winding of the 2nd strip to remain CCW. Now based on what I just outline above, I fixed up my code to insert the correct number of degenerate triangles between strips and got the above image which while is an improvement over the original, it's still in no way correct (the object should look like Canada and the US). So now I'm stuck and I have to assume that if my above understandings are correct, that the htgen program is not generating strips with consistent winding orders (the objects render perfectly fine if I turn off culling). If that's the case, it looks like I'll have try generating strips myself because I literally cannot find another suitable application that will generate triangle strips. So in the interest of saving myself even more headache and time spent on writing my own application to generate triangle strips, I wanted to confirm that my understanding of the subject is indeed correct.

Graphics and GPU Programming Programming OpenGL

Started by Blips August 11, 2011 09:22 PM

10 comments, last by rip-off 11 years, 6 months ago

Pratiksha Jhalani

101

September 28, 2012 07:01 AM

Say you've got 2 strips, strip 1 has vertexes 0, 1, 2, 3 and strip 2 has vertexes 4, 5, 6, 7. To render them with GL_TRIANGLES you use indexes 0, 1, 2, 1, 3, 2, 4, 5, 6, 5, 7, 6 for your glDrawElements call. No need to make any changes to the vertexes. That'll do it.

The number of indexes you need for each strip is always (numverts - 2) * 3 and just remember to follow the winding order rules, reversing the order for alternate tris that make up the strip. Code for writing out indexes might look something like this:

for (int i = 2; i < stripverts; i++) { indexes[totalindexes++] = totalverts + i - 2; indexes[totalindexes++] = (i & 1) ? (totalverts + i) : (totalverts + i - 1); indexes[totalindexes++] = (i & 1) ? (totalverts + i - 1) : (totalverts + i); } totalverts += stripverts;

Then just ensure that all of your vertexes for all strips are in a single big array (or single VBO, as appropriate) and your draw call is:

glDrawElements (GL_TRIANGLES, totalindexes, GL_UNSIGNED_WHATEVER, indexes); // _WHATEVER is _SHORT or _INT, replace indexes with 0 for a VBO

OK, it's more indexes, but indexes are much smaller than full vertexes, the duplicate vertexes will be in your cache and so won't need to be retransformed, and you'll find that the overall submission is lighter than with degenerate triangles.