# Triangle Strip Stitching question

Hi,

I’m reading about Rendering Geometry on the wiki, and I’m not quite sure what the differences are between “Triangle Strips” vs. “Polygons with 3 vertices”. Also, out of curiousity, the metalball conversion to polygon is to take much CPU, but requires one batch to render metaball… I do not quite understand this… Is this saying that while metaballs are deforming it would be costly to CPU due to conversions, but while it does not deform it does not cost much?

Thanks!

A Polygon with 3 vertices is a triangle, ofcourse.

Here is a link to a better description of a triangle strip
http://en.wikipedia.org/wiki/Triangle_strip

It’s a single primitive that is described by a chain of vertices. It results in multiple triangles getting draw by a single primitive.

I guess there are actually 4 costs involved with rendering

1. The CPU may need to convert the data to a format OpenGL can render.
2. The CPU needs to copy the geometry data to the GPU.
3. The CPU needs to tell the GPU how to render the geometry.
4. The GPU needs to render the geometry.

When a metaball isn’t changing it only needs to be converted once and then the data sits in polygonal format ready to be rendered. So cost #1 goes away in this case.
Cost #2 and #3 are always cheap for metaballs because at this point they are triangle strips that can be rendered in 1 batch.

I’ll update the wiki

I just updated the article with more info, split it into 5 costs infact.

Article on Touch Wiki says:

Any number of triangle strips can be rendered as one batch. This is done using a technique called Triangle Strip Stitching.

I have 2 questions:

1. It’s very important for me to understand, do i need to merge all the sops(with tristrips) into 1 sop or it will be the same efficiency as separate multiple sops?

2. “technique called Triangle Strip Stitching” - it’s an automatic thing or it’s needed to be done manually? If it’s manually, how it can be done? I couldn’t find any advices about it on Touch wiki.

Different SOPs will always be rendered as different batches, they can’t be merged together easily.
You dont need to worry about triangle strip stitching, it’s always done automatically if your SOP only has triangle strips.

great, thanks for your help!