why we need <vertices>
I don't understand why we need <vertices> to keep POSITION and then let primitive_elements hold VERTEX. is it has any evident?
I'm try to program imported COLLADA, any suggestion on this issue?
This has been discussed several times before.
I agree that the <vertices> element is basically worthless, but it's part of Collada so you need to deal with it in your importer. When a primitive element like <triangles> references a <vertices>, the <vertices> will contain <input> elements that reference the <source>s that contain the geometry data. I believe this is explained in the spec.