Beginner question about vertex arrays in OpenGL

Question

Is there a special order in which vertices are entered into a vertex array? Currently I'm drawing single textures like this:

glBindTexture(GL_TEXTURE_2D, texName);
glVertexPointer(2, GL_FLOAT, 0, vertices);
glTexCoordPointer(2, GL_FLOAT, 0, coordinates);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

where vertices has four "xy pairs". This is working fine.

As a test I doubled the sizes of the vertices and coordinates arrays and changed the last line above to:

glDrawArrays(GL_TRIANGLE_STRIP, 0, 8);

since vertices now contains eight "xy pairs". I do see two textures (the second is intentionally offset from the first). However the textures are now distorted. I've tried passing GL_TRIANGLES to glDrawArrays instead of GL_TRIANGLE_STRIP but this doesn't work either. I'm so new to OpenGL that I thought it's best to just ask here :-)

Cheers!

Answer 1

You've got the two quads joined by the tri-strip. You could either do two separate glDrawArrays with the first being (0,4) and the 2nd being (4,4) to split the two quads up, or as you say, do triangles instead.

If you switch to triangles then you'll need to either duplicate up vertices, or if you use indices to index, you'll need to specify the 3 corners of each triangle.

Tri-strips work by taking the first 2 vertices specified then draw a triangle to the 3rd. Subsequently every extra vertex results in another triangle where it makes a triangle using the last 2 vertices and the new vertex. Some platforms support providing an index value allowing you to "reset" the tri-strip and effectively allowing you to split a sequence into multiple tri-strips with a single call.