glIndexPointer & glDrawArrays

Instead of using vertex arrays, I’m using vertex buffers to specify the data for glDrawArrays.

I am using 3 buffers of the same size. I am updating the buffers from a framebuffer object by glReadPixels. The framebuffer object has 3 textures attached of size width*height in RGBA format and type GL_FLOAT.

According to the following code, the index_buffer should dictate in what sequence the points from the vertex_buffer are drawn. But instead the problem is that the points are drawn in a sequential manner from the vertex buffer.

glEnableClientState(GL_COLOR_ARRAY);
glBindBufferARB(GL_ARRAY_BUFFER, color_buffer);
glColorPointer(4, GL_FLOAT, 0, 0);

glEnableClientState(GL_INDEX_ARRAY);
glBindBufferARB(GL_ARRAY_BUFFER, index_buffer);
glIndexPointer(GL_FLOAT, 4, 0);

glBindBufferARB(GL_ARRAY_BUFFER, vertex_buffer);
glVertexPointer(4, GL_FLOAT, 0, 0);
glEnableClientState(GL_VERTEX_ARRAY);

glDrawArrays(GL_POINTS, 0, width*height);

You may want to try out glDrawElements instead of glDrawArrays

Index array is something completely different (it is about color index).

What you are looking for is glDrawRangedElements in combination with a GL_ELEMENT_ARRAY_BUFFER

thanks for the quick reply.
I had one more query. Since I have my index buffer of float values, it will not work with glDrawRangedElements as it does not take float for indices ?
and is there a way to specify the stride for the indices (i.e. read the index after “stride” number of bytes/elements) ?

No, you will have to use tightly packed integers.