Opengl: triangles with element buffer object (EBO) aka GL_ELEMENT_ARRAY

I am trying to render a kinect depth map in real time and in 3D using openGL in an efficient way to possibly scale up and use multiple kinects.

A frame from the kinect gives 640*480 3D coordinates. X and Y are static and Z vary each frame depending on the depth of what the kinect films.

I am trying to modify my GL_ARRAY_BUFFER but partially since the X and Y don't change, I just need to change the Z part of the buffer. This is easy yet, I can use glBufferSubData or glMapBuffer to partially change the buffer and I thus decided to put all X values together, all Y togethers and all Z together at the end, I can thus change in one block the whole Z values.

The problem is the following: Since I have a cloud points of vertices, I want to draw triangles from them and the easy way I found was using a GL_ELEMENT_ARRAY_BUFFER which prevents repeating vertices multiple times. But GL_ELEMENT_ARRAY_BUFFER reads from the buffer X,Y,Z in an automatic way. Like you give the indice 0 to the GL_ELEMENT_ARRAY_BUFFER, I'd like him to take his X from the first X element in the buffer, his Y from the first Y element in the buffer and his Z from the first Z element in the buffer. Since the vertices coordinates are not arranged in a continuous fashion, it doesn't work.

Is there an alternative to specify to the GL_ELEMENT_ARRAY_BUFFER how to interprete the indices?

I tried to find a way to glBufferSubData in a disparate way (not big continuous chunk of memory but rater changing an element in the buffer every 3 steps, but this seems not optimal)

CodePudding user response：

I'm not entirely sure what the problem is here? Indices stored within a GL_ELEMENT_ARRAY_BUFFER can be used to index multiple buffers at the same time. Just set up your separated vertex buffers in your VAO:

glBindBuffer(GL_ARRAY_BUFFER, vbo_X);
glVertexAttribPointer(0, 1, GL_FLOAT, GL_FALSE, sizeof(float), 0); //< x

glBindBuffer(GL_ARRAY_BUFFER, vbo_Y);
glVertexAttribPointer(1, 1, GL_FLOAT, GL_FALSE, sizeof(float), 0); //< y

glBindBuffer(GL_ARRAY_BUFFER, vbo_Z);
glVertexAttribPointer(2, 1, GL_FLOAT, GL_FALSE, sizeof(float), 0); //< z

Set your indices and draw:

glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, indices_vbo);
glDrawElements(GL_TRIANGLES, num_indices, GL_UNSIGNED_INT, 0);

And then just recombine the vertex data in your vertex shader

layout(location = 0) in float x_value;
layout(location = 1) in float y_value;
layout(location = 2) in float z_value;

uniform mat4 mvp;

void main() {
  gl_Position = mvp * vec4(x_value, y_value, z_value, 1.0);
}