Question

When I bind my buffers to attributes for my shaders, they seem to be getting flipped.

So, I’ve got a vertex shader:

precision highp float;

uniform mat4 projection_matrix;
uniform mat4 modelview_matrix;

in vec3 in_position;
in vec3 in_color;

out vec3 ex_Color;

void main(void)
{ 
  gl_Position = projection_matrix * modelview_matrix * vec4(in_position, 1);
  ex_Color = in_color;
}

and a fragment shader

precision highp float;

in vec3 ex_Color;

out vec4 out_frag_color;

void main(void)
{
  out_frag_color = vec4(ex_Color, 1.0);
}

Nothing too complicated. There are two inputs: one for vertex locations, and one for colors. (As a newb, I didn’t want to deal with textures or light yet.)

Now, in my client code, I put data into two arrays of vectors, positionVboData and colorVboData, and I create the VBOs…

GL.GenBuffers(1, out positionVboHandle);
GL.BindBuffer(BufferTarget.ArrayBuffer, positionVboHandle);
GL.BufferData<Vector3>(BufferTarget.ArrayBuffer,
                       new IntPtr(positionVboData.Length * Vector3.SizeInBytes),
                       positionVboData, BufferUsageHint.StaticDraw);

GL.GenBuffers(1, out colorVboHandle);
GL.BindBuffer(BufferTarget.ArrayBuffer, colorVboHandle);
GL.BufferData<Vector3>(BufferTarget.ArrayBuffer,
                new IntPtr(colorVboData.Length * Vector3.SizeInBytes),
                colorVboData, BufferUsageHint.StaticDraw);

and then, I would expect the following code to work to bind the vbos to the attributes for the shaders:

    GL.EnableVertexAttribArray(0);
    GL.BindBuffer(BufferTarget.ArrayBuffer, positionVboHandle);
    GL.VertexAttribPointer(0, 3, VertexAttribPointerType.Float, true, Vector3.SizeInBytes, 0);
    GL.BindAttribLocation(shaderProgramHandle, 0, "in_position");

    GL.EnableVertexAttribArray(1);
    GL.BindBuffer(BufferTarget.ArrayBuffer, colorVboHandle);
    GL.VertexAttribPointer(1, 3, VertexAttribPointerType.Float, true, Vector3.SizeInBytes, 0);
    GL.BindAttribLocation(shaderProgramHandle, 1, "in_color");

But, in fact I have to swap positionVboHandle and colorVboHandle in the last code sample and then it works perfectly. But that seems backwards to me. What am I missing?

---

Update

Something weird is going on. If I change the vertex shader to this:

precision highp float;

uniform mat4 projection_matrix;
uniform mat4 modelview_matrix;

in vec3 in_position;
in vec3 in_color;

out vec3 ex_Color;

void main(void)
{
  gl_Position = projection_matrix * modelview_matrix * vec4(in_position, 1);
  //ex_Color = in_color;
  ex_Color = vec3(1.0, 1.0, 1.0);
}"

And make no other changes (other than the fix suggested to move the program link after all the set up, it loads the correct attribute, the vertex positions, into in_position rather than into in_color.

Answer 1

So, with Mārtiņš Možeiko’s help, I was able to figure this out. I was calling BindAttribLocation before LinkProgram correctly. However, I wasn’t calling GL.CreateProgram() BEFORE I was binding any of the attribute locations.