OpenGL ES 2.0: glVertexAttribPointer vs vec3

Question

I'm trying to draw some points with GL_POINTS, and have it work as fast as possible. I have two variables that need to be passed to the shader - a position (vec2) as well as a point size (float) - and I'm not sure of the quickest way to pass them over.

Before I was using glVertexAttribPointer two times, the first time to pass the position and the second to pass the size. This lets me write the vertex shader code as follows:

attribute mediump vec4 Position;
attribute lowp float Size;

void main(void) {
  gl_PointSize = Size;
  gl_Position = Position;
}

I thought this was good as it worked and looked nice, but then I read somewhere that you should try to keep glVertexAttribPointer calls to a minimum. This led me to decide to pass the position and the size in a single vec3, which makes it so I only need to do one glVertexAttribPointer call.

However, this means I need to rewrite the vertex shader to this:

attribute mediump vec3 Position;

void main(void) {
  gl_PointSize = Position.z;
  gl_Position = vec4(Position.x, Position.y, 0.0, 1.0);
}

To me this looks uglier but it makes the glVertexAttribPointer calls much simpler.

My question is, does passing it as a vec3 actually make a performance difference, and if it does, is there a way for me to rewrite it to look a bit less ugly? I was hoping to do something like

gl_Position = vec4(vec2(Position));

But that doesn't work sadly.

You could use swizzle operators. gl_Position = vec4(Position.xy, 0.0, 1.0);. About the performance: You already implemented it. So why not profile it and test if it got faster? — BDL
I'll try profiling, but I'll have to go dig around for a spare iPad or iPhone — MysteryPancake

MysteryPancake MysteryPancake · Accepted Answer · 2017-03-02T06:57:08

I tried profiling it and it gave pretty similar results, however, it did give me some odd errors involving overlapping data and out of bounds. I think I'll stick to doing it without vec3.

OpenGL ES 2.0: glVertexAttribPointer vs vec3

1 Answers