in/out keywords in GLSL

Question

I don't really understand how to use the in / out keywords in GLSL, and google is being uncharacteristically unhelpful.

What exactly do they do? How would I use them if, for example, I want to pass a varying variable set per vertex to the fragment shader?

Literally every tutorial I find uses the varying / attribute keywords and that's not helpful.

Answer 1

The storage qualifiers in and out actually have a purpose that contains and supersedes that of varying and attribute. They define what variables are respectively *in*puts and *out*puts for the shader. See the GLSL 4.2 reference card page 7:

• in: linkage into shader from previous stage
• out: linkage out of a shader to next stage
• attribute: same as in for vertex shader
• varying: same as out for vertex shader, same as in for fragment shader

With the side note that the latter two are sort of deprecated: they are not present in the 4.2 core profile, only in the compatibility profile.

What exactly do they do?

As for usage, take the vertex shader from An intro to modern OpenGL. Chapter 2.2: Shaders:

#version 110    
attribute vec2 position;    
varying vec2 texcoord;    
void main()
{
    gl_Position = vec4(position, 0.0, 1.0);
    texcoord = position * vec2(0.5) + vec2(0.5);
}

It should be rewritten in 4.2 core as:

#version 420
in vec2 position;    
out vec2 texcoord;    
void main()
{
    gl_Position = vec4(position, 0.0, 1.0);
    texcoord = position * vec2(0.5) + vec2(0.5);
}

Unhelpful tutorials

I'm guessing the main reason you find "outdated" tutorial code is that not everyone has access to GLSL 3.3+ compatible hardware. Regardless, for a good and more up to date tutorial I'll gladly point you in the direction of Nicol Bolas' Learning Modern 3D Graphics Programming.