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 inputs and outputs 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.

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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);
}

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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I'm guessing the main reason you find "outdated" tutorial code is that not everyone has access to GLSL 3.3+ compatible hardware.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.