Fragment Shaders - Praise for OpenGL ES 30 Programming Guide - page 289

Figure 10-4 Linear Fog on Torus in PVRShaman

Example 10-2 provides the code for the vertex shader that computes the
distance to the eye.

Example 10-2 Vertex Shader for Computing Distance to Eye

#version 300 es
uniform mat4 u_matViewProjection;
uniform mat4 u_matView;
uniform vec4 u_eyePos;

in vec4 a_vertex;
in vec2 a_texCoord0;

out vec2 v_texCoord;
out float v_eyeDist;

void main( void )
{
// Transform vertex to view space
vec4 vViewPos = u_matView * a_vertex;
// Compute the distance to eye
v_eyeDist = sqrt( (vViewPos.x - u_eyePos.x) *
(vViewPos.x - u_eyePos.x) +
(vViewPos.y - u_eyePos.y) *
(vViewPos.y - u_eyePos.y) +
(vViewPos.z - u_eyePos.z) *
(vViewPos.z - u_eyePos.z) );

gl_Position = u_matViewProjection * a_vertex;
v_texCoord = a_texCoord0.xy;
}