Fragment Shaders - Praise for OpenGL ES 30 Programming Guide - page 295

Example 10-6 User Clip Plane Fragment Shader

#version 300 es
precision mediump float;
in float v_clipDist;
layout( location = 0 ) out vec4 outColor;
void main( void )
{
// Reject fragments behind the clip plane
if( v_clipDist < 0.0 )
discard;
outColor = vec4( 0.5, 0.5, 1.0, 0.0 );
}

As you can see, if the v_clipDist varying variable is negative, this
means the fragment is behind the clip plane and must be discarded.
Otherwise, the fragment is processed as usual. This simple example just
demonstrates the computations needed to implement user clip planes.
You can easily implement multiple user clip planes by simply computing
multiple clip distances and having multiple discard tests.

Summary

This chapter introduced implementing several rendering techniques
using fragment shaders. We focused on implementing fragment shaders
that accomplish techniques that were part of fixed-function OpenGL
ES 1.1. Specifically, we showed you how to implement multitexturing,
linear fog, alpha test, and user clip planes. The number of shading
techniques that become possible when using programmable fragment
shaders is nearly limitless. This chapter gave you grounding in how to
develop some fragment shaders that you can build on to create more
sophisticated effects.

Now we are just ready to introduce a number of advanced rendering
techniques. The next topics to cover before getting there are what
happens after the fragment shader—namely, per-fragment operations and
framebuffer objects. These topics are covered in the next two chapters.