Fragment Shaders - Praise for OpenGL ES 30 Programming Guide - page 287

Figure 10-3 Multitextured Quad

Example 10-1 Multitexture Fragment Shader

#version 300 es
precision mediump float;
in vec2 v_texCoord;
layout(location = 0) out vec4 outColor;
uniform sampler2D s_baseMap;
uniform sampler2D s_lightMap;
void main()
{
vec4 baseColor;
vec4 lightColor;

baseColor = texture( s_baseMap, v_texCoord );
lightColor = texture( s_lightMap, v_texCoord );
// Add a 0.25 ambient light to the texture light color
outColor = baseColor * (lightColor + 0.25);
}

The fragment shader has two samplers, one for each of the textures. The
relevant code for setting up the texture units and samplers follows.

// Bind the base map
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, userData->baseMapTexId);
// Set the base map sampler to texture unit 0
glUniformli(userData->baseMapLoc, 0);