How can I pass multiple textures to a single shader?

Question

I am using freeglut, GLEW and DevIL to render a textured teapot using a vertex and fragment shader. This is all working fine in OpenGL 2.0 and GLSL 1.2 on Ubuntu 14.04.

Answer 1

It is very simple, really. All you need is to bind the sampler to some texture unit with glUniform1i. So for your code sample, assuming the two uniform samplers:

uniform sampler2D DecalTex;  // The texture  (we'll bind to texture unit 0)
uniform sampler2D BumpTex;   // The bump-map (we'll bind to texture unit 1)

In your initialization code:

// Get the uniform variables location. You've probably already done that before...
decalTexLocation = glGetUniformLocation(shader_program, "DecalTex");
bumpTexLocation  = glGetUniformLocation(shader_program, "BumpTex");

// Then bind the uniform samplers to texture units:
glUseProgram(shader_program);
glUniform1i(decalTexLocation, 0);
glUniform1i(bumpTexLocation,  1);

OK, shader uniforms set, now we render. To do so, you will need the usual glBindTexture plus glActiveTexture:

glActiveTexture(GL_TEXTURE0 + 0); // Texture unit 0
glBindTexture(GL_TEXTURE_2D, decalTexHandle);

glActiveTexture(GL_TEXTURE0 + 1); // Texture unit 1
glBindTexture(GL_TEXTURE_2D, bumpHandle);

// Done! Now you render normally.

And in the shader, you will use the textures samplers just like you already do:

vec4 a = texture2D(DecalTex, tc);
vec4 b = texture2D(BumpTex,  tc);

Note: For techniques like bump-mapping, you only need one set of texture coordinates, since the textures are the same, only containing different data. So you should probably pass texture coordinates as a vertex attribute.

Answer 2

instead of using:

glUniform1i(decalTexLocation, 0);
glUniform1i(bumpTexLocation,  1);

in your code, you can have:

layout(binding=0) uniform sampler2D DecalTex;  
// The texture  (we'll bind to texture unit 0)
layout(binding=1)uniform sampler2D BumpTex;   
// The bump-map (we'll bind to texture unit 1)

in your shader. That also mean you don't have to query for the location.