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.

Now, I want to apply a bump map to the teapot. My lecturer evidently doesn't brew his own tea, and so doesn't know they're supposed to be smooth. Anyway, I found a nice-looking tutorial on old-school bump mapping that includes a fragment shader that begins:

uniform sampler2D DecalTex; //The texture
uniform sampler2D BumpTex; //The bump-map 

What they don't mention is how to pass two textures to the shader in the first place.

Previously I

//OpenGL cpp file
glBindTexture(GL_TEXTURE_2D, textureHandle);

//Vertex shader
gl_TexCoord[0] = gl_TextureMatrix[0] * gl_MultiTexCoord0;

//Fragment shader
gl_FragColor = color * texture2D(DecalTex,gl_TexCoord[0].xy);

so now I

//OpenGL cpp file
glBindTexture(GL_TEXTURE_2D, textureHandle);
glBindTexture(GL_TEXTURE_2D, bumpHandle);

//Vertex shader
gl_TexCoord[0] = gl_TextureMatrix[0] * gl_MultiTexCoord0;
gl_TexCoord[1] = gl_TextureMatrix[1] * gl_MultiTexCoord1;

//Fragment shader
gl_FragColor = color * texture2D(BumpTex,gl_TexCoord[0].xy);
//no bump logic yet, just testing I can use texture 1 instead of texture 0

but this doesn't work. The texture disappears completely (effectively the teapot is white). I've tried GL_TEXTURE_2D_ARRAY, glActiveTexture and few other likely-seeming but fruitless options.

After sifting through the usual mixed bag of references to OpenGL and GLSL new and old, I've come to the conclusion that I probably need glGetUniformLocation. How exactly do I use this in the OpenGL cpp file to pass the already-populated texture handles to the fragment shader?

• How to pass an array of textures with different sizes to GLSL?
• Passing Multiple Textures from OpenGL to GLSL shader
• Multiple textures in GLSL - only one works

(This is homework so please answer with minimal code fragments (if at all). Thanks!)

Failing that, does anyone have a tea cosy mesh?

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.