Cube mapping and skyboxes

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Cube mapping and skyboxes

Introduction
Cube map examples
Uploading cube map textures
Texture parameters
What is a skybox?
Skybox shader code
Further reading

1. Introduction

Cube-map textures are used for
- making skyboxes
- faking reflections on objects
A cube-map is comprised of 6 individual texture images
OpenGL treats them as one texture

It can be mipmapped:


    glGenerateMipmap( GL_TEXTURE_CUBE_MAP );

Cube-map requires 3 texture coordinates: S, T, and R
The R coordinates identifies a specific image among the 6 provided

2. Cube map examples

Cube map background applied:

Reflection cube map:

3. Uploading cube map textures

Create and bind texture buffer

Upload images by glTexImage2D(). For example,


    glTexImage2D(
        GL_TEXTURE_CUBE_MAP_POSITIVE_X, 0, GL_RGBA,
        w, h, 0, GL_BGR, GL_UNSIGNED_BYTE, bitmap_data );

where texture targets are


        GL_TEXTURE_CUBE_MAP_POSITIVE_X
        GL_TEXTURE_CUBE_MAP_NEGATIVE_X
        GL_TEXTURE_CUBE_MAP_POSITIVE_Y
        GL_TEXTURE_CUBE_MAP_NEGATIVE_Y
        GL_TEXTURE_CUBE_MAP_POSITIVE_Z
        GL_TEXTURE_CUBE_MAP_NEGATIVE_Y

4. Texture parameters

Use GL_TEXTURE_CUBE_MAP target texture
Continue using magnification and minification filters

Specify how to wrap each texture coordinate. For example,


glTexParameteri( GL_TEXTURE_CUBE_MAP, GL_TEXTURE_MAG_FILTER, GL_LINEAR );
glTexParameteri( GL_TEXTURE_CUBE_MAP, GL_TEXTURE_MIN_FILTER, GL_LINEAR );
glTexParameteri( GL_TEXTURE_CUBE_MAP, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE );
glTexParameteri( GL_TEXTURE_CUBE_MAP, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE );
glTexParameteri( GL_TEXTURE_CUBE_MAP, GL_TEXTURE_WRAP_R, GL_CLAMP_TO_EDGE );

5. What is a skybox?

Skybox is a giant box with texture image on every side
It surrounds the camera and never moves anywhere
It rotates with the rest of the world
The fragment shader uses sampleCube variable to sample the texels

6. Skybox shader code

Vertex shader


#version 150

in vec4 vPosition; // vertex pos
uniform mat4 mP;   // perpsective
uniform mat4 mV;   // view
uniform mat4 mM;   // model

out vec3 texCoords;

void main( void )
{
    texCoords = normalize( vPosition.xyz );
    gl_Position = mP*mV*mM*vPosition;
}

Fragment shader


#version 150

out vec4 vFragColor;
uniform samplerCube cubeMap;
in vec3 texCoords;

void main( void )
{
    vFragColor = texture( cubeMap, texCoords );
}

7. Further reading

Cube map reflections, OpenGL Superbible: openglsuperbible.com