OpenGL教程：14. 立方体贴图和天空盒

立方体贴图是指一个立方体的贴图，一共有六个面。这种贴图一般是立方体的展开图：

但是OpenGL不会自动裁剪这个贴图，所以你需要手动裁剪，然后读取。

假如裁剪后得到的序列如下：

vector<std::string> faces;
{
    "right.jpg",
    "left.jpg",
    "top.jpg",
    "bottom.jpg",
    "front.jpg",
    "back.jpg"
};

我们写一个函数来读取：

unsigned int loadSkyBox(std::vector<std::string> &faces) {
    unsigned int textureID;
    glGenTextures(1, &textureID);
    glBindTexture(GL_TEXTURE_CUBE_MAP, textureID);

    int width, height, nrComponents;
    for (unsigned int i = 0; i < faces.size(); i++) {
        unsigned char *data = stbi_load(faces[i].c_str(), &width, &height, &nrComponents, 0);
        if (data) {
            glTexImage2D(GL_TEXTURE_CUBE_MAP_POSITIVE_X + i,
                         0, GL_RGB, width, height, 0, GL_RGB, GL_UNSIGNED_BYTE, data);
            stbi_image_free(data);
        } else {
            std::cout << "Error: Fail to load skybox !\n";
            stbi_image_free(data);
            exit(0);
        }
    }
    glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_MAG_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);
    return textureID;
}

顺序分别是：

// loads a cubemap texture from 6 individual texture faces
// order:
// +X (right)
// -X (left)
// +Y (top)
// -Y (bottom)
// +Z (front) 
// -Z (back)

之后记得激活texture：

glActiveTexture(GL_TEXTURE0);
  glBindTexture(GL_TEXTURE_CUBE_MAP, skyBoxTextureID);

利用VAO、VBO：

glGenVertexArrays(1, &skyBoxVAO);
glBindVertexArray(skyBoxVAO);
glGenBuffers(1, &skyBoxVBO);
glBindBuffer(GL_ARRAY_BUFFER, skyBoxVBO);
glBufferData(GL_ARRAY_BUFFER,
             sizeof(skyBoxVertices),
             &skyBoxVertices,
             GL_STATIC_DRAW);
glEnableVertexAttribArray(0);
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(float), (void *) (nullptr));

绘制：

glDepthFunc(GL_EQUAL);
  glBindVertexArray(skyBoxVAO);
  glDrawArrays(GL_TRIANGLES, 0, 36);
  glDepthFunc(GL_LESS);

效果：

完整代码：

void initSkyBox() {
    std::vector<std::string> faces{
            "textures/skybox/right.jpg",
            "textures/skybox/left.jpg",
            "textures/skybox/top.jpg",
            "textures/skybox/bottom.jpg",
            "textures/skybox/front.jpg",
            "textures/skybox/back.jpg"
    };
    float skyBoxVertices[] = {
            // positions
            -1.0f, 1.0f, -1.0f,
            -1.0f, -1.0f, -1.0f,
            1.0f, -1.0f, -1.0f,
            1.0f, -1.0f, -1.0f,
            1.0f, 1.0f, -1.0f,
            -1.0f, 1.0f, -1.0f,

            -1.0f, -1.0f, 1.0f,
            -1.0f, -1.0f, -1.0f,
            -1.0f, 1.0f, -1.0f,
            -1.0f, 1.0f, -1.0f,
            -1.0f, 1.0f, 1.0f,
            -1.0f, -1.0f, 1.0f,

            1.0f, -1.0f, -1.0f,
            1.0f, -1.0f, 1.0f,
            1.0f, 1.0f, 1.0f,
            1.0f, 1.0f, 1.0f,
            1.0f, 1.0f, -1.0f,
            1.0f, -1.0f, -1.0f,

            -1.0f, -1.0f, 1.0f,
            -1.0f, 1.0f, 1.0f,
            1.0f, 1.0f, 1.0f,
            1.0f, 1.0f, 1.0f,
            1.0f, -1.0f, 1.0f,
            -1.0f, -1.0f, 1.0f,

            -1.0f, 1.0f, -1.0f,
            1.0f, 1.0f, -1.0f,
            1.0f, 1.0f, 1.0f,
            1.0f, 1.0f, 1.0f,
            -1.0f, 1.0f, 1.0f,
            -1.0f, 1.0f, -1.0f,

            -1.0f, -1.0f, -1.0f,
            -1.0f, -1.0f, 1.0f,
            1.0f, -1.0f, -1.0f,
            1.0f, -1.0f, -1.0f,
            -1.0f, -1.0f, 1.0f,
            1.0f, -1.0f, 1.0f
    };

    glGenVertexArrays(1, &skyBoxVAO);
    glBindVertexArray(skyBoxVAO);
    glGenBuffers(1, &skyBoxVBO);
    glBindBuffer(GL_ARRAY_BUFFER, skyBoxVBO);
    glBufferData(GL_ARRAY_BUFFER,
                 sizeof(skyBoxVertices),
                 &skyBoxVertices,
                 GL_STATIC_DRAW);
    glEnableVertexAttribArray(0);
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(float), (void *) (nullptr));

    skyBoxTextureID = loadSkyBox(faces);
    skyBoxShader->use();
    skyBoxShader->setInt("skyBox", 0);
    glActiveTexture(GL_TEXTURE0);
    glBindTexture(GL_TEXTURE_CUBE_MAP, skyBoxTextureID);
}
unsigned int loadSkyBox(std::vector<std::string> &faces) {
    unsigned int textureID;
    glGenTextures(1, &textureID);
    glBindTexture(GL_TEXTURE_CUBE_MAP, textureID);

    int width, height, nrComponents;
    for (unsigned int i = 0; i < faces.size(); i++) {
        unsigned char *data = stbi_load(faces[i].c_str(), &width, &height, &nrComponents, 0);
        if (data) {
            glTexImage2D(GL_TEXTURE_CUBE_MAP_POSITIVE_X + i,
                         0, GL_RGB, width, height, 0, GL_RGB, GL_UNSIGNED_BYTE, data);
            stbi_image_free(data);
        } else {
            std::cout << "Error: Fail to load skybox !\n";
            stbi_image_free(data);
            exit(0);
        }
    }
    glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_MAG_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);
    return textureID;
}
void displaySkyBox() {
    view = glm::mat4(glm::mat3(camera.getView()));// remove translation from the view matrix
    skyBoxShader->use();
    skyBoxShader->set4Matrix("view", view);
    skyBoxShader->set4Matrix("projection", projection);

    glDepthFunc(GL_EQUAL);
    glBindVertexArray(skyBoxVAO);
    glDrawArrays(GL_TRIANGLES, 0, 36);
    glDepthFunc(GL_LESS);
}

着色器：

//vsh
#version 330 core
layout (location = 0) in vec3 aPos;

out vec3 TexCoords;
//out vec3 position;
uniform mat4 view;
uniform mat4 projection;

void main(){
    TexCoords = aPos;
    vec4 pos = projection * view * vec4(aPos, 1.0);
    gl_Position = pos.xyww;
}

//fsh
#version 330 core
in vec3 TexCoords;
out vec4 FragColor;
uniform samplerCube skyBox;

void main(){
    FragColor = texture(skyBox, TexCoords);
}