OpenGL基础12：进入3D世界

一、坐标系

我们都学过坐标系，根据z轴的方向，一般有两种不同的坐标系表示方法：

其中拇指是x轴正方向，食指是y轴正方向，中指是z轴正方形

OpenGL约定是右手坐标系，而在标准化设备坐标系(NDCSpace)中OpenGL使用的是左手坐标系

还记得上一章的这个公式嘛： $V_{\text {clip}}=M_{\text {projection}} \cdot M_{\text {view}} \cdot M_{\text {model}} \cdot V_{\text {local}}$

再看看下面的这个顶点着色器，是不是有种亲切感

#version 330 core
layout (location = 0) in vec3 position;
layout (location = 1) in vec4 color;
layout (location = 2) in vec2 texture;
out vec4 colorIn;
out vec2 texIn;
uniform mat4 model;             //模型矩阵
uniform mat4 view;              //观察矩阵
uniform mat4 projection;        //投影矩阵
void main()
{
    gl_Position = projection * view * model * vec4(position, 1.0);
    colorIn = color;
    texIn = vec2(texture.x, 1.0f - texture.y);
}

是的，我们将片段着色器修改如此，就至少可以开始生成我们想要的简单3D图形了

二、进入3D世界

依然是在 OpenGL基础8：SOIL库这一章的代码上进行扩展，我们让那张拥有纹理的矩形在3D世界中“躺下”

效果如下：（有那种感觉了）

（顶点着色器在上面，片段着色器和Shader.h依旧不用改）

很明显可以看出来，我们用的投影方式是透视投影（近大远小）：

glm::mat4 projection = glm::mat4(1.0f);
projection = glm::perspective(glm::radians(45.0f), (GLfloat)WIDTH / (GLfloat)HEIGHT, 0.1f, 100.0f);
GLint projLoc = glGetUniformLocation(shaderYellow.Program, "projection");
glUniformMatrix4fv(projLoc, 1, GL_FALSE, glm::value_ptr(projection));

而对于模型，我们让它倾斜了一定的角度（向后倾倒55°）：

glm::mat4 model = glm::mat4(1.0f);
model = glm::rotate(model, glm::radians(-55.0f), glm::vec3(1.0f, 0.0f, 0.0f));
GLint modelLoc = glGetUniformLocation(shaderYellow.Program, "model");
glUniformMatrix4fv(modelLoc, 1, GL_FALSE, glm::value_ptr(model));

如果我们不设观察矩阵，那么我们相当于就是在(0, 0, 0)这个位置观察世界，然而问题就来了，我们的这个矩形中心点正是(0, 0, 0)，如果这样的话就会出现我们的“眼睛”和这个矩形过于近甚至重叠的情况

如果你玩过不少游戏，想象一下你卡了BUG，进入了游戏的某块箱子又或者是卡进了墙中，那么你就会发现你的视线可以直接穿过这面墙，这正是因为透视矩阵的near值设置大小超过了你与物体的距离

我们当然要避免这种情况，当然不是为了避免卡BUG，而是“往后退”，暂时定死在(0, 0, 3)的这个位置观察我们例子中的矩形，~~为什么这里说是“暂时定死”呢，因为我们肯定是在移动中观察世界的，后面我们也可以让“摄像机”动起来~~

也可以这样理解：观察矩阵即是摄像机的位置！但是由于相对关系，这个位置必然是是取反后的结果

因此我们的观察矩阵设置如下：

glm::mat4 view = glm::mat4(1.0f);
view = glm::translate(view, glm::vec3(0.0f, 0.0f, -3.0f));
GLint viewLoc = glGetUniformLocation(shaderYellow.Program, "view");
glUniformMatrix4fv(viewLoc, 1, GL_FALSE, glm::value_ptr(view));

搞定，完整代码如下：

#include<iostream>
#include<opengl/glew.h>
#define GLEW_STATIC
#include<GLFW/glfw3.h>
#include<glm/glm.hpp>
#include<glm/gtc/matrix_transform.hpp>
#include<glm/gtc/type_ptr.hpp>
#include"Shader.h"
#include<opengl/freeglut.h>
#include<SOIL.h>

void key_callback(GLFWwindow* window, int key, int scancode, int action, int mode);
const GLuint WIDTH = 800, HEIGHT = 600;

int main()
{
    glfwInit();
    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
    glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);
    glfwWindowHint(GLFW_RESIZABLE, GL_FALSE);

    GLFWwindow* window = glfwCreateWindow(WIDTH, HEIGHT, "LearnOpenGL", nullptr, nullptr);
    glfwMakeContextCurrent(window);
    glfwSetKeyCallback(window, key_callback);
    glewExperimental = GL_TRUE;
    glewInit();

    int width, height;
    glfwGetFramebufferSize(window, &width, &height);
    glViewport(0, 0, width, height);

    Shader shaderYellow("VShader.txt", "FShaderY.txt");

    GLfloat vertices[] =
    {
        -0.5f, -0.5f, 0.0f, 0.0f, 0.0f, 1.0f, 0.0f, 0.0f,
        0.5f, -0.5f, 0.0f, 0.0f, 1.0f, 0.0f, 1.0f, 0.0f,
        -0.5f, 0.5f, 0.0f, 1.0f, 0.0f, 0.0f, 0.0f, 1.0f,
        0.5f, 0.5f, 0.0f, 0.0f, 0.0f, 1.0f, 1.0f, 1.0f
    };
    GLuint indices[] =
    {
        0, 1, 2,        //用前3个顶点绘制第一个三角形
        1, 2, 3         //用后3个顶点绘制第二个三角形
    };
    GLuint VBO, EBO, VAO, texture;
    glGenVertexArrays(1, &VAO);
    glGenBuffers(1, &VBO);
    glGenBuffers(1, &EBO);
    glGenTextures(1, &texture);

    glBindVertexArray(VAO);
    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO);
    glBindTexture(GL_TEXTURE_2D, texture);


    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 8 * sizeof(GLfloat), (GLvoid*)0);
    glEnableVertexAttribArray(0);
    glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 8 * sizeof(GLfloat), (GLvoid*)(3 * sizeof(GLfloat)));
    glEnableVertexAttribArray(1);
    glVertexAttribPointer(2, 2, GL_FLOAT, GL_FALSE, 8 * sizeof(GLfloat), (GLvoid*)(6 * sizeof(GLfloat)));
    glEnableVertexAttribArray(2);

    int picWidth, picHeight;
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    unsigned char* image = SOIL_load_image("Texture/wallpaper.jpg", &picWidth, &picHeight, 0, SOIL_LOAD_RGB);
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, picWidth, picHeight, 0, GL_RGB, GL_UNSIGNED_BYTE, image);
    glGenerateMipmap(GL_TEXTURE_2D);
    SOIL_free_image_data(image);
    glBindTexture(GL_TEXTURE_2D, 0);

    glBindBuffer(GL_ARRAY_BUFFER, 0);
    glBindVertexArray(0);

    GLint transformLoc = glGetUniformLocation(shaderYellow.Program, "transform");
    while (!glfwWindowShouldClose(window))
    {
        glfwPollEvents();
        glClearColor(0.2f, 0.3f, 0.3f, 1.0f);
        glClear(GL_COLOR_BUFFER_BIT);

        glBindTexture(GL_TEXTURE_2D, texture);
        shaderYellow.Use();

        glm::mat4 model = glm::mat4(1.0f);
        glm::mat4 view = glm::mat4(1.0f);
        glm::mat4 projection = glm::mat4(1.0f);
        model = glm::rotate(model, glm::radians(-55.0f), glm::vec3(1.0f, 0.0f, 0.0f));
        view = glm::translate(view, glm::vec3(0.0f, 0.0f, -3.0f));
        projection = glm::perspective(glm::radians(45.0f), (GLfloat)WIDTH / (GLfloat)HEIGHT, 0.1f, 100.0f);
        GLint modelLoc = glGetUniformLocation(shaderYellow.Program, "model");
        GLint viewLoc = glGetUniformLocation(shaderYellow.Program, "view");
        GLint projLoc = glGetUniformLocation(shaderYellow.Program, "projection");
        glUniformMatrix4fv(modelLoc, 1, GL_FALSE, glm::value_ptr(model));
        glUniformMatrix4fv(viewLoc, 1, GL_FALSE, glm::value_ptr(view));
        glUniformMatrix4fv(projLoc, 1, GL_FALSE, glm::value_ptr(projection));

        glBindVertexArray(VAO);
        glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);

        glBindVertexArray(0);
        glfwSwapBuffers(window);
    }
    glDeleteVertexArrays(1, &VAO);
    glDeleteBuffers(1, &VBO);
    glDeleteBuffers(1, &EBO);
    glfwTerminate();
    return 0;
}

void key_callback(GLFWwindow* window, int key, int scancode, int action, int mode)
{
    if (key == GLFW_KEY_ESCAPE && action == GLFW_PRESS)
        glfwSetWindowShouldClose(window, GL_TRUE);
}

也可以通过各种调整矩阵来看下不同的效果