基于C++的OpenGL 03 之纹理

1. 概述

本文基于C++语言，描述OpenGL的纹理

前置知识可参考：

• 基于C++的OpenGL 02 之着色器 - 当时明月在曾照彩云归 - 博客园 (cnblogs.com)

笔者这里不过多描述每个名词、函数和细节，更详细的文档可以参考：

• 纹理 - LearnOpenGL CN (learnopengl-cn.github.io)

2. 纹理使用流程

参考：纹理 - LearnOpenGL CN (learnopengl-cn.github.io)

OpenGL中纹理使用流程大致如下：

• 加载图片数据
• 创建纹理对象
• 绑定纹理对象
• 使用图片数据生成纹理
• 设置纹理坐标
• 在顶点着色器中传递纹理
• 在片段着色器中采用纹理
• （绘制时）激活纹理并绑定纹理

3. 具体流程

3.1 加载图片数据

使用stb_image.h实现图片数据的读取

参考官方说明：nothings/stb: stb single-file public domain libraries for C/C++ (github.com)

使用以下方式导入：

#define STB_IMAGE_IMPLEMENTATION
#include "stb_image.h"

加载图片数据：

int width, height, nrChannels;
unsigned char *data = stbi_load("container.jpg", &width, &height, &nrChannels, 0);

3.2 创建纹理对象

纹理对象也是使用ID进行引用：

unsigned int texture;
glGenTextures(1, &texture);

3.3 绑定纹理对象

绑定纹理对象，进行之后的纹理配置：

glBindTexture(GL_TEXTURE_2D, texture);

进行纹理配置：

// 为当前绑定的纹理对象设置环绕、过滤方式
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);

3.4 使用图片数据生成纹理

通过glTexImage2D()生成纹理：

glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, width, height, 0, GL_RGB, GL_UNSIGNED_BYTE, data);
glGenerateMipmap(GL_TEXTURE_2D);

3.5 设置纹理坐标

纹理坐标：

float vertices[] = {
//     ---- 位置 ----       ---- 颜色 ----     - 纹理坐标 -
     0.5f,  0.5f, 0.0f,   1.0f, 0.0f, 0.0f,   1.0f, 1.0f,   // 右上
     0.5f, -0.5f, 0.0f,   0.0f, 1.0f, 0.0f,   1.0f, 0.0f,   // 右下
    -0.5f, -0.5f, 0.0f,   0.0f, 0.0f, 1.0f,   0.0f, 0.0f,   // 左下
    -0.5f,  0.5f, 0.0f,   1.0f, 1.0f, 0.0f,   0.0f, 1.0f    // 左上
};

现在内存中的坐标格式：

指定纹理坐标属性：

glVertexAttribPointer(2, 2, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)(6 * sizeof(float)));
glEnableVertexAttribArray(2);

3.6 在顶点着色器中传递纹理坐标

在顶点着色器中编写GLSL实现数据传递：

#version 330 core
layout (location = 0) in vec3 aPos;
layout (location = 1) in vec3 aColor;
layout (location = 2) in vec2 aTexCoord;

out vec3 ourColor;
out vec2 TexCoord;

void main()
{
    gl_Position = vec4(aPos, 1.0);
    ourColor = aColor;
    TexCoord = aTexCoord;
}

3.7 在片段着色器中采用纹理

在片段着色器中接收纹理坐标与纹理：

#version 330 core
out vec4 FragColor;

in vec3 ourColor;
in vec2 TexCoord;

uniform sampler2D ourTexture;

void main()
{
    FragColor = texture(ourTexture, TexCoord);
}

使用GLSL内置的texture函数来采样纹理的颜色

3.8 激活纹理并绑定纹理（绘制时）

激活纹理单元并绑定纹理数据：

glActiveTexture(GL_TEXTURE0); // 在绑定纹理之前先激活纹理单元
glBindTexture(GL_TEXTURE_2D, texture);

4. 代码总结

一个简单的纹理绘制流程完整代码如下：

#include <glad/glad.h>
#include <GLFW/glfw3.h>
#include <iostream>
#include <math.h>
#include "Shader.hpp"
#define STB_IMAGE_IMPLEMENTATION
#include "stb_image.h"

void framebuffer_size_callback(GLFWwindow *window, int width, int height);
void process_input(GLFWwindow *window);
unsigned int *renderInit();
void render(unsigned int shaderProgram, unsigned int VAO, unsigned int texture);
bool checkCompile(unsigned int shader);
bool checkProgram(unsigned int shaderProgram);

int main()
{
    glfwInit();
    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
    GLFWwindow *window = glfwCreateWindow(800, 600, "texture", nullptr, nullptr);

    if (window == nullptr)
    {
        std::cout << "Faild to create window" << std::endl;
        glfwTerminate();
    }
    glfwMakeContextCurrent(window);

    if (!gladLoadGLLoader((GLADloadproc)glfwGetProcAddress))
    {
        std::cout << "Faild to initialize glad" << std::endl;
        return -1;
    }
    glad_glViewport(0, 0, 800, 600);
    glfwSetFramebufferSizeCallback(window, framebuffer_size_callback);

    unsigned int *arr = renderInit();
    while (!glfwWindowShouldClose(window))
    {
        process_input(window);

        // render

        std::cout << arr[0] << " " << arr[1] << " " << arr[2] << " " << arr[3] << std::endl;
        render(arr[0], arr[1], arr[3]);

        glfwSwapBuffers(window);
        glfwPollEvents();
    }

    glDeleteProgram(arr[0]);
    glDeleteVertexArrays(1, &arr[1]);
    glDeleteBuffers(1, &arr[2]);

    glfwTerminate();
    return 0;
}

void framebuffer_size_callback(GLFWwindow *window, int width, int height)
{
    glViewport(0, 0, width, height);
}

void process_input(GLFWwindow *window)
{
    if (glfwGetKey(window, GLFW_KEY_ESCAPE) == GLFW_PRESS)
    {
        glfwSetWindowShouldClose(window, true);
    }
}

unsigned int *renderInit()
{
    unsigned int VAO;
    glGenVertexArrays(1, &VAO);
    glBindVertexArray(VAO);

    unsigned int texture;
    glGenTextures(1, &texture);
    glBindTexture(GL_TEXTURE_2D, texture);
    // 为当前绑定的纹理对象设置环绕、过滤方式
    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);
    // 加载并生成纹理
    int width, height, nrChannels;
    unsigned char *data = stbi_load("../container.jpg", &width, &height, &nrChannels, 0);
    if (data)
    {
        glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, width, height, 0, GL_RGB, GL_UNSIGNED_BYTE, data);
        glGenerateMipmap(GL_TEXTURE_2D);
    }
    else
    {
        std::cout << "Failed to load texture" << std::endl;
    }
    stbi_image_free(data);

    float vertices[] = {
        //     ---- 位置 ----       ---- 颜色 ----     - 纹理坐标 -
        0.5f, 0.5f, 0.0f, 1.0f, 0.0f, 0.0f, 1.0f, 1.0f,   // 右上
        0.5f, -0.5f, 0.0f, 0.0f, 1.0f, 0.0f, 1.0f, 0.0f,  // 右下
        -0.5f, -0.5f, 0.0f, 0.0f, 0.0f, 1.0f, 0.0f, 0.0f, // 左下
        -0.5f, 0.5f, 0.0f, 1.0f, 1.0f, 0.0f, 0.0f, 1.0f   // 左上
    };
    unsigned int VBO;
    glGenBuffers(1, &VBO);
    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);

    unsigned int indices[] = {
        0, 1, 3, // first triangle
        1, 2, 3  // second triangle
    };
    unsigned int EBO;
    glGenBuffers(1, &EBO);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);

    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void *)0);
    glEnableVertexAttribArray(0);
    glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void *)(3 * sizeof(float)));
    glEnableVertexAttribArray(1);
    glVertexAttribPointer(2, 2, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void *)(6 * sizeof(float)));
    glEnableVertexAttribArray(2);

    Shader shaderProgram = Shader("../test.vs", "../test.fs");
    shaderProgram.use();

    glUniform1i(glGetUniformLocation(shaderProgram.ID, "texture1"), 0); // 手动设置
    return new unsigned int[4]{shaderProgram.ID, VAO, VBO, texture};
}
void render(unsigned int shaderProgram, unsigned int VAO, unsigned int texture)
{
    glClearColor(0.2, 0.3, 0.3, 1.0);
    glClear(GL_COLOR_BUFFER_BIT);

    glActiveTexture(GL_TEXTURE0);
    glBindTexture(GL_TEXTURE_2D, texture);

    glUseProgram(shaderProgram);

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

• Shader.hpp见 基于C++的OpenGL 02 之着色器 - 当时明月在曾照彩云归 - 博客园 (cnblogs.com)

• 图片container下载自：https://learnopengl-cn.github.io/img/01/06/container.jpg

顶点着色器test.vs：

#version 330 core
layout (location = 0) in vec3 aPos;
layout (location = 1) in vec3 aColor;
layout (location = 2) in vec2 aTexCoord;

out vec3 ourColor;
out vec2 TexCoord;

void main()
{
    gl_Position = vec4(aPos, 1.0);
    ourColor = aColor;
    TexCoord = aTexCoord;
}

片段着色器test.fs：

#version 330 core
out vec4 FragColor;

in vec3 ourColor;
in vec2 TexCoord;

uniform sampler2D texture1;

void main()
{
    FragColor = texture(texture1, TexCoord);
}

编译代码并运行：

5. 参考资料

[1]纹理 - LearnOpenGL CN (learnopengl-cn.github.io)

[2]nothings/stb: stb single-file public domain libraries for C/C++ (github.com)

[3]OpenGL学习笔记（五）纹理 - 知乎 (zhihu.com)