1.纹理采样：根据纹理坐标获取像素值

2.纹理环绕：当纹理坐标超出默认范围时纹理的重复方式，OpenGL提供了以下几种方式

对纹理环绕方式进行设置：

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_MIRRORED_REPEAT); 

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_MIRRORED_REPEAT);

float borderColor[] = { 1.0f, 1.0f, 0.0f, 1.0f }; 

glTexParameterfv(GL_TEXTURE_2D, GL_TEXTURE_BORDER_COLOR, borderColor);

3.纹理过滤：纹理坐标不依赖于分辨率，可以是任意浮点值，纹理像素映射到纹理坐标称为过滤。

    两种纹理过滤方式：

        （1） GL_NEAREST：选取中心点距离纹理坐标最近的像素值作为纹理的颜色

        （2） GL_LINEAR： 它会基于纹理坐标附近的纹理像素，计算出一个插值，近似出这些纹理像素之间的颜色

    设置纹理被放大或缩小时的过滤方式

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST); 

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);

4.多级渐远纹理： 它简单来说就是一系列的纹理图像，后一个纹理图像是前一个的二分之一。多级渐远纹理背后的理念很简单：距观察者的距离超过一定的阈值，OpenGL会使用不同的多级渐远纹理，即最适合物体的距离的那个（仅用于纹理被缩小时）

    多级渐远纹理的过滤方式：

设置多级渐远纹理的过滤方式：

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR); 
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);

5.加载纹理：使用文件图像加载库stb_image.h，在源文件中引入stb_image.h并定义 STB_IMAGE_IMPLEMENTATION即可使用

#define STB_IMAGE_IMPLEMENTATION 
#include "stb_image.h"
main(){int width, height, nrChannels;unsigned char *data = stbi_load("container.jpg", &width, &height, &nrChannels, 0);//释放图像内存stbi_image_free(data);
}

6.创建纹理对象并配置：

       （1）创建纹理对象，使用ID进行引用unsigned int texture;glGenTextures(1,&texture);（2）绑定纹理到target，之后的任何纹理指令都会配置当前绑定的纹理（使用多个纹理时，需要配置完一个纹理之后，再绑定别的纹理进行配置）glBindTexture(GL_TEXTURE_2D,texture);（3）设置纹理参数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);（4）翻转y轴stbi_set_flip_vertically_on_load(true);（5）使用图像数据生成纹理glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, width, height, 0, GL_RGB,  GL_UNSIGNED_BYTE, data);（6）自动生成多级渐远纹理glGenerateMipmap(GL_TEXTURE_2D);

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

• 第一个参数指定了纹理目标(Target)。设置为GL_TEXTURE_2D意味着会生成与当前绑定的纹理对象在同一个目标上的纹理（任何绑定到GL_TEXTURE_1D和GL_TEXTURE_3D的纹理不会受到影响）。

• 第二个参数为纹理指定多级渐远纹理的级别，如果你希望单独手动设置每个多级渐远纹理的级别的话。这里我们填0，也就是基本级别。

• 第三个参数告诉OpenGL我们希望把纹理储存为何种格式。我们的图像只有RGB值，因此我们也把纹理储存为RGB值。

• 第四个和第五个参数设置最终的纹理的宽度和高度。我们之前加载图像的时候储存了它们，所以我们使用对应的变量。

• 下个参数应该总是被设为0（历史遗留的问题）。

• 第七第八个参数定义了源图的格式和数据类型。我们使用RGB值加载这个图像，并把它们储存为char(byte)数组，我们将会传入对应值。

• 最后一个参数是真正的图像数据

7.使用纹理

    （1）主程序中：在绘制前将纹理绑定至target

glBindTexture(GL_TEXTURE_2D, texture);
glBindVertexArray(VAO); 
glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);

（2）shader中：

顶点着色器将纹理坐标传出给片元着色器

layout (location = 2) in vec2 aTexCoord;
out vec2 TexCoord;
void main() { gl_Position = vec4(aPos, 1.0); ourColor = aColor; TexCoord = aTexCoord; 
}

片元着色器中根据纹理target和纹理坐标进行采样

in vec2 TexCoord;
uniform sampler2D ourTexture; 
void main() { FragColor = texture(ourTexture, TexCoord); 
}

8.使用纹理单元：纹理在shader中定义类型为uniform，通过uniform1i我们可以将纹理绑定至不同纹理单元，在绘制前激活不同的纹理单元并绑定至target即可使用，默认激活纹理单元是GL_TEXTURE0

//设置纹理单元
unsigned int tex1Location  = glGetUniformLocation(shaderProgram,"ourTexture1");
glUniform1i(tex1Location,0);
unsigned int tex2Location  = glGetUniformLocation(shaderProgram,"ourTexture2");
glUniform1i(tex2Location,1);//激活纹理单元
glActiveTexture(GL_TEXTURE0); 
//绑定至target
glBindTexture(GL_TEXTURE_2D,texture);
glActiveTexture(GL_TEXTURE1);
glBindTexture(GL_TEXTURE_2D,texture2);

9.使用两幅纹理进行混合：

#define STB_IMAGE_IMPLEMENTATION
#include "stb_image.h"
#include <glad.h>
#include <glfw3.h>
#include <iostream>
extern void framebuffer_size_callback(GLFWwindow* window, int width, int height);
extern void processInput(GLFWwindow *window);
extern unsigned int SCR_WIDTH = 800;
extern unsigned int SCR_HEIGHT = 600;
const char * vertexShaderSource_TextureTest ="#version 330 core\n""layout(location = 0)in vec3 aPos;\n""layout(location = 2)in vec2 aTexCoord;\n ""layout(location = 1)in vec3 aColor;\n""out vec3 myColor;\n""out vec2 TexCoord;\n""void main(){\n""gl_Position = vec4(aPos,1.0f);\n""TexCoord = vec2(aTexCoord.x,aTexCoord.y);\n""myColor = aColor;\n""}\n";
const char * fragementShaderSource_TextureTest ="#version 330 core\n""out vec4 FragColor;\n""in vec2 TexCoord;\n""in vec3 myColor;\n""uniform sampler2D ourTexture1;\n""uniform sampler2D ourTexture2;\n""void main(){\n""FragColor =  mix(texture(ourTexture1,TexCoord),texture(ourTexture2,TexCoord),0.2);\n""}\n";
int DrawATexture(){//初始化窗口和OpenGLglfwInit();glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);GLFWwindow* window = glfwCreateWindow(SCR_WIDTH, SCR_HEIGHT, "LearnOpenGL",  NULL, NULL);if (window == NULL){std::cout << "Failed to create GLFW window" << std::endl;glfwTerminate();return -1;}glfwMakeContextCurrent(window);glfwSetFramebufferSizeCallback(window, framebuffer_size_callback);// glad: load all OpenGL function pointers// ---------------------------------------if (!gladLoadGLLoader((GLADloadproc)glfwGetProcAddress)){std::cout << "Failed to initialize GLAD" << std::endl;return -1;}//顶点着色器//创建着色器对象unsigned int vertexShader;vertexShader = glCreateShader(GL_VERTEX_SHADER);//着色器填充源代码glShaderSource(vertexShader,1,&vertexShaderSource_TextureTest,NULL);//编译着色器glCompileShader(vertexShader);//检测着色器是否编译成功，并查看错误信息int success;char infoLog[512];glGetShaderiv(vertexShader,GL_COMPILE_STATUS,&success);   //检测是否编译成功if(!success){glGetShaderInfoLog(vertexShader, 512, NULL, infoLog);std::cout << "ERROR::SHADER::VERTEX::COMPILATION_FAILED\n" << infoLog <<  std::endl;}//片元着色器unsigned int fragmentShader;fragmentShader = glCreateShader(GL_FRAGMENT_SHADER);glShaderSource(fragmentShader,1,&fragementShaderSource_TextureTest,NULL);glCompileShader(fragmentShader);//检测着色器是否编译成功，并查看错误信息int success_f;char infoLog_f[512];glGetShaderiv(fragmentShader,GL_COMPILE_STATUS,&success_f);      //检测是否编译成功if(!success){glGetShaderInfoLog(fragmentShader, 512, NULL, infoLog_f);std::cout << "ERROR::SHADER::VERTEX::COMPILATION_FAILED\n" << infoLog <<  std::endl;}//创建程序对象int shaderProgram;shaderProgram = glCreateProgram();//为程序对象分配着色器对象glAttachShader(shaderProgram,vertexShader);glAttachShader(shaderProgram,fragmentShader);//链接程序对象glLinkProgram(shaderProgram);//检测链接着色器程序是否失败glGetProgramiv(shaderProgram,GL_LINK_STATUS,&success);if(!success){glGetProgramInfoLog(shaderProgram,512,NULL,infoLog);std::cout<<"ERROR::PROGRAM::LINK_FAILED"<<infoLog<<std::endl;}//激活程序对象glUseProgram(shaderProgram);//删除着色器对象glDeleteShader(vertexShader);glDeleteShader(fragmentShader);//创建纹理对象unsigned int texture,texture2;glGenTextures(1,&texture);glGenTextures(1,&texture2);//绑定纹理glBindTexture(GL_TEXTURE_2D,texture);//设置纹理参数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);//翻转y轴stbi_set_flip_vertically_on_load(true);//使用stb_image加载图像int width,height,nrChannels;unsigned char * data = stbi_load("rabbit.jpg",&width,&height,&nrChannels,0);if(!data){std::cout<<"图片读取失败"<<std::endl;}//使用图像数据生成纹理glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, width, height, 0, GL_RGB,  GL_UNSIGNED_BYTE, data);//自动生成多级渐远纹理glGenerateMipmap(GL_TEXTURE_2D);glBindTexture(GL_TEXTURE_2D,texture2);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 * data2 = stbi_load("3.png",&width,&height,&nrChannels,0);if(!data2){std::cout<<"图片2读取失败"<<std::endl;}glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA,  GL_UNSIGNED_BYTE, data2);glGenerateMipmap(GL_TEXTURE_2D);//释放图像内存stbi_image_free(data);stbi_image_free(data2);//顶点属性数组float vertices[] = {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};//索引数组unsigned int indices[] = {0,1,3,1,2,3};//使用VAO,VBO,EBOunsigned int VAO,VBO,EBO;glGenVertexArrays(1,&VAO);glGenBuffers(1,&VBO);glGenBuffers(1,&EBO);glBindVertexArray(VAO);glBindBuffer(GL_ARRAY_BUFFER,VBO);glBindBuffer(GL_ELEMENT_ARRAY_BUFFER,EBO);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(float),(void *)0);glVertexAttribPointer(1,3,GL_FLOAT,GL_FALSE,8*sizeof(float),(void  *)(3*sizeof(float)));glVertexAttribPointer(2,2,GL_FLOAT,GL_FALSE,8*sizeof(float),(void  *)(6*sizeof(float)));//为纹理设置纹理单元unsigned int tex1Location  = glGetUniformLocation(shaderProgram,"ourTexture1");glUniform1i(tex1Location,0);unsigned int tex2Location  = glGetUniformLocation(shaderProgram,"ourTexture2");glUniform1i(tex2Location,1);glEnableVertexAttribArray(0);glEnableVertexAttribArray(1);glEnableVertexAttribArray(2);glBindBuffer(GL_ARRAY_BUFFER,0);glBindVertexArray(0);while (!glfwWindowShouldClose(window)){processInput(window);glClearColor(0.2f, 0.3f, 0.3f, 1.0f);glClear(GL_COLOR_BUFFER_BIT);glActiveTexture(GL_TEXTURE0); // 在绑定纹理之前先激活纹理单元glBindTexture(GL_TEXTURE_2D,texture);glActiveTexture(GL_TEXTURE1);glBindTexture(GL_TEXTURE_2D,texture2);glUseProgram(shaderProgram);//使用VAO进行绘制glBindVertexArray(VAO);glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT,0);glfwSwapBuffers(window);glfwPollEvents();}glDeleteVertexArrays(1, &VAO);glDeleteBuffers(1, &VBO);glDeleteBuffers(1,&EBO);return 0;
}