OpenGL：使用FBO为渲染对象并从GPU取出存图

OpenGL 使用FBO为渲染对象并从GPU取出存图的代码

#include "gl/glew.h"
#include "gl/glut.h"
#include <fstream>
#define isize 512
const char* vertexShaderSource = "#version 460 \n"
"layout (location = 0) in vec3 aPos;\n"
"void main()\n"
"{\n"
"   gl_Position = vec4(aPos.x, aPos.y, aPos.z, 1.0);\n"
"}\0";
const char* fragmentShaderSource = "#version 460 \n"
"out vec4 FragColor;\n"
"void main()\n"
"{\n"
"   FragColor = vec4(0.0f, 1.0f, 1.0f, 1.0f);\n"
"}\n\0";

void saveToBMP24(void* packBuffer, const std::string& img);
int main(int argc, char** argv)
{
    argc;argv;

    
    //初始化窗口以及渲染上下文


    //创建viewport
    int iViewportIndex = 0;
    float fv4Viewport[4] = {0.0f,0.0f,512.0f,512.0f};
    glViewportIndexedfv(iViewportIndex, fv4Viewport);
    //创建FBO绑定的纹理
    GLuint texName;
    glGenTextures(1, &texName);
    glActiveTexture(GL_TEXTURE0);
    glBindTexture(GL_TEXTURE_2D, texName);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_BASE_LEVEL, 0);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAX_LEVEL, 0);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    glTexImage2D(GL_TEXTURE_2D, GLint(0), GL_RGBA, isize, isize, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);
    
    //创建FOB
    GLuint FramebufferName;
    glGenFramebuffers(1, &FramebufferName);
    glBindFramebuffer(GL_FRAMEBUFFER, FramebufferName);
    glFramebufferTexture(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, texName, 0);
    
    //三角形的三个顶点数据
    float vertices[] = {
        -0.5f, -0.5f, 0.0f, // left  
        0.5f, -0.5f, 0.0f, // right 
        0.0f, 0.5f, 0.0f  // top   
    };

    //创建VAO VBO 
    unsigned int VBO, VAO;
    glGenVertexArrays(1, &VAO);
    glGenBuffers(1, &VBO);
    glBindVertexArray(VAO);

    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(float), (void*)0);
    glEnableVertexAttribArray(0);
    
    //初始化program，shader
    int vertexShader = glCreateShader(GL_VERTEX_SHADER);
    glShaderSource(vertexShader, 1, &vertexShaderSource, NULL);
    glCompileShader(vertexShader);

    int fragmentShader = glCreateShader(GL_FRAGMENT_SHADER);
    glShaderSource(fragmentShader, 1, &fragmentShaderSource, NULL);
    glCompileShader(fragmentShader);

    int shaderProgram = glCreateProgram();
    glAttachShader(shaderProgram, vertexShader);
    glAttachShader(shaderProgram, fragmentShader);
    glLinkProgram(shaderProgram);

    //清背景
    glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
    glClear(GL_COLOR_BUFFER_BIT);
    //设置绘制buffer，数组形式设置
    GLuint a[] = { GL_COLOR_ATTACHMENT0 };
    glDrawBuffers(1, a);
    glUseProgram(shaderProgram);
    //绑定VAO
    glBindVertexArray(VAO);
    //绘制三角形
    glDrawArrays(GL_TRIANGLES, 0, 3);
    
    //备份
    int iOldPixelPackAlignment(1);
    glGetIntegerv(GL_PACK_ALIGNMENT, &iOldPixelPackAlignment);
    //设置新的对齐
    glPixelStorei(GL_PACK_ALIGNMENT, 1);
    //绑定fbo，读取数据
    glBindFramebuffer(GL_READ_FRAMEBUFFER, FramebufferName);
    glReadBuffer(GL_COLOR_ATTACHMENT0);
    void* pImage = new char[isize * isize * 4];
    glReadPixels(0, 0, isize, isize,
        GL_RGBA, GL_UNSIGNED_BYTE, pImage);
    glBindFramebuffer(GL_READ_FRAMEBUFFER, 0);
    //存图 bmp格式
    saveToBMP24(pImage, "/usr/sss1.bmp");
  
    return 0;
}



void saveToBMP24(void* packBuffer, const std::string& img)
{
    using namespace std;
    //读入一个bmp格式文件，创建头用，todo：尺寸和位数对应
    fstream in("/usr/123.bmp", ios_base::in | ios_base::binary);
    if (!in)
    {
        return;
    }
    char bmpHeader[54] = { 0 };
    in.read(bmpHeader, sizeof(bmpHeader));

    char* pBuffer = new char[isize * isize *3];
    in.read(pBuffer, isize * isize * 3);
    in.close();
    
    fstream out(img.c_str(), ios_base::out | ios_base::binary | ios_base::trunc);
    if (!out)
    {
        in.close();
        delete[] pBuffer;
        return;
    }
    
    //RGB数据拷贝，A丢弃
    for (size_t i = 0; i < isize*isize; i++)
    {
        int num = i * 4;
        int numI = i * 3;
        pBuffer[numI] = ((char*)packBuffer)[num + 2];
        pBuffer[numI+1] = ((char*)packBuffer)[num + 1];
        pBuffer[numI+2] = ((char*)packBuffer)[num ];
    }

    *((int*)&bmpHeader[0x12]) = isize;
    *((int*)&bmpHeader[0x16]) = isize;
    
    //bmp对齐
    int totals = (isize) * 3;
    while (totals % 4 != 0)
        ++totals;
    totals = totals * (isize);
    out.write(bmpHeader, sizeof(bmpHeader));
    
    out.write(pBuffer, totals);
    out.close();
    delete[]pBuffer;
}