c++ - OpenGL 离屏渲染和使用 glfw 的隐藏窗口读取像素值

Question

我需要将 3d 模型渲染成多个 View ，并且图像尺寸比屏幕分辨率大得多，所以我无法打开这么大的窗口，也不需要打开窗口来查看渲染结果。我只想将 3d 模型渲染成许多具有不同视点的图像。

我找到这个教程，http://www.opengl-tutorial.org/intermediate-tutorials/tutorial-14-render-to-texture/ ，我尝试使用 glGetTexImage 和 glReadPixels 从 fbo 读取像素值。

问题是如果窗口没有被隐藏，就OK了。如果我使用

设置隐藏窗口

glfwWindowHint(GLFW_VISIBLE, GL_FALSE);

我只能使用 glGetTexImage 或 glReadPixels 从 fbo 读取背景颜色。并且使用 glDrawElements 绘制的模型消失了。

任何人都可以帮助我或给我举个例子吗？谢谢!

这是我读取 fbo 的代码:(如果窗口没有隐藏，它工作得很好，我的代码的其余部分与 http://www.opengl-tutorial.org/intermediate-tutorials/tutorial-14-render-to-texture/ 相同)

这是我的代码:

if (!glfwInit())
{
    fprintf(stderr, "Failed to initialize GLFW\n");
    getchar();
    return -1;
}

glfwWindowHint(GLFW_SAMPLES, 1);
glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 4);
glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 5);
glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE); // To make MacOS happy; should not be needed
glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);
glfwWindowHint(GLFW_VISIBLE, GL_TRUE);

// Open a window and create its OpenGL context
int windowWidth = 4000;
int windowHeight = 3000;

window = glfwCreateWindow(windowWidth, windowHeight, "Tutorial 14 - Render To Texture", NULL, NULL);
if (window == NULL) {
    fprintf(stderr, "Failed to open GLFW window. If you have an Intel GPU, they are not 3.3 compatible. Try the 2.1 version of the tutorials.\n");
    getchar();
    glfwTerminate();
    return -1;
}
glfwMakeContextCurrent(window);

// Initialize GLEW
glewExperimental = true; // Needed for core profile
if (glewInit() != GLEW_OK) {
    fprintf(stderr, "Failed to initialize GLEW\n");
    getchar();
    glfwTerminate();
    return -1;
}

// Ensure we can capture the escape key being pressed below
glfwSetInputMode(window, GLFW_STICKY_KEYS, GL_TRUE);
// Hide the mouse and enable unlimited mouvement
glfwSetInputMode(window, GLFW_CURSOR, GLFW_CURSOR_DISABLED);

// Set the mouse at the center of the screen
glfwPollEvents();
glfwSetCursorPos(window, windowWidth / 2, windowHeight / 2);

// Dark blue background
glClearColor(0.0f, 0.0f, 0.0f, 0.0f);

// Enable depth test
glEnable(GL_DEPTH_TEST);
// Accept fragment if it closer to the camera than the former one
glDepthFunc(GL_LESS);

// Cull triangles which normal is not towards the camera
glEnable(GL_CULL_FACE);

GLuint VertexArrayID;
glGenVertexArrays(1, &VertexArrayID);
glBindVertexArray(VertexArrayID);

// Create and compile our GLSL program from the shaders
GLuint programID = LoadShaders("StandardShadingRTT.vertexshader", "StandardShadingRTT.fragmentshader");

// Get a handle for our "MVP" uniform
GLuint MatrixID = glGetUniformLocation(programID, "MVP");
GLuint ViewMatrixID = glGetUniformLocation(programID, "V");
GLuint ModelMatrixID = glGetUniformLocation(programID, "M");

// Load the texture
GLuint Texture = loadDDS("uvmap.DDS");

// Get a handle for our "myTextureSampler" uniform
GLuint TextureID = glGetUniformLocation(programID, "myTextureSampler");

// Read our .obj file
std::vector<glm::vec3> vertices;
std::vector<glm::vec2> uvs;
std::vector<glm::vec3> normals;
bool res = loadOBJ("suzanne.obj", vertices, uvs, normals);

std::vector<unsigned short> indices;
std::vector<glm::vec3> indexed_vertices;
std::vector<glm::vec2> indexed_uvs;
std::vector<glm::vec3> indexed_normals;
indexVBO(vertices, uvs, normals, indices, indexed_vertices, indexed_uvs, indexed_normals);

// Load it into a VBO
GLuint vertexbuffer;
glGenBuffers(1, &vertexbuffer);
glBindBuffer(GL_ARRAY_BUFFER, vertexbuffer);
glBufferData(GL_ARRAY_BUFFER, indexed_vertices.size() * sizeof(glm::vec3), &indexed_vertices[0], GL_STATIC_DRAW);
GLuint uvbuffer;
glGenBuffers(1, &uvbuffer);
glBindBuffer(GL_ARRAY_BUFFER, uvbuffer);
glBufferData(GL_ARRAY_BUFFER, indexed_uvs.size() * sizeof(glm::vec2), &indexed_uvs[0], GL_STATIC_DRAW);
GLuint normalbuffer;
glGenBuffers(1, &normalbuffer);
glBindBuffer(GL_ARRAY_BUFFER, normalbuffer);
glBufferData(GL_ARRAY_BUFFER, indexed_normals.size() * sizeof(glm::vec3), &indexed_normals[0], GL_STATIC_DRAW);
// Generate a buffer for the indices as well
GLuint elementbuffer;
glGenBuffers(1, &elementbuffer);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, elementbuffer);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, indices.size() * sizeof(unsigned short), &indices[0], GL_STATIC_DRAW);
// Get a handle for our "LightPosition" uniform
glUseProgram(programID);
GLuint LightID = glGetUniformLocation(programID, "LightPosition_worldspace");


// ---------------------------------------------
// Render to Texture - specific code begins here
// ---------------------------------------------

// The framebuffer, which regroups 0, 1, or more textures, and 0 or 1 depth buffer.
GLuint FramebufferName = 0;
glGenFramebuffers(1, &FramebufferName);
glBindFramebuffer(GL_FRAMEBUFFER, FramebufferName);

// The texture we're going to render to
GLuint renderedTexture;
glGenTextures(1, &renderedTexture);

// "Bind" the newly created texture : all future texture functions will modify this texture
glBindTexture(GL_TEXTURE_2D, renderedTexture);

// Give an empty image to OpenGL ( the last "0" means "empty" )
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, windowWidth, windowHeight, 0, GL_RGB, GL_UNSIGNED_BYTE, 0);

// Poor filtering
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);

// Set "renderedTexture" as our colour attachement #0
glFramebufferTexture(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, renderedTexture, 0);
// Set the list of draw buffers.
GLenum DrawBuffers[1] = { GL_COLOR_ATTACHMENT0 };
glDrawBuffers(1, DrawBuffers); // "1" is the size of DrawBuffers
// Always check that our framebuffer is ok
if (glCheckFramebufferStatus(GL_FRAMEBUFFER) != GL_FRAMEBUFFER_COMPLETE)
    return false;

// Render to our framebuffer
glBindFramebuffer(GL_FRAMEBUFFER, FramebufferName);
glViewport(0, 0, windowWidth, windowHeight); // Render on the whole framebuffer, complete from the lower left corner to the upper right
// Clear the screen
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
// Use our shader
glUseProgram(programID);
// Compute the MVP matrix from keyboard and mouse input
computeMatricesFromInputs();
glm::mat4 ProjectionMatrix = getProjectionMatrix();
glm::mat4 ViewMatrix = getViewMatrix();
glm::mat4 ModelMatrix = glm::mat4(1.0);
glm::mat4 MVP = ProjectionMatrix * ViewMatrix * ModelMatrix;

// Send our transformation to the currently bound shader, 
// in the "MVP" uniform
glUniformMatrix4fv(MatrixID, 1, GL_FALSE, &MVP[0][0]);
glUniformMatrix4fv(ModelMatrixID, 1, GL_FALSE, &ModelMatrix[0][0]);
glUniformMatrix4fv(ViewMatrixID, 1, GL_FALSE, &ViewMatrix[0][0]);

glm::vec3 lightPos = glm::vec3(4, 4, 4);
glUniform3f(LightID, lightPos.x, lightPos.y, lightPos.z);

// Bind our texture in Texture Unit 0
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, Texture);
// Set our "myTextureSampler" sampler to user Texture Unit 0
glUniform1i(TextureID, 0);

// 1rst attribute buffer : vertices
glEnableVertexAttribArray(0);
glBindBuffer(GL_ARRAY_BUFFER, vertexbuffer);
glVertexAttribPointer(
    0,                  // attribute
    3,                  // size
    GL_FLOAT,           // type
    GL_FALSE,           // normalized?
    0,                  // stride
    (void*)0            // array buffer offset
    );

// 2nd attribute buffer : UVs
glEnableVertexAttribArray(1);
glBindBuffer(GL_ARRAY_BUFFER, uvbuffer);
glVertexAttribPointer(
    1,                                // attribute
    2,                                // size
    GL_FLOAT,                         // type
    GL_FALSE,                         // normalized?
    0,                                // stride
    (void*)0                          // array buffer offset
    );

// 3rd attribute buffer : normals
glEnableVertexAttribArray(2);
glBindBuffer(GL_ARRAY_BUFFER, normalbuffer);
glVertexAttribPointer(
    2,                                // attribute
    3,                                // size
    GL_FLOAT,                         // type
    GL_FALSE,                         // normalized?
    0,                                // stride
    (void*)0                          // array buffer offset
    );

// Index buffer
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, elementbuffer);

// Draw the triangles !
glDrawElements(
    GL_TRIANGLES,      // mode
    indices.size(),    // count
    GL_UNSIGNED_SHORT, // type
    (void*)0           // element array buffer offset
    );

unsigned char *image = new unsigned char[windowWidth * windowHeight * 3];
glBindFramebuffer(GL_FRAMEBUFFER, FramebufferName);
glReadBuffer(GL_COLOR_ATTACHMENT0);
glReadPixels(0, 0, windowWidth, windowHeight, GL_RGB, GL_UNSIGNED_BYTE, image);

cv::Mat img;
img = cv::Mat::zeros(windowHeight, windowWidth, CV_8UC3);
for (int i = 0; i < windowHeight; i ++) {
    for (int j = 0; j < windowWidth; j ++) {
        int index = (i * windowWidth + j) * 3;
        cv::Vec3b px;
        px.val[0] = image[index + 0];
        px.val[1] = image[index + 1];
        px.val[2] = image[index + 2];
        img.at<cv::Vec3b>(i, j) = px;
    }
}

cv::imwrite("test.png", img);

当窗口可见时效果很好

glfwWindowHint(GLFW_VISIBLE, GL_FALSE);

但是如果我把它设置为GL_TRUE，我就不能再得到结果了!

谁能帮帮我？非常感谢!

Answer 1

你检查过这个例子了吗？它使用帧缓冲区

https://www.opengl.org/wiki/Framebuffer_Object_Examples#Quick_example.2C_render_to_texture_.282D.29

下面的代码(以防将来链接断开):

   //RGBA8 2D texture, 24 bit depth texture, 256x256
   glGenTextures(1, &color_tex);
   glBindTexture(GL_TEXTURE_2D, color_tex);
   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_NEAREST);
   glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
   //NULL means reserve texture memory, but texels are undefined
   glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, 256, 256, 0, GL_BGRA, GL_UNSIGNED_BYTE, NULL);
   //-------------------------
   glGenFramebuffersEXT(1, &fb);
   glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fb);
   //Attach 2D texture to this FBO
   glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, GL_TEXTURE_2D, color_tex, 0);
   //-------------------------
   glGenRenderbuffersEXT(1, &depth_rb);
   glBindRenderbufferEXT(GL_RENDERBUFFER_EXT, depth_rb);
   glRenderbufferStorageEXT(GL_RENDERBUFFER_EXT, GL_DEPTH_COMPONENT24, 256, 256);
   //-------------------------
   //Attach depth buffer to FBO
   glFramebufferRenderbufferEXT(GL_FRAMEBUFFER_EXT, GL_DEPTH_ATTACHMENT_EXT, GL_RENDERBUFFER_EXT, depth_rb);
   //-------------------------
   //Does the GPU support current FBO configuration?
   GLenum status;
   status = glCheckFramebufferStatusEXT(GL_FRAMEBUFFER_EXT);
   switch(status)
   {
      case GL_FRAMEBUFFER_COMPLETE_EXT:
      cout<<"good";
       default:
      HANDLE_THE_ERROR;
   }
   //-------------------------
   //and now you can render to GL_TEXTURE_2D
   glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fb);
   glClearColor(0.0, 0.0, 0.0, 0.0);
   glClearDepth(1.0f);
   glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
   //-------------------------
   glViewport(0, 0, 256, 256);
   glMatrixMode(GL_PROJECTION);
   glLoadIdentity();
   glOrtho(0.0, 256.0, 0.0, 256.0, -1.0, 1.0); 
   glMatrixMode(GL_MODELVIEW);
   glLoadIdentity();
   //-------------------------
   glDisable(GL_TEXTURE_2D);
   glDisable(GL_BLEND);
   glEnable(GL_DEPTH_TEST);
   //-------------------------
   //**************************
   //RenderATriangle, {0.0, 0.0}, {256.0, 0.0}, {256.0, 256.0}
   //Read http://www.opengl.org/wiki/VBO_-_just_examples
   RenderATriangle();
   //-------------------------
   GLubyte pixels[4*4*4];
   glReadPixels(0, 0, 4, 4, GL_BGRA, GL_UNSIGNED_BYTE, pixels);
   //pixels 0, 1, 2 should be white
   //pixel 4 should be black
   //----------------
   //Bind 0, which means render to back buffer
   glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);

代码清理:

   //Delete resources
   glDeleteTextures(1, &color_tex);
   glDeleteRenderbuffersEXT(1, &depth_rb);
   //Bind 0, which means render to back buffer, as a result, fb is unbound
   glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);
   glDeleteFramebuffersEXT(1, &fb);