c++ - 离屏渲染到帧缓冲区

Question

我有一个简单的场景，它渲染了一个网格、一个平面和一个立方体。我也有两个不同的着色器。一个是平面着色器，它以随机颜色渲染对象，另一个是噪声着色器，它渲染具有噪声效果的对象。我希望能够:

通过噪声着色器渲染平面和立方体，使用平面着色器渲染网格。

一旦用户单击场景，我想仅使用平面着色器将整个场景渲染到(屏幕外)渲染缓冲区并打印通过 glReadPixels 获取的单击颜色.

继续使用两个着色器渲染场景。

基本上我想实现颜色选择。不过，它似乎不适用于立方体和飞机。像素值始终是来自噪声着色器的灰度值。

这是我的绘图程序的样子:

void Draw(const bool offscreen = false)
{
    glClearColor(0.2f, 0.3f, 0.3f, 1.0f);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glm::mat4 projection = glm::perspective(glm::radians(camera.zoom), (float)viewport_width / (float)viewport_height, 0.1f, 100.0f);
    glm::mat4 view = camera.get_view_matrix();

    if (offscreen)
    {
        flat_shader.use();
        flat_shader.set_mat4("projection", projection);
        flat_shader.set_mat4("view", view);
        grid.Draw(&flat_shader);
        box.Draw_offscreen(&flat_shader);
        plane.Draw_offscreen(&flat_shader);
    }
    else
    {
        noise_shader.use();
        noise_shader.set_mat4("projection", projection);
        noise_shader.set_mat4("view", view);
        noise_shader.set_float("iTime", delta_time);
        plane.Draw(&noise_shader);
        box.Draw(&noise_shader);

        flat_shader.use();
        flat_shader.set_mat4("projection", projection);
        flat_shader.set_mat4("view", view);
        grid.Draw(&flat_shader);
    }

    glfwSwapBuffers(window);
}

所以如果屏幕外是 false场景看起来像这样(正常渲染):



这就是 offscreen 时的场景。是 true :



这就是我创建 fbo 的方式:

void init_offscreen_buffer()
{
    glGenFramebuffers(1, &fbo_off);
    glGenRenderbuffers(1, &render_buf);
    glBindRenderbuffer(GL_RENDERBUFFER, render_buf);
    glRenderbufferStorage(GL_RENDERBUFFER, GL_RGBA, viewport_width, viewport_height);
    glBindFramebuffer(GL_DRAW_FRAMEBUFFER, fbo_off);
    glFramebufferRenderbuffer(GL_DRAW_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, render_buf);
    // I also checked for FRAMEBUFFER_COMPLETE
    glBindFramebuffer(GL_DRAW_FRAMEBUFFER, 0);
}

现在当用户点击我运行的场景时 pick_color_id在点击的像素上打印颜色。

void pick_color_id(double x, double y)
{
    glBindFramebuffer(GL_DRAW_FRAMEBUFFER, fbo_off);

    Draw(true);

    GLubyte pixel_color[4];
    glReadBuffer(GL_COLOR_ATTACHMENT0);
    glReadPixels(x, 800 - y - 1, 1, 1, GL_RGBA, GL_UNSIGNED_BYTE, pixel_color);
    cout << "---------------------------------------------------------" << endl;
    cout << "Mouse click position:  " << x << "; " << y << endl;
    cout << "Target pixel color:    " << (unsigned int)pixel_color[0] << ";" << (unsigned int)pixel_color[1] << ";" << (unsigned int)pixel_color[2] << endl;
    cout << "---------------------------------------------------------" << endl;
    glBindFramebuffer(GL_DRAW_FRAMEBUFFER, 0);
}

正如我在绑定(bind)缓冲区后看到的那样，它应该是渲染目标，它应该包含平面和立方体的平面颜色。实际上，它总是像在噪声着色器中一样打印灰度颜色。

我认为我的 fbo 设置或使用有问题(可能两者都有)。我错过了什么？

Answer 1

glReadPixels从帧缓冲区读取日期，因此帧缓冲区绑定(bind)的目标必须是 GL_READ_FRAMEBUFFER不是 GL_DRAW_FRAMEBUFFER :

glBindFramebuffer(GL_DRAW_FRAMEBUFFER, fbo_off);

Draw(true);

GLubyte pixel_color[4];

glBindFramebuffer(GL_READ_FRAMEBUFFER, fbo_off);
glReadBuffer(GL_COLOR_ATTACHMENT0);
glReadPixels(x, 800 - y - 1, 1, 1, GL_RGBA, GL_UNSIGNED_BYTE, pixel_color);

分别

glBindFramebuffer(GL_FRAMEBUFFER, fbo_off);

Draw(true);

GLubyte pixel_color[4];

glReadBuffer(GL_COLOR_ATTACHMENT0);
glReadPixels(x, 800 - y - 1, 1, 1, GL_RGBA, GL_UNSIGNED_BYTE, pixel_color);

我建议启用 Debug Output用于查找 OpenGL 错误。例如:

#include <iostream>

void GLAPIENTRY DebugCallback( 
    unsigned int  source,
    unsigned int  type,
    unsigned int  id,
    unsigned int  severity, 
    int           length,
    const char   *message,
    const void   *userParam )
{
   std::cout << message << std::endl;
}

void init_opengl_debug() {
    glDebugMessageCallback(&DebugCallback, nullptr );
    glDebugMessageControl(GL_DONT_CARE, GL_DONT_CARE, GL_DONT_CARE, 0, nullptr, GL_TRUE);
    glEnable(GL_DEBUG_OUTPUT);
    glEnable(GL_DEBUG_OUTPUT_SYNCHRONOUS);
}