如何将 FBO 的文本附件绘制到默认帧缓冲区？

Question

我想使用片段着色器输出到 FBO，然后将其纹理附件绘制到默认帧缓冲区。 最终，我希望能够输出到一个 FBO，然后使用另一个着色器将其传递给另一个 FBO，依此类推。 但我认为让它在默认帧缓冲区上工作是一个很好的第一步，特别是对于调试着色器的输出。

我不确定我做错了什么，我已经设置了一个小程序来演示它。 它与 learnopengl.com 上的完整示例大致相同： https ://learnopengl.com/Getting-started/Textures

该程序绑定到 FBO，然后使用着色器在单击光标的位置绘制一个点。 如果没有点击它，它会将其绘制到默认位置。 然后它绑定到默认的帧缓冲区并使用另一个着色器（assign.frag.glsl）来绘制 FBO 的纹理附件。

无论如何，这就是我想要它做的......问题是我只是得到一个黑屏，好像FBO的纹理附件从未被写入或者它以某种方式没有被绑定到着色器中的统一sampler2D。 如果我注释掉“assignProgram.use();” 线然后我看到我用鼠标单击的点。 这让我感到惊讶，因为光标着色器的输出去了 FBO……但我在默认帧缓冲区中看到了它。 即使我注释掉“glBindTexture(GL_TEXTURE_2D, tex0);” 我仍然可以看到那个点。

这是顶点着色器（即使我将它排除在程序之外也会出现问题）：

#version 330 core
layout (location = 0) in vec3 aPos;

void main()
{
    gl_Position = vec4(aPos, 1.0);
}

光标点着色器：

#version 330 core

uniform vec2 cursor;
uniform float rdx;

out vec4 FragColor;

void main()
{

    float distance = distance(cursor, gl_FragCoord.xy) * rdx;
    distance = step(0.02, distance);
    vec3 colour = vec3(distance);

    FragColor = vec4(colour, 1.0f);
}

分配着色器：

#version 330 core

uniform sampler2D mytexture;
uniform float rdx;

out vec4 FragColor;

void main()
{
    FragColor = texture2D(mytexture, gl_FragCoord.xy);
    FragColor.a = 1.0f;
}

和 C++ 代码：

#include <glad/glad.h>
#include <GLFW/glfw3.h>

#include <learnopengl/shader_s.h>

#include <iostream>

void framebuffer_size_callback(GLFWwindow* window, int width, int height);
void processInput(GLFWwindow* window);

// settings
const unsigned int SCR_WIDTH = 800;
const unsigned int SCR_HEIGHT = 800;

int main()
{
    // glfw: initialize and configure
    glfwInit();
    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
    glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);

    // glfw window creation
    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);

    if (!gladLoadGLLoader((GLADloadproc)glfwGetProcAddress))
    {
        std::cout << "Failed to initialize GLAD" << std::endl;
        return -1;
    }

    Shader cursorProgram("default.vert.glsl", "cursor.frag.glsl"); // you can name your shader files however you like
    Shader assignProgram("default.vert.glsl", "assign.frag.glsl"); // you can name your shader files however you like

    float vertices[] = {
       1.f,  1.f, 0.0f,  // top right
        1.f, -1.f, 0.0f,  // bottom right
        -1.f, -1.f, 0.0f,  // bottom left
        -1.f,  1.f, 0.0f,   // top left 
    };

    unsigned int indices[] = {  // note that we start from 0!
        0, 1, 3,   // first triangle
        1, 2, 3    // second triangle
    };

    unsigned int VBO, VAO, EBO;
    glGenVertexArrays(1, &VAO);
    glGenBuffers(1, &VBO);
    glGenBuffers(1, &EBO); 

    // bind the Vertex Array Object first, then bind and set vertex buffer(s), and then configure vertex attributes(s).
    glBindVertexArray(VAO);

    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);

    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);

    // position attribute
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(float), (void*)0);
    glEnableVertexAttribArray(0);

    glBindVertexArray(0);

    unsigned int tex0, fbo0;

    glGenTextures(1, &tex0);
    glBindTexture(GL_TEXTURE_2D, tex0);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);

    glGenFramebuffers(1, &fbo0);
    glBindFramebuffer(GL_FRAMEBUFFER, fbo0);
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, SCR_WIDTH, SCR_HEIGHT, 0, GL_RGBA, GL_UNSIGNED_BYTE, nullptr);
    glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, tex0, 0);
    glViewport(0, 0, SCR_WIDTH, SCR_HEIGHT);
    glClearColor(0.f, 0.f, 0.f, 0.f);
    glClear(GL_COLOR_BUFFER_BIT);
    if (glCheckFramebufferStatus(GL_FRAMEBUFFER) != GL_FRAMEBUFFER_COMPLETE)
        std::cout << "uh oh :(" << std::endl;


    glBindFramebuffer(GL_FRAMEBUFFER, 0);

    float rdx = 1.f / SCR_WIDTH;

    while (!glfwWindowShouldClose(window))
    {
        // input
        processInput(window);


        // Draw to FBO
        glBindFramebuffer(GL_FRAMEBUFFER, fbo0);
        GLenum targets[1] = { GL_COLOR_ATTACHMENT0 };
        glDrawBuffers(1, &targets[0]);

        cursorProgram.use();
        cursorProgram.setFloat("rdx", rdx);
        
        if (glfwGetMouseButton(window, GLFW_MOUSE_BUTTON_LEFT) == GLFW_PRESS)
        {
            double x = 0, y = 0;
            glfwGetCursorPos(window, &x, &y);
            cursorProgram.setVec2("cursor", (float)x, SCR_HEIGHT - (float)y);
        }
        else
        {
            cursorProgram.setVec2("cursor", 0.f, 0.f);
        }

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


        // Draw fbo0 to default buffer
        glBindFramebuffer(GL_FRAMEBUFFER, 0);
        assignProgram.use();
        assignProgram.setFloat("rdx", rdx);
        assignProgram.setInt("mytexture", 0);
        glActiveTexture(GL_TEXTURE0);
        glBindTexture(GL_TEXTURE_2D, tex0);

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

        Sleep(16);

        // glfw: swap buffers and poll IO events (keys pressed/released, mouse moved etc.)
        glfwSwapBuffers(window);
        glfwPollEvents();
    }

    // optional: de-allocate all resources once they've outlived their purpose:
    glDeleteVertexArrays(1, &VAO);
    glDeleteBuffers(1, &VBO);

    // glfw: terminate, clearing all previously allocated GLFW resources.
    glfwTerminate();
    return 0;
}

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

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

Answer 1

您必须通过glTexParameter设置纹理缩小功能（ GL_TEXTURE_MIN_FILTER ）：

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);

如果您没有生成的贴图（与glGenerateMipmap ）这是一套重要GL_TEXTURE_MIN_FILTER 。 由于默认过滤器是GL_NEAREST_MIPMAP_LINEAR ，如果您没有将最小化函数更改为GL_NEAREST或GL_LINEAR ，则纹理将为“Mipmap Incomplete”。

---

您错过了在赋值着色器gl_FragCoord.xy乘以rdx ：

FragColor = texture2D(mytexture, gl_FragCoord.xy);

FragColor = texture2D(mytexture, gl_FragCoord.xy * rdx);