如何在单独的视口和默认的框架缓冲区中使用框架缓冲区？

Question

我想了解Framebuffers是如何工作的。AFAIK，AFAIK缓冲器可以将渲染存储为纹理，具有存储深度信息的渲染缓冲区和存储颜色信息的颜色缓冲区。这里有一个默认的框架缓冲区，然后是我可能为存储单独呈现而创建的任何其他框架缓冲区。现在，我希望在整个屏幕上呈现默认的框架缓冲区，而在一个小的侧视图端口上，应该有一个由我的新框架缓冲区存储的呈现。在下面的源代码中没有发生这种情况。

代码：

#include <GL/glew.h>  
#include <GLFW/glfw3.h> 
#include <stdio.h>

#define WINDOW_WIDTH 640
#define WINDOW_HEIGHT 480

int main() {
    GLFWwindow *window = NULL;
    GLuint vao;
    GLuint vbo;

    GLfloat points[] = { 0.0f, 0.5f, 0.0f, 0.5f, -0.5f, 0.0f, -0.5f, -0.5f, 0.0f };

    const char *vertex_shader = "#version 410\n"
                                "in vec3 vp;"
                                "void main () {"
                                "  gl_Position = vec4(vp, 1.0);"
                                "}";

    const char *fragment_shader = "#version 410\n"
                                "out vec4 frag_colour;"
                                "void main () {"
                                "  frag_colour = vec4(0.5, 0.0, 0.5, 1.0);"
                                "}";

    GLuint vert_shader, frag_shader;
    GLuint shader_programme;

    if ( !glfwInit() ) {
        fprintf( stderr, "ERROR: could not start GLFW3\n" );
        return 1;
    }

    glfwWindowHint( GLFW_CONTEXT_VERSION_MAJOR, 4 );
    glfwWindowHint( GLFW_CONTEXT_VERSION_MINOR, 6 );
    glfwWindowHint( GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE );
    glfwWindowHint( GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE );

    window = glfwCreateWindow( WINDOW_WIDTH, WINDOW_HEIGHT, "Hello Triangle", NULL, NULL );
    if ( !window ) {
        fprintf( stderr, "ERROR: could not open window with GLFW3\n" );
        glfwTerminate();
        return 1;
    }
    glfwMakeContextCurrent( window );

    glewExperimental = GL_TRUE;
    glewInit();

    glEnable( GL_DEPTH_TEST ); /* enable depth-testing */
    glDepthFunc( GL_LESS );

    glGenBuffers( 1, &vbo );
    glBindBuffer( GL_ARRAY_BUFFER, vbo );
    glBufferData( GL_ARRAY_BUFFER, 9 * sizeof( GLfloat ), points, GL_STATIC_DRAW );
    glGenVertexArrays( 1, &vao );
    glBindVertexArray( vao );
    glEnableVertexAttribArray( 0 );
    glBindBuffer( GL_ARRAY_BUFFER, vbo );
    glVertexAttribPointer( 0, 3, GL_FLOAT, GL_FALSE, 0, NULL );

    vert_shader = glCreateShader( GL_VERTEX_SHADER );
    glShaderSource( vert_shader, 1, &vertex_shader, NULL );
    glCompileShader( vert_shader );
    frag_shader = glCreateShader( GL_FRAGMENT_SHADER );
    glShaderSource( frag_shader, 1, &fragment_shader, NULL );
    glCompileShader( frag_shader );
    shader_programme = glCreateProgram();
    glAttachShader( shader_programme, frag_shader );
    glAttachShader( shader_programme, vert_shader );
    glLinkProgram( shader_programme );


    GLuint g_fb;
    GLuint g_fb_tex;
    glGenFramebuffers( 1, &g_fb );
    glGenTextures( 1, &g_fb_tex );
    glBindTexture( GL_TEXTURE_2D, g_fb_tex );
    glTexImage2D( GL_TEXTURE_2D, 0, GL_RGBA, WINDOW_WIDTH/2, WINDOW_HEIGHT/2, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL );
    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 );
    glBindFramebuffer( GL_FRAMEBUFFER, g_fb );
    glFramebufferTexture2D( GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, g_fb_tex, 0 );
    GLuint rb = 0;
    glGenRenderbuffers( 1, &rb );
    glBindRenderbuffer( GL_RENDERBUFFER, rb );
    glRenderbufferStorage( GL_RENDERBUFFER, GL_DEPTH_COMPONENT, WINDOW_WIDTH/2, WINDOW_HEIGHT/2 );
    glFramebufferRenderbuffer( GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, rb );
    GLenum draw_bufs[] = { GL_COLOR_ATTACHMENT0 };
    glDrawBuffers( 1, draw_bufs );
    glBindFramebuffer( GL_FRAMEBUFFER, 0 );



    while ( !glfwWindowShouldClose( window ) ) {
        glBindFramebuffer(GL_FRAMEBUFFER, g_fb);
        glViewport(0, 0, WINDOW_WIDTH/2, WINDOW_HEIGHT/2);
        glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );
        glUseProgram( shader_programme );
        glBindVertexArray( vao );
        glDrawArrays( GL_TRIANGLES, 0, 3 );

        glBindFramebuffer(GL_FRAMEBUFFER, 0);
        glViewport(0, 0, WINDOW_WIDTH, WINDOW_HEIGHT);
        glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );
        glUseProgram( shader_programme );
        glBindVertexArray( vao );
        glDrawArrays( GL_TRIANGLES, 0, 3 );

        glfwPollEvents();
        glfwSwapBuffers( window );
    }

    glfwTerminate();
    return 0;
}

在这里，只呈现最后提到的缓冲区。我该怎么纠正呢？此外，我正在写一个场景类和渲染类在我的项目。是否应该在每个呈现器对象中存储框架缓冲区，还是应该将任何呈现器实例的所有帧缓冲区存储在场景类中？主要是，存储多个fbos需要多少存储/计算？

编辑：我在答案中提到了新的纹理，但我仍然只得到最后渲染的结果。请指出错误。我是否需要为新的帧缓冲区使用新的着色器？如果是的话，为什么不能重复使用早期的着色器呢？

代码：

#include <GL/glew.h>  
#include <GLFW/glfw3.h> 
#include <stdio.h>

#define WINDOW_WIDTH 640
#define WINDOW_HEIGHT 480

int main() {
    GLFWwindow *window = NULL;
    GLuint vao;
    GLuint vbo;

    GLfloat points[] = { 0.0f, 0.5f, 0.0f, 0.5f, -0.5f, 0.0f, -0.5f, -0.5f, 0.0f };

    const char *vertex_shader = "#version 410\n"
                                "in vec3 vp;"
                                "void main () {"
                                "  gl_Position = vec4(vp, 1.0);"
                                "}";

    const char *fragment_shader = "#version 410\n"
                                "out vec4 frag_colour;"
                                "void main () {"
                                "  frag_colour = vec4(0.5, 0.0, 0.5, 1.0);"
                                "}";

    GLuint vert_shader, frag_shader;
    GLuint shader_programme;

    if ( !glfwInit() ) {
        fprintf( stderr, "ERROR: could not start GLFW3\n" );
        return 1;
    }

    glfwWindowHint( GLFW_CONTEXT_VERSION_MAJOR, 4 );
    glfwWindowHint( GLFW_CONTEXT_VERSION_MINOR, 6 );
    glfwWindowHint( GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE );
    glfwWindowHint( GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE );

    window = glfwCreateWindow( WINDOW_WIDTH, WINDOW_HEIGHT, "Hello Triangle", NULL, NULL );
    if ( !window ) {
        fprintf( stderr, "ERROR: could not open window with GLFW3\n" );
        glfwTerminate();
        return 1;
    }
    glfwMakeContextCurrent( window );

    glewExperimental = GL_TRUE;
    glewInit();

    glEnable( GL_DEPTH_TEST ); /* enable depth-testing */
    glDepthFunc( GL_LESS );

    glGenBuffers( 1, &vbo );
    glBindBuffer( GL_ARRAY_BUFFER, vbo );
    glBufferData( GL_ARRAY_BUFFER, 9 * sizeof( GLfloat ), points, GL_STATIC_DRAW );
    glGenVertexArrays( 1, &vao );
    glBindVertexArray( vao );
    glEnableVertexAttribArray( 0 );
    glBindBuffer( GL_ARRAY_BUFFER, vbo );
    glVertexAttribPointer( 0, 3, GL_FLOAT, GL_FALSE, 0, NULL );

    vert_shader = glCreateShader( GL_VERTEX_SHADER );
    glShaderSource( vert_shader, 1, &vertex_shader, NULL );
    glCompileShader( vert_shader );
    frag_shader = glCreateShader( GL_FRAGMENT_SHADER );
    glShaderSource( frag_shader, 1, &fragment_shader, NULL );
    glCompileShader( frag_shader );
    shader_programme = glCreateProgram();
    glAttachShader( shader_programme, frag_shader );
    glAttachShader( shader_programme, vert_shader );
    glLinkProgram( shader_programme );


    GLuint g_fb;
    GLuint g_fb_tex;
    glGenFramebuffers( 1, &g_fb );
    glGenTextures( 1, &g_fb_tex );
    glBindTexture( GL_TEXTURE_2D, g_fb_tex );
    glTexImage2D( GL_TEXTURE_2D, 0, GL_RGBA, WINDOW_WIDTH/2, WINDOW_HEIGHT/2, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL );
    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 );
    glBindFramebuffer( GL_FRAMEBUFFER, g_fb );
    glFramebufferTexture2D( GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, g_fb_tex, 0 );
    GLuint rb = 0;
    glGenRenderbuffers( 1, &rb );
    glBindRenderbuffer( GL_RENDERBUFFER, rb );
    glRenderbufferStorage( GL_RENDERBUFFER, GL_DEPTH_COMPONENT, WINDOW_WIDTH/2, WINDOW_HEIGHT/2 );
    glFramebufferRenderbuffer( GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, rb );
    GLenum draw_bufs[] = { GL_COLOR_ATTACHMENT0 };
    glDrawBuffers( 1, draw_bufs );
    glBindFramebuffer( GL_FRAMEBUFFER, 0 );



    while ( !glfwWindowShouldClose( window ) ) {
        glBindFramebuffer(GL_FRAMEBUFFER, g_fb);
        glViewport(0, 0, WINDOW_WIDTH/2, WINDOW_HEIGHT/2);
        glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );
        glUseProgram( shader_programme );
        glBindVertexArray( vao );
        glDrawArrays( GL_TRIANGLES, 0, 3 );

        glBindFramebuffer(GL_FRAMEBUFFER, 0);
        glViewport(0, 0, WINDOW_WIDTH, WINDOW_HEIGHT);
        glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );
        glUseProgram( shader_programme );
        glBindVertexArray( vao );
        glDrawArrays( GL_TRIANGLES, 0, 3 );


        GLuint m_vbo, m_vao;
        GLfloat points[] = { 
            -0.5f,  0.5f, 0.0f, 
             0.5f,  0.5f, 0.0f, 
            -0.5f, -0.5f, 0.0f,
            -0.5f, -0.5f, 0.0f,
             0.5f,  0.5f, 0.0f,
             0.5f, -0.5f, 0.0f
        };

        glGenBuffers(1, &m_vbo);
        glBindBuffer(GL_ARRAY_BUFFER, m_vbo);
        glBufferData(GL_ARRAY_BUFFER, 12 * sizeof(GLfloat), points, GL_STATIC_DRAW);
        glGenVertexArrays(1, &m_vao);
        glBindVertexArray(m_vao);
        glEnableVertexAttribArray(0);
        glBindBuffer(GL_ARRAY_BUFFER, m_vbo);
        glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, NULL);
        glBindTexture(GL_TEXTURE_2D, g_fb_tex);

        glViewport(0, 0, WINDOW_WIDTH/2, WINDOW_HEIGHT/2);
        glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );
        glUseProgram( shader_programme );
        glBindVertexArray( m_vao );
        glDrawArrays( GL_TRIANGLES, 0, 6 );

        glfwPollEvents();
        glfwSwapBuffers( window );
    }

    glfwTerminate();
    return 0;
}

Answer 1

呈现循环的第一个块呈现给g_fb。结果是g_fb_tex。如果您想在窗口(= framebuffer 0)的“一个小视口”上显示此纹理，则应该呈现一个大小适当的矩形，并使用从g_fb_tex中获取的阴影。