带有 FBO 附件的 glreadpixels

Posted 2023-02-22

【中文标题】带有 FBO 附件的 glreadpixels

【英文标题】：glreadpixels with FBO attachment

【发布时间】：2014-06-17 20:27:13

【问题描述】：

好的，所以我有这个带有深度缓冲区和 4 个颜色附件的 FBO。我想在我持有光标的地方读取 dept 缓冲区和来自第四个颜色附件的值。我似乎没有管理任何一个。 我写了这个类：

class Picker3D
    public:
bool useAtachment;
GLenum  atachment;
float PickerLastSignature;
point3f PickerLastPos;

Picker3D( bool useAtachment = true , GLenum atachment = GL_COLOR_ATTACHMENT3 ) : useAtachment(useAtachment) , atachment(atachment) 



void SetObjectSignature( float signature , GLSL_Prog prog )
    glUniform1f( glGetUniformLocation( prog.PR , "Signature" ) , signature );


float getSignature( float x , float y , float height )
    glGetError();
    if( useAtachment )
        glReadBuffer( atachment );

    cout << " Error message1 : " << glGetError() << endl ;

    float *pixels = new float[4];
    glReadPixels( x , height - y , 1 , 1 , GL_RGB , GL_FLOAT , pixels );
    PickerLastSignature = pixels[1];

    cout << " Error message2 : " << glGetError() << endl ;

    return PickerLastSignature;


point3f get3DPosition( float x , float y , float height )
    //glReadBuffer( GL_DEPTH_ATTACHMENT );

    double depth;
    glReadPixels( x , y , 1 , 1 , GL_DEPTH_COMPONENT , GL_FLOAT , &depth );

    cout << depth << endl;

    int viewport[4];
    float *modelview;
    double projection[16];
    double ModelView[16];

    modelview = mat4f::GetTopMatrix().returnTransposedMatrix().returnFloatArray();
    for( int i = 0 ; i < 16 ; i++ )
        ModelView[i] = modelview[i];
    

    glGetDoublev(GL_PROJECTION_MATRIX, projection);
    glGetIntegerv(GL_VIEWPORT, viewport);

    double x1 , y1 , z1;

    gluUnProject( x , height - y , depth , ModelView  , projection , viewport , &x1 , &y1 , &z1 );

    PickerLastPos = point3f( x1 , y1 , z1 );
    return PickerLastPos;

;

我就这样使用它

//在主循环中

BindFrameBuffer(); 
drawStuff(); 
Picker.get3DPosition( x , y , height );
Picker.getSignature( x , y , height );
UnbindBuffer();
drawSecondPass(); 
endDrawSecondPass();
// check Values of PickerLastPos and PickerLastSignature 
SwapBuffers();  

虽然它在屏幕上工作，但深度没有改变，getSignature 在与 glReadBuffer 的行给出错误 1282（我认为无效操作）。

主循环是：

               glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT );

            /// deferred pass

            DefFBO->drawFBO();
            GeometryShader.UseNow();

            glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT );

            // load the view matrix and set the camera
            cam.camera_set();
            cam.camera_get( GeometryShader );

            // load identity for model matrix
            mat4f::LoadIdentity();

            /// Geometry Drawing

            mat4f::PushMatrix();

                mat4f::Translate( 0 , 0 , -10 );
                Picker.SetObjectSignature( 1.0f , GeometryShader );
                T90.obj_rend( GeometryShader );
                Picker.SetObjectSignature( 2.0f , GeometryShader );
                Town.obj_rend( GeometryShader );

            mat4f::PopMatrix();

            Picker.get3DPosition( Window.Mouse->mouse_x , Window.Mouse->mouse_y , Window.Height );

            /// setting the deferred read from buffer part

            // last read from  framebuffer
            Picker.getSignature( Window.Mouse->mouse_x , Window.Mouse->mouse_y , Window.Height );

            // unbind FBO
            DefFBO->readFBO();

            // use fragment shader, for lights and for post processing
            FragmentShader.UseNow();

            // send windows width and height and bind multiple textures
            DefFBO->send_to_GLSL( FragmentShader.PR );

            // send view matrix to the shader
            cam.camera_get( FragmentShader );

            glDepthMask(GL_FALSE);
            glEnable(GL_DEPTH_TEST);

            glUniform3f( glGetUniformLocation( FragmentShader.PR , "CameraPosition" ) , cam.x , cam.y , cam.z );

            /// final draw
            DefFBO->sendGlobalLight( FragmentShader , point3f( 0 , 1 , 0 ) , point3f( 0.3 , 0.3 , 0.3 ) , point3f( 0.9 , 0.9 , 0.9 ) );

            cout << Picker.PickerLastPos << endl;

            DefFBO->end_read();

            glEnable(GL_DEPTH_TEST);
            glDepthMask(GL_TRUE);

【问题讨论】：

只是想指出我在您注释掉的一行中注意到的一点：//glReadBuffer( GL_DEPTH_ATTACHMENT )。这将不起作用，使用该命令设置的读取缓冲区是颜色缓冲区，因此实际上只有 FRONT/BACK/LEFT/RIGHT（或这些的某种组合）和GL_COLOR_ATTACHMENTi 是有效值。但是，缺少太多代码无法评论其他任何内容；实际的 FBO 设置（例如，附在哪里）甚至没有列出。 GL_INVALID_OPERATION 让我相信你绑定了错误的 FBO，或者它实际上没有附加到 GL_COLOR_ATTACHMENT3。

【参考方案1】：

正如@Andon 指出的那样，我似乎必须绑定 fbo 以从中获取数据。我正在与 GL_DRAW_FRAMEBUFFER 绑定。

【讨论】：

是的，这就是将GL_FRAMEBUFFER 分成GL_DRAW_FRAMEBUFFER 和GL_READ_FRAMEBUFFER 的想法。它允许您独立控制绘制到哪个帧缓冲区以及从哪个帧缓冲区读取。能够做到这一点对于读取和写入帧缓冲区的操作至关重要，尤其是glBlitFramebuffer()。但它也适用于其他读取操作，例如glReadPixels()。