I want to do offscreen rendering, but have no luck
These are the codes, I have no any idea why the FBO(frame buffer object)
always give me a blank image

What I want to do is, the texture after render will write to the FBO
after that, I could transfer the data of fbo into QImage by toImage()

.hpp
Qt Code: 
  1. #ifndef OFFSCREENEXP_HPP
  2. #define OFFSCREENEXP_HPP
  3.  
  4. #include <QOpenGLFunctions>
  5. #include <QWindow>
  6.  
  7. class QImage;
  8. class QOpenGLContext;
  9.  
  10. class offScreenExp : public QWindow, protected QOpenGLFunctions
  11. {
  12. public:
  13.     explicit offScreenExp(QWindow *parent = 0);
  14.  
  15.     QImage render();
  16.  
  17. private:
  18.     QOpenGLContext *context_;
  19. };
  20.  
  21. #endif // OFFSCREENEXP_HPP
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.cpp
Qt Code: 
  1. #include <QImage>
  2. #include <QOpenGLBuffer>
  3. #include <QOpenGLContext>
  4. #include <QOpenGLFramebufferObject>
  5. #include <QOpenGLShaderProgram>
  6. #include <QString>
  7. #include <QWidget>
  8.  
  9. #include <QDebug>
  10.  
  11. #include "offScreenExp.hpp"
  12.  
  13. offScreenExp::offScreenExp(QWindow *parent) :
  14.     QWindow(parent),
  15.     context_(nullptr)
  16. {
  17.     setSurfaceType(QWindow::OpenGLSurface);
  18.     setFormat(QSurfaceFormat());
  19.     create();
  20. }
  21.  
  22. QImage offScreenExp::render()
  23. {
  24.     //create the context
  25.     if (!context_) {
  26.         context_ = new QOpenGLContext(this);
  27.         QSurfaceFormat format;
  28.         context_->setFormat(format);
  29.  
  30.         if (!context_->create())
  31.             qFatal("Cannot create the requested OpenGL context!");
  32.     }
  33.  
  34.     context_->makeCurrent(this);
  35.     initializeOpenGLFunctions();
  36.  
  37.     //load image
  38.     QString const prefix("/Users/Qt/program/experiment_apps_and_libs/openGLTest/simpleGPGPU/");
  39.     QImage img(prefix + "images/emili.jpg");
  40.     if(img.isNull()){
  41.       qFatal("image is null");
  42.     }    
  43.  
  44.     //if(glCheckFramebufferStatus(fbo.handle()) != GL_FRAMEBUFFER_COMPLETE){
  45.     //    qDebug()<<"frame buffer error";
  46.     //}
  47.     qDebug()<<"has opengl fbo : "<<QOpenGLFramebufferObject::hasOpenGLFramebufferObjects();
  48.  
  49.     //use two triangles two cover whole screen
  50.     std::vector<GLfloat> const vertex{
  51.         -1.0f,  1.0f, 0.0f, 1.0f,
  52.         1.0f,  1.0f, 0.0f, 1.0f,
  53.         -1.0f, -1.0f, 0.0f, 1.0f,
  54.         1.0f, 1.0f, 0.0f, 1.0f,
  55.         1.0f, -1.0f, 0.0f, 1.0f,
  56.         -1.0f, -1.0f, 0.0f, 1.0f,
  57.     };
  58.  
  59.     //initialize vbo
  60.     QOpenGLBuffer buffer(QOpenGLBuffer::VertexBuffer);
  61.     buffer.create();
  62.     buffer.setUsagePattern(QOpenGLBuffer::StaticDraw);
  63.     buffer.bind();
  64.     buffer.allocate(&vertex[0], vertex.size() * sizeof(GLfloat) );
  65.     buffer.release();
  66.  
  67.     //create texture
  68.     GLuint rendered_texture;
  69.     glGenTextures(1, &rendered_texture);
  70.  
  71.     // "Bind" the newly created texture : all future texture functions will modify this texture
  72.     glBindTexture(GL_TEXTURE_2D, rendered_texture);
  73.  
  74.     //naive solution, better encapsulate the format in a function
  75.     if(img.format() == QImage::Format_Indexed8){
  76.         glTexImage2D(GL_TEXTURE_2D, 0, GL_RED, img.width(), img.height(), 0, GL_RED, GL_UNSIGNED_BYTE, img.scanLine(0));
  77.     }else if(img.format() == QImage::Format_RGB888){
  78.         glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, img.width(), img.height(), 0, GL_RGB, GL_UNSIGNED_BYTE, img.scanLine(0));
  79.     }else{
  80.         QImage temp = img.convertToFormat(QImage::Format_RGB888);
  81.         glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, img.width(), img.height(), 0, GL_RGB, GL_UNSIGNED_BYTE, temp.scanLine(0));
  82.     }
  83.  
  84.     glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
  85.     glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
  86.     glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
  87.     glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
  88.  
  89.     glBindTexture(GL_TEXTURE_2D, 0);
  90.  
  91.     //compile and link program
  92.     QOpenGLShaderProgram program;
  93.     if(!program.addShaderFromSourceCode(QOpenGLShader::Vertex,
  94.                                         "attribute highp vec4 qt_Vertex;"
  95.                                         "varying highp vec2 qt_TexCoord0;"
  96.  
  97.                                         "void main(void)"
  98.                                         "{"
  99.                                         "   gl_Position =  qt_Vertex;"
  100.                                         "   qt_TexCoord0 = (qt_Vertex.xy + vec2(1.0)) * 0.5;"
  101.                                         "}")){
  102.         qDebug()<<"QOpenGLShader::Vertex error : " + program.log();
  103.     }
  104.  
  105.     // Compile fragment shader
  106.     if (!program.addShaderFromSourceCode(QOpenGLShader::Fragment,
  107.                                          "uniform sampler2D source;"
  108.                                          "varying highp vec2 qt_TexCoord0;"
  109.  
  110.                                          "vec3 toGray(vec3 rgb)"
  111.                                          "{"
  112.                                          " return vec3(rgb.r * 0.299 + rgb.g * 0.587 + rgb.b * 0.114);"
  113.                                          "}"
  114.  
  115.                                          "void main(void)"
  116.                                          "{"
  117.                                          "vec3 gray = toGray(texture2D(source, qt_TexCoord0).rgb);"
  118.                                          "gl_FragColor = vec4(gray, 0.0);"
  119.                                          "}"
  120.                                          )){
  121.         qDebug()<<"QOpenGLShader::Fragment error : " + program.log();
  122.     }
  123.  
  124.     // Link shader pipeline
  125.     if (!program.link()){
  126.         qDebug()<<"link error : " + program.log();
  127.     }
  128.  
  129.     //render the texture as usual
  130.     glClearColor(0, 0, 0, 1);
  131.     glClear(GL_COLOR_BUFFER_BIT);
  132.     glViewport(0, 0, img.width(), img.height());
  133.  
  134.     program.bind();
  135.     buffer.bind();
  136.     program.enableAttributeArray("qt_Vertex");
  137.     program.setAttributeBuffer( "qt_Vertex", GL_FLOAT, 0, 4);
  138.  
  139.     glActiveTexture(GL_TEXTURE0);
  140.     glBindTexture(GL_TEXTURE_2D, rendered_texture);
  141.  
  142.     //bind and create fbo
  143.     QOpenGLFramebufferObject fbo(img.size());
  144.     qDebug()<<"bind success? : "<<fbo.bind();
  145.  
  146.     glDrawArrays(GL_TRIANGLES, 0, buffer.size());
  147.  
  148.     program.disableAttributeArray("qt_Vertex");
  149.     program.release();
  150.     glActiveTexture(0);
  151.     glBindTexture(GL_TEXTURE_2D, 0);
  152.     buffer.release();
  153.  
  154.     QImage result = fbo.toImage();
  155.     fbo.release();
  156.     context_->doneCurrent();
  157.  
  158.     return result;
  159. }
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