Hi all,
i'm trying to capture an image from a QGLWidget's back buffer (so, no redraw must be seen from user) into a QImage having a transparent background, but the alpha value always return 255. Here's what i did (till now):

I have reimplemented QGLWidget to myGLWidget:

Qt Code: 
  1. myGLWidget::myGLWidget
  2. {
  3.     setAutoBufferSwap(false);
  4.     takingSnapshot = false;  // member variable. When taking snapshot turns true
  5. }
  6.  
  7. void myGLWidget::initializeGL()
  8. {
  9.     glClearColor(1., 1., 1., 0.);
  10. }
  11.  
  12. void myGLWidget::resizeGL(int ww, int hh)
  13. {
  14.     glViewport(0, 0, (GLint)ww, (GLint)hh);
  15. }
  16.  
  17. void myGLWidget::paintGL()
  18. {
  19.     qDebug() << "paintGL";
  20.  
  21.     glPushAttrib(GL_TRANSFORM_BIT);
  22.  
  23.     glMatrixMode(GL_PROJECTION);
  24.     glPushMatrix();
  25.     glLoadIdentity();
  26.     glMatrixMode(GL_MODELVIEW);
  27.     glPushMatrix();
  28.     glLoadIdentity();
  29.     glOrtho(-1., 1., -1., 1., -1., 1.);
  30.  
  31.  
  32.     /* Clear screen */
  33.     if (takingSnapshot) {
  34.         glEnable(GL_BLEND);
  35.         glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
  36.         glClearColor(0., 0., 1., 0.0f);
  37.         glClear(GL_COLOR_BUFFER_BIT);
  38.         glDisable(GL_BLEND);
  39.     } else {
  40.         glClearColor(1., 1., 0., 1.);
  41.         glClear(GL_COLOR_BUFFER_BIT);
  42.     }
  43.  
  44.  
  45.     /* Begin Draw */
  46.     glColor4f(1., 0., 0., 1.);
  47.     glLineWidth(10);
  48.  
  49.     glBegin(GL_LINES);
  50.     glVertex2f(-0.3, 0.0);
  51.     glVertex2f(0.3, 0.0);
  52.     glEnd();
  53.  
  54.     glEnable(GL_BLEND);
  55.     glColor4f(0., 1., 0., 0.4);
  56.     glRotatef(90., 0., 0., 1.);
  57.     glBegin(GL_LINES);
  58.     glVertex2f(-0.3, 0.0);
  59.     glVertex2f(0.3, 0.0);
  60.     glEnd();
  61.  
  62.     glPopMatrix();
  63.     glMatrixMode(GL_PROJECTION);
  64.     glPopMatrix();
  65.  
  66.     glPopAttrib();
  67.     glDisable(GL_BLEND);
  68.  
  69.     if (!takingSnapshot) {
  70.         swapBuffers();
  71.     } else {
  72.         qDebug() << "Finished drawing after snapshot\n";
  73.     }
  74. }
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And here is my mainWindow ( MainWindow : public QMainWindow):

Qt Code: 
  1. MainWindow::MainWindow(QWidget *parent)
  2.     : QMainWindow(parent)
  3. {
  4.     mainFrame = new QFrame(this);
  5.     setCentralWidget(mainFrame);
  6.  
  7.     mainLay = new QVBoxLayout(mainFrame);
  8.     mainFrame->setLayout(mainLay);
  9.  
  10.     glw = new myGLWidget(mainFrame);
  11.     snap = new QPushButton("Snap", mainFrame);
  12.     label = new QLabel(mainFrame);
  13.  
  14.     mainLay->addWidget(glw);
  15.     mainLay->addWidget(snap);
  16.     mainLay->addWidget(label);
  17.  
  18.     mainFrame->show();
  19.     glw->show();
  20.     snap->show();
  21.     label->show();
  22.  
  23.     QObject::connect(snap,SIGNAL(clicked()),this,SLOT(takeImage()));
  24. }
  25.  
  26. static QImage createQImage(const unsigned char *image2, int ww, int hh)
  27. {
  28.             /* Do some stuff here with "image2" */
  29.             /* and return the final image */
  30. }
  31.  
  32. void MainWindow::takeImage()
  33. {
  34.     int ww = glw->width();
  35.     int hh = glw->height();
  36.     QImage finalImage;
  37.     GLubyte *image = (GLubyte *)malloc(ww * hh * sizeof(GLubyte) * 4);
  38.  
  39.     glw->takingSnapshot = true;
  40.     glPushAttrib(GL_PIXEL_MODE_BIT | GL_COLOR_BUFFER_BIT);  // readBuffer + drawBuffer
  41.     glReadBuffer(GL_BACK);
  42.     glDrawBuffer(GL_BACK);
  43.     qDebug() << "takeImage, will call updateGL";
  44.  
  45.     glw->updateGL();
  46.     glReadPixels(0,0,ww,hh,GL_RGBA,GL_UNSIGNED_BYTE,image);
  47.  
  48.     finalImage = createQImage(image, ww, hh);
  49.  
  50.     if (!finalImage.isNull()) {
  51.         QPixmap pix;
  52.         pix.convertFromImage(finalImage);
  53.         label->setPixmap(pix);
  54.     }
  55.     glPopAttrib();
  56.     glw->takingSnapshot = false;
  57.     if (image) free(image);
  58. }
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In function "createQImage", when i loop the pixels of "image2" and test the alpha value, as i said before, it is always 255! In order to check if i do something wrong with the created QImage in "createQImage", i forced a block of pixels to have 0 alpha and my final QImage had that block with 100% transparency.

Any ideas?