Snap openGL image with transparent background

[aftos]

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:

myGLWidget::myGLWidget
{
    setAutoBufferSwap(false);
    takingSnapshot = false;  // member variable. When taking snapshot turns true
}
 
void myGLWidget::initializeGL()
{
    glClearColor(1., 1., 1., 0.);
}
 
void myGLWidget::resizeGL(int ww, int hh)
{
    glViewport(0, 0, (GLint)ww, (GLint)hh);
}
 
void myGLWidget::paintGL()
{
    qDebug() << "paintGL";
 
    glPushAttrib(GL_TRANSFORM_BIT);
 
    glMatrixMode(GL_PROJECTION);
    glPushMatrix();
    glLoadIdentity();
    glMatrixMode(GL_MODELVIEW);
    glPushMatrix();
    glLoadIdentity();
    glOrtho(-1., 1., -1., 1., -1., 1.);
 
 
    /* Clear screen */
    if (takingSnapshot) {
        glEnable(GL_BLEND);
        glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
        glClearColor(0., 0., 1., 0.0f);
        glClear(GL_COLOR_BUFFER_BIT);
        glDisable(GL_BLEND);
    } else {
        glClearColor(1., 1., 0., 1.);
        glClear(GL_COLOR_BUFFER_BIT);
    }
 
 
    /* Begin Draw */
    glColor4f(1., 0., 0., 1.);
    glLineWidth(10);
 
    glBegin(GL_LINES);
    glVertex2f(-0.3, 0.0);
    glVertex2f(0.3, 0.0);
    glEnd();
 
    glEnable(GL_BLEND);
    glColor4f(0., 1., 0., 0.4);
    glRotatef(90., 0., 0., 1.);
    glBegin(GL_LINES);
    glVertex2f(-0.3, 0.0);
    glVertex2f(0.3, 0.0);
    glEnd();
 
    glPopMatrix();
    glMatrixMode(GL_PROJECTION);
    glPopMatrix();
 
    glPopAttrib();
    glDisable(GL_BLEND);
 
    if (!takingSnapshot) {
        swapBuffers();
    } else {
        qDebug() << "Finished drawing after snapshot\n";
    }
}

myGLWidget::myGLWidget
{
    setAutoBufferSwap(false);
    takingSnapshot = false;  // member variable. When taking snapshot turns true
}

void myGLWidget::initializeGL()
{
    glClearColor(1., 1., 1., 0.);
}

void myGLWidget::resizeGL(int ww, int hh)
{
    glViewport(0, 0, (GLint)ww, (GLint)hh);
}

void myGLWidget::paintGL()
{
    qDebug() << "paintGL";

    glPushAttrib(GL_TRANSFORM_BIT);

    glMatrixMode(GL_PROJECTION);
    glPushMatrix();
    glLoadIdentity();
    glMatrixMode(GL_MODELVIEW);
    glPushMatrix();
    glLoadIdentity();
    glOrtho(-1., 1., -1., 1., -1., 1.);


    /* Clear screen */
    if (takingSnapshot) {
        glEnable(GL_BLEND);
        glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
        glClearColor(0., 0., 1., 0.0f);
        glClear(GL_COLOR_BUFFER_BIT);
        glDisable(GL_BLEND);
    } else {
        glClearColor(1., 1., 0., 1.);
        glClear(GL_COLOR_BUFFER_BIT);
    }


    /* Begin Draw */
    glColor4f(1., 0., 0., 1.);
    glLineWidth(10);

    glBegin(GL_LINES);
    glVertex2f(-0.3, 0.0);
    glVertex2f(0.3, 0.0);
    glEnd();

    glEnable(GL_BLEND);
    glColor4f(0., 1., 0., 0.4);
    glRotatef(90., 0., 0., 1.);
    glBegin(GL_LINES);
    glVertex2f(-0.3, 0.0);
    glVertex2f(0.3, 0.0);
    glEnd();

    glPopMatrix();
    glMatrixMode(GL_PROJECTION);
    glPopMatrix();

    glPopAttrib();
    glDisable(GL_BLEND);

    if (!takingSnapshot) {
        swapBuffers();
    } else {
        qDebug() << "Finished drawing after snapshot\n";
    }
}

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And here is my mainWindow ( MainWindow : public QMainWindow):

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    mainFrame = new QFrame(this);
    setCentralWidget(mainFrame);
 
    mainLay = new QVBoxLayout(mainFrame);
    mainFrame->setLayout(mainLay);
 
    glw = new myGLWidget(mainFrame);
    snap = new QPushButton("Snap", mainFrame);
    label = new QLabel(mainFrame);
 
    mainLay->addWidget(glw);
    mainLay->addWidget(snap);
    mainLay->addWidget(label);
 
    mainFrame->show();
    glw->show();
    snap->show();
    label->show();
 
    QObject::connect(snap,SIGNAL(clicked()),this,SLOT(takeImage()));
}
 
static QImage createQImage(const unsigned char *image2, int ww, int hh)
{
            /* Do some stuff here with "image2" */
            /* and return the final image */
}
 
void MainWindow::takeImage()
{
    int ww = glw->width();
    int hh = glw->height();
    QImage finalImage;
    GLubyte *image = (GLubyte *)malloc(ww * hh * sizeof(GLubyte) * 4);
 
    glw->takingSnapshot = true;
    glPushAttrib(GL_PIXEL_MODE_BIT | GL_COLOR_BUFFER_BIT);  // readBuffer + drawBuffer
    glReadBuffer(GL_BACK);
    glDrawBuffer(GL_BACK);
    qDebug() << "takeImage, will call updateGL";
 
    glw->updateGL();
    glReadPixels(0,0,ww,hh,GL_RGBA,GL_UNSIGNED_BYTE,image);
 
    finalImage = createQImage(image, ww, hh);
 
    if (!finalImage.isNull()) {
        QPixmap pix;
        pix.convertFromImage(finalImage);
        label->setPixmap(pix);
    }
    glPopAttrib();
    glw->takingSnapshot = false;
    if (image) free(image);
}

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    mainFrame = new QFrame(this);
    setCentralWidget(mainFrame);

    mainLay = new QVBoxLayout(mainFrame);
    mainFrame->setLayout(mainLay);

    glw = new myGLWidget(mainFrame);
    snap = new QPushButton("Snap", mainFrame);
    label = new QLabel(mainFrame);

    mainLay->addWidget(glw);
    mainLay->addWidget(snap);
    mainLay->addWidget(label);

    mainFrame->show();
    glw->show();
    snap->show();
    label->show();

    QObject::connect(snap,SIGNAL(clicked()),this,SLOT(takeImage()));
}

static QImage createQImage(const unsigned char *image2, int ww, int hh)
{
            /* Do some stuff here with "image2" */
            /* and return the final image */
}

void MainWindow::takeImage()
{
    int ww = glw->width();
    int hh = glw->height();
    QImage finalImage;
    GLubyte *image = (GLubyte *)malloc(ww * hh * sizeof(GLubyte) * 4);

    glw->takingSnapshot = true;
    glPushAttrib(GL_PIXEL_MODE_BIT | GL_COLOR_BUFFER_BIT);  // readBuffer + drawBuffer
    glReadBuffer(GL_BACK);
    glDrawBuffer(GL_BACK);
    qDebug() << "takeImage, will call updateGL";

    glw->updateGL();
    glReadPixels(0,0,ww,hh,GL_RGBA,GL_UNSIGNED_BYTE,image);

    finalImage = createQImage(image, ww, hh);

    if (!finalImage.isNull()) {
        QPixmap pix;
        pix.convertFromImage(finalImage);
        label->setPixmap(pix);
    }
    glPopAttrib();
    glw->takingSnapshot = false;
    if (image) free(image);
}

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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?

[aftos]

Bah! I solved it. Alpha buffer is disabled by default. I needed to enable it. It works fine