QFont的有效PointSize

Question

I have a QOpenGLWidget in which I draw text with a QPainter. I would like to implement a snapshot feature by rendering the widget content into a QOpenGLFrameBuffer and converting the frame buffer into a QImage.

Unfortunately, if I set the font's point size too high (> 46), the text appears as a black bar in the snapshot, while in the widget it is displayed correctly. See below an example snapshot where the block over the line is supposed to be text with font size > 46.

Here is the simplified code to render the image (it should work, because it is correctly displayed in the QOpenGLWidget):

void renderSomething(const int x, const int y, const QString & str, const int fontSize) {
    // 1. draw the line
    // (calculate min_x, max_x, y and z values)
    glLineWidth(3);
    glColor3f(0., 0., 0.);
    glBegin(GL_LINES);
    glVertex3f(min_x, y, z);
    glVertex3f(max_x, y, z);
    glEnd();

    // 2. draw the text
    GLint gl_viewport[4];
    glGetIntegerv(GL_VIEWPORT, gl_viewport);
    backup_gl_state();
    QOpenGLPaintDevice paintDevice(gl_viewport[2], gl_viewport[3]);
    QPainter painter(&paintDevice);
    painter.setFont(QFont(painter.font().family(), fontSize);
    painter.setPen(Qt::black);
    painter.drawText(x, y, str);
    painter.end();
    restore_gl_state();
}

Here is the code for storing a snapshot:

void Viewport::takeSnapshot(const QString & path, const int size) {
    glPushAttrib(GL_VIEWPORT_BIT);
    glViewport(0, 0, size, size); // since size varies I want the text to scale with it
    QOpenGLFramebufferObject fbo(size, size, QOpenGLFramebufferObject::Depth);
    fbo.bind();
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    renderer.renderSomething(100, 100, "Test", 50); // Here is the render call!

    QImage fboImage(fbo.toImage());
    QImage image(fboImage.constBits(), fboImage.width(), fboImage.height(), QImage::Format_RGB32);
    image.save(path);
    glPopAttrib();
    fbo.bindDefault();
    fbo.release();
}

EDIT

I found a workaround by using QPainter to directly draw the text on the QImage instead of drawing into the FBO. No black bar anymore, but it complicates my code and I would be happy to find a real solution...

Answer 1

The solution was simple: Re-read the documentation on QOpenGLFrameBufferObject in which it says:

Create the QOpenGLFrameBufferObject instance with the CombinedDepthStencil attachment if you want QPainter to render correctly.

I only attached a depth buffer. The correct initialization would be:

QOpenGLFramebufferObject fbo(size, size, QOpenGLFramebufferObject::CombinedDepthStencil);