Zooming image

[MTK358]

I have a QLabel (displaying an image) inside a QScrollArea. How do I find out by how much to scale the image to make it fit with no scrollbars? There will also be a zoom slider to adjust it manually.

You can tell me is using QGraphicsView is a better idea, I'm just not familiar with it.

[JohannesMunk]

Have you tried QLabel::setScaledContents(true);?

I didn't get the Scrollarea at first! There is an example that does just what you want:

http://doc.qt.nokia.com/latest/widgets-imageviewer.html

Joh

[MTK358]

That's a nice link, but I still have problems:

Scrollbars don't appear when the image is larger than the viewport.

Switching back to fit mode doesn't work.

The image's aspect ratio isn't retained in fit mode.

It uses a smooth scaling algorithm which takes a ridiculously long time to complete.

Here is some relavent code:


void MainWindow::fitButton(bool on) // note that fitButton is checkable
{
ui->imageScrollArea->setWidgetResizable(on);
ui->actualSizeButton->setDisabled(on);
ui->zoomSlider->setDisabled(on);
ui->zoomLabel->setDisabled(on);
}

void MainWindow::actualSizeButton()
{
ui->zoomSlider->setValue(100);
}

void MainWindow::zoomSlider(int value)
{
QSize size = ui->imageLabel->pixmap()->size();
size.setWidth((size.width() * value) / 100);
size.setHeight((size.height() * value) / 100);
ui->imageLabel->resize(size);
}

[JohannesMunk]

http://www.qtcentre.org/threads/31058-Change-the-size-of-pixmap-in-a-label

Subclass QLabel. And handle the resizeEvent yourself with fast transformation turned on.

Joh

[MTK358]

Good idea, it will solve both the speed and aspect ratio problems. I'm going to implement it now.

[MTK358]

Wait a minute — what do I do in the resizeEvent()? Where to store the resized image?

Also, it should still handle text, for "file format not supported" messages and stuff like that.

[wysota]

I would subclass QAbstractScrollArea and implement everything as a single custom widget. Placing a QLabel in a QScrollArea will cause you problems sooner or later.

[MTK358]

The problem is that I'm using a Designer form. How would I put my own widget right in the middle of a layout?

[wysota]

How would I put my own widget right in the middle of a layout?
Use the "Promote to..." feature of Designer - put a QFrame on the form, right click the frame, choose "Promote to..." and enter your custom widget's data.

[MTK358]

The Promote feature is great! I didn't know about it.

But OK, I created a custom widget containing the scroll area, label, and zoom controls, but it still has the same issues. It just behaves very strangely and so inconsistently that I can't really even explain it.

[wysota]

I told you to subclass QAbstractScrollArea and implement the functionality from scratch, not to use a custom widget composed of a scroll area and a label. You will have to get your hands dirty here.

[MTK358]

OK, I looked but I don't understand what methods to reimplement and how to paint to the viewport.

[wysota]

OK, I looked but I don't understand what methods to reimplement and how to paint to the viewport.

Reimplement at least paintEvent() and optionally scrollContentsBy().

[MTK358]

OK, but how do I get a QPainter for the viewport widget?

[wysota]

You create it in the paint event like usual.

[MTK358]

Finally, I did it.

Anyway, it works great now, but I would really like it so that the user can scroll by dragging the mouse in the viewport. How would I do that?

[SixDegrees]

You're gradually reinventing QGraphicsView here. Just use that; you get all of the features you've brought up so far for free.

[MTK358]

You're gradually reinventing QGraphicsView here. Just use that; you get all of the features you've brought up so far for free.

I'm not very familiar with QGraphicsView but maybe...

Anyway, why didn't anyone else offer that?

[wysota]

Anyway, why didn't anyone else offer that?
I'm myself against using Graphics View for displaying a single image. In my opinion it gives too much overhead without giving anything else in return.