How to show part of an image using QT?

Question

So, this is my problem: I have this very big image, and I want to show only a specific part of it. After the user pressing a specific key I want the image to move, showing another part of it. The transition from one part of the image to another have to be smooth, animated.

I tried using a QLabel to show the image but it always shows the center of the image, and I do not really know how to make the animation. What would you guys suggest?

Answer 1

Interesting question. Here is something I just tested and seems to work.

Add a QGraphicsView with dimensions the dimensions of the part of the image you want to display, eg 100x100. Create a QGraphicsScene and add it to the view:

QGraphicsScene* pScene = new QGraphicsScene(this);
ui->graphicsView->setScene(pScene);

Now add your image into the scene. In my case I has an image in my resource file. The trick is to set the sceneRect to the position you want to display. I wanted to display an 100x100 part of the image starting from 0,300 :

pItem = pScene->addPixmap(QPixmap::fromImage(QImage(":/photos/image")));
pScene->setSceneRect(0,300,100,100);

In order to test the smooth moving I added a button which when clicked is triggering a slot called move. This slot simply updates the sceneRect. In my simple example I just move the image 100 pixels right. In a real world scenario you could also move it diagonally or vertically and check the image limits.

void move()
{
    for (unsigned i=currentX; i<currentX + 100; i++)
    {
    ui->graphicsView->scene()->setSceneRect(i,300,100,100);
    qApp->processEvents();
    }
    currentX += 100;
}

Notice the currentX variable. It is nothing more than the last image position. Also we must call the processEvents in order to "see" the image moving smoothly.

Answer 2

You could use QPixmap::copy( int x, int y, int width, int height ) to copy a region of the image and display that.