view image on button click. qt

Question

I created a label in my main window and its size is set to 200x300px. now on a button click i need to view a image in it. here is the slot function of the button.

 void MainWindow::function1(){
QImage img;
img.loadFromData("test.jpg");
img = img.scaled(200, 300, Qt::KeepAspectRatio, Qt::SmoothTransformation);
label->setPixmap(QPixmap::fromImage(img));
}

the problem is this compiles without errors, but not displaying the image. insted when the button is pressed it says QImage::scaled: Image is a null image in the IDE im new in qt. I would be very thankful for any advice.

thank you

Answer 1

Using a QImage is necessary if you are processing images outside of your GUI thread. Also the loadFromData() function looks like it is used for passing in a large array of bytes, not the name of the file.

Here is how I would do it with a QPixmap.

QPixmap pix;

bool loaded = pix.load("test.jpg");

if(loaded == false)
{
    label->setText("Failed to load test.jpg from" + QDir::currentPath());
}
else
{
    pix = pix.scaled(200, 300, Qt::KeepAspectRatio, Qt::SmoothTransformation);
    label->setPixmap(pix);
}

You may also want to replace 200 and 300 with label->width() and label->height(). Scaling a pixmap this way looks the best, but you can also just set the pixmap and then use label->setScaledContents(true)

Answer 2

instead of

img.loadFromData("test.jpg");

use

img.load("test.jpg");

if you're loading from a QIODevice.