How to display a picture

Payx

I got this error with your code :

C:\Users\David\Documents\projet\mainwindow.cpp:17: erreur : redefinition of 'MainWindow::MainWindow(QWidget*)'
MainWindow::MainWindow(QWidget *parent) :
^

? Offline

You need to add #include <QPixmap> to the other includes and add these three lines...

QPixmap pm("C:/Users/Patrick Wieland/Documents/Kochen/bento.jpg"); // <- path to image file
ui->label_2->setPixmap(pm);
ui->label_2->setScaledContents(true);

to your existing MainWindow constructor.

Payx

Okay sorry.

I did it but nothing appear in my MainWindow

? Offline

Did you put in the correct path to your image file? The path in the code above is only valid on my harddrive ;-) Also, did you add a QLabel to your MainWindow? I added a QLabel with the designer in Qt Creator and the name of that label is label_2.

Payx

I put this "file:///C:/Users/David/Documents/addddd/r.jpg"

Yes i added a Qlabel with the .ui and named label (dont worry i changed the name in my code)

? Offline

Only type "C:/Users/David/Documents/addddd/r.jpg", without the "file://". The pixmap constructor takes a file path and not an URL.

Payx

Its okay it works ! Thanks a lot for your time

To works now with the Qlabel and for example i would like to calculate the numbers of pixels (number of x pixels and y pixels) do you have any tutorial for this ?

? Offline

@Payx Add another QLabel (here its name is label_3) to the MainWindow. We can use that label to display the size of the Pixmap. When adding the label in Qt Creator's Designer, make the label wide enough so that all the text fits in.

In the MainWindow constructor in mainwindow.cpp:

const QSize s = pm.size(); // <- get size of the Pixmap
ui->label_3->setText( QString("Size: %1, %2").arg(s.width()).arg(s.height()) );

Edit: Pixmap has a lot of built-in stuff, see QPixmap Class for more. QPixmap is there to display images, to actually put them on the screen. If you want to manipulate an image, you don't use a pixmap for that. Use a QImage then, see: QImage Class.

So, this is the workflow:

Create a QImage from a file: QImage img("C:/path/filename.jpg");
Do something with that image, like change the color of some pixels.
Create a QPixmap from that QImage: QPixmap pm = QPixmap::fromImage(img);
Show the Pixmap in a QLabel: ui->label->setPixmap(pm);

Payx

So if i understand, i use QImage for doing whatever i want with this image.
And i use Qpixmax just for display in a label ?

(sorry for my english im not bilingual)

Payx

So if i want to calculate the size of my picture i use :

QSize QImage::size() const

The question is : how to use it ? there is no example

mrjj

@Payx said in How to display a picture:

QImage::size()

It returns a
http://doc.qt.io/qt-5/qsize.html
and you can ask it height / width etc
http://doc.qt.io/qt-5/qsize.html#height

const QSize MySize = theImage->size();

int h=MySize.height();

etc.
Update:
@wieland already showed ;)

Payx

Thanks but how to return it ?

""ui->label_3->setText( QString("Size: %1, %2").arg(s.width()).arg(s.height()) );""

there is not more simple ?

mrjj

@Payx
Well You need to convert to text anyway so it will become

ui->label_3->setText( "Size: " + QString::number(s.width()) +" "+ QString::number(s.width()) );

Not sure its better.

Thanks but how to return it ?
What we do there is to convert to text and set that text in some label.
That is not really returning it.

returning it would be something like

QSize SomeFunc() {
return pm.size();
}

Payx

Thanks it works. I prefer : ui->label_3->setText( "Size: " + QString::number(s.width()) +" "+ QString::number(s.height()) );
because i understand all.

Then if i want to use height and width for other calcul i can use :
s.width() * 5 ?

If i want to cut my picture in many square or rectangle i have to do :

int h = s.width() and work with h thats right ?

mrjj

@Payx

int h = s.width() and work with h thats right ?
Yes that's a copy of the current width. ( so w is better name ;)
but its makes the calculation easier to read than using s.width() directly.

Note to actually cut the image, you can use
http://doc.qt.io/qt-5/qimage.html#copy

Payx

Okay !

but : QImage QImage::copy(const QRect &rectangle = QRect()) const
with this code we can work only in one rectangle no ?

for example i thought cut my picture in pixels 4*4 and have a new image with resolution / 4 :

i have to use a code like this :

for (i=0,i<s.width()+1,i=i+4)
for (j=0,j<s.height()+1,j=j+4)

end

end

mrjj

@Payx

int step=4;
for (i=0,i<s.width()+1,i=i+step)
  for (j=0,j<s.height()+1,j=j+step)
  just use  QImage QImage::copy(int x, int y, int width, int height) const

This will generate small images of each 4x4

Pradeep Kumar

@Payx said in How to display a picture:

I put this "file:///C:/Users/David/Documents/addddd/r.jpg"

Yes i added a Qlabel with the .ui and named label (dont worry i changed the name in my code)

A small info, u can also place image in resource file of the project , so no need to worry about absolute path.

Payx

@mrjj said in How to display a picture:

@Payx

int step=4;
for (i=0,i<s.width()+1,i=i+step)
  for (j=0,j<s.height()+1,j=j+step)
  just use  QImage QImage::copy(int x, int y, int width, int height) const

This will generate small images of each 4x4

Okay i will test that.

I got an other question, with that code : "QImage img("C:/path/filename.jpg");" i have to put the destination of the file (sorry for my english)

but if i want to create a file explorer (for example in facebook we can change a picture with a file explorer and choose what picture i want)
what can i do ?

mrjj

but if i want to create a file explorer (for example in facebook we can change a picture with a file explorer and choose what picture i want)

Im not 100% sure what you ask, but I give a guess:

Instead of using a fixed path in the program you can use
http://doc.qt.io/qt-5/qfiledialog.html
to let user browser and select an image.

Like in this example
http://doc.qt.io/qt-5/qtwidgets-widgets-imageviewer-example.html

so
QImage img("C:/path/filename.jpg"
becomes

QString imagefile = QFileDialog::getOpenFileNames(this, tr("Files"), QDir::currentPath(), tr("*.jpg *.png"));
QImage img(imagefile);

Hope that is what you asked about.