Display an image in a c++ program

JYG61

Hello,
I just started to use qt for programming in C++, i tried some classical widgets (edit, buttons etc..) but i need to display an image on the main window, i found an code exemple
QLabel *labelImage = new QLabel();
QImage image("tonImage.jpg");
labelImage->setPixmap(QPixmap::fromImage(image));
that i added atfer
ui->setupUi(this);
in my MainWindow creator
it just display an empty window, i checked using the isNull function of the QImage the check that my program find the jpeg file and i don't know why he does not display it.
I had a quick look at the documentation about QImage and QLabel, there are hundreds of functions, just the syntax is described but i found no clear information about that each function does.
Thanks to help me
JY Giroud

SGaist

Hi and welcome to devnet,

labelImage is a new widget on which you didn't set a parent nor did you add to any layout that you may have in you ui file.
If you already have a QLabel through the ui object, then call setPixmap on that label and remove labelImage.

JYG61

Thanks for your answer,
I tried to add a label from the widget box, i added a widget called 'label' but it added a 'text label' what is the name of the widget i need to add from the widget box ?

JYG61

Hello,
I did my first tests in C++ with QT using a free online training and i don't remember where i found it, is there anybody that can help me to find it again ?
Thanks

SGaist

@JYG61 said in Display an image in a c++ program:

Thanks for your answer,
I tried to add a label from the widget box, i added a widget called 'label' but it added a 'text label' what is the name of the widget i need to add from the widget box ?

The name is a bit misleading but that is the correct widget.

In your code you can simply do:

setupUi(this):
QImage image("tonImage.jpg");
ui->label->setPixmap(QPixmap::fromImage(image));

JYG61

It works fine, thanks a lot but my problem is a bit more complicated, i need to load the QImage object directly from a memory buffer that contain the RGB value (8 bit per sample) of every pixel, do you know which method i should use to do this ?

SGaist

Look at this constructor using the QImage::Format_RGB888 image format.

Please take the time to read the documentation, especially the part about the buffer lifetime.

JYG61

Hello,
I tried this code :
unsigned char tab[400];
int i;
for (i=0;i<400;i++) tab[i]=i%256;
ui->setupUi(this);
//QImage image("/home/x/passeport.jpg");
QImage image (tab, 100, 100, QImage::Format_RGB888, nullptr, nullptr);
ui->label->setPixmap(QPixmap::fromImage(image));
I just replaced le loading of the image file (commented) by an unsigned char[400], there are no errors at compilation but when i try to start it, it crashes immediately

Christian Ehrlicher

@JYG61 said in Display an image in a c++ program:

it crashes immediately

What do you expect?
An rgb image with 100x100 needs a little bit more memory than 400bytes... 100 * 100 * 3 I would guess.

JYG61

Oups
Sorry to disturb with that stupid question, i just missed a factor 100 (as it seems to be 4 bytes aligned), i just added two zeros and it works, of course i need to put a real image in my memory bloc but that is nt a problem.
Thanks

JYG61

Is there a way to mark this topic solved ?

jsulm

@JYG61 See "Topics Tools" at the top