QImageWriter and EXIF

samdol · wrote on 11 Oct 2017, 22:12

Hi,

I used QImageReader to read image from file and QImageWriter to save imge to file. The issue is that when I save image to file using QImageWriter, it looses EXI
F information of image. Is there any way in Qt I don't loose EXIF information when I save image to file?

SGaist · 12 Oct 2017, 09:07

Hi,

There was at some point the QExifImageHeader class that might be of interest.

samdol · S SGaist 11 Oct 2017, 22:34

@SGaist
Thanks, I have checked it, but there seems some bug for Gps info. For example, when I run following code, I think exif values should not change.

    QExifImageHeader header;
    header.loadFromJpeg("C:/myfile.jpg");
    QExifValue value_GpsLatitude = header.value(QExifImageHeader::GpsLatitude);
    QExifValue value_ImageWidth = header.value(QExifImageHeader::ImageWidth);
    QExifValue value_Make = header.value(QExifImageHeader::Make);
    QExifValue value_Model = header.value(QExifImageHeader::Model);
    header.setValue(QExifImageHeader::GpsLatitude, value_GpsLatitude);
    header.setValue(QExifImageHeader::ImageWidth, value_ImageWidth);
    header.setValue(QExifImageHeader::Make, value_Make);
    header.setValue(QExifImageHeader::Model, value_Model);
    header.saveToJpeg("C:/myfile.jpg");

But when I run it, GpsLatitude value has changed. Do you think it is a bug?
And how can I read GpsLatitude?
When I run the following,

   qDebug()<<"laa: "<< QString::number(value_GpsLatitude.toRational().first)+"/"+QString::number(value_GpsLatitude.toRational().second);

I always get "0/0" which is not correct.

Pablo J. Rogina · 13 Oct 2017, 01:08

@samdol what about this EXIF helper code?

SGaist · 13 Oct 2017, 01:19

Might be, I can't confirm, I don't have images containing such data to test.

samdol · wrote on 13 Oct 2017, 01:08

@Pablo-J.-Rogina
Thank you, I used it before. but it only reads exif information. Because I want to preserve EXIF after save image, I need a method to read/write exif.

samdol · 17 Dec 2018, 16:13

@SGaist
I found that Reading Gps information should be like this,

  qDebug()<<"laa: "<< QString::number(value_GpsLatitude.toRationalVector().at(0).first)+"/"+QString::number(value_GpsLatitude.toRationalVector().at(0).second);
  qDebug()<<"laa: "<< QString::number(value_GpsLatitude.toRationalVector().at(1).first)+"/"+QString::number(value_GpsLatitude.toRationalVector().at(1).second);
  qDebug()<<"laa: "<< QString::number(value_GpsLatitude.toRationalVector().at(2).first)+"/"+QString::number(value_GpsLatitude.toRationalVector().at(2).second);

But I still don't understand why it changes Gps value after

header.saveToJpeg("C:/myfile.jpg");

It changes even without

header.setValue(QExifImageHeader::GpsLatitude, value_GpsLatitude);

It seems a bug.
You may get test jpg file with gps_tag from here

Luisa · S samdol 13 Oct 2017, 01:19

@samdol I have the same problem.

If I load metadata and then I save without changing anything I read incorrect value.
Have you find a solution?

QExifImageHeader * source = new QExifImageHeader();
QExifValue *vale;
source->loadFromJpeg(filedest);
source->saveToJpeg(filedest);
QExifImageHeader * dest = new QExifImageHeader();
dest->loadFromJpeg(filedest);

Thanke you for your help

Luisa

SGaist · 18 Dec 2018, 08:17

IIRC, when I took a look at it, there was an issue with handling some of the tags. I can't remember exactly what it was.

Luisa · 18 Dec 2018, 08:36

@SGaist Do you know a library that I could use to read / write exif files? I need to cross compile it for a system embedded. I tried exiv2 with poor results