How to get a picture from QString in Qt

Christian Ehrlicher

And how do you transfer the data?

Mikeeeeee

I use TcpSocket

Gojir4

I think it's easier if you use QDataStream.
I never tested but it seems quite straightforward

Write

QDataStream out(&socket);   // serialize the data 
out << myImage;             // serialize an image

Read

QDataStream in(&socket);    // read the data serialized from the socket
QImage myImage;
in >> myImage;              // extract image

aha_1980

@mikeeeeee said in How to get a picture from QString in Qt:

I use TcpSocket

Then you don't need QString. You need QByteArray, probably QDataStream as @Gojir4 pointed out.

QString imageString = QString::fromStdString(bArray.toStdString());

Sorry, but never do that!

QString imageString = QString::fromUtf8(bArray); is most often correct, sometimes fromLatin1() or fromLocal8Bit() might be needed too.

But as said, your problem cannot be solved with QString.

Regards

Mikeeeeee

But I pass the picture to json , and with the picture I pass additional information to the server. So I need a QString.

SGaist

Hi,

As already explained by my fellows, use QByteArray. QString is for string manipulation, it's not what you are doing. You are sending data through network. Therefore you are creating a base64 QByteArray that represent your image's binary data. There's no need for any QString in between.

Mikeeeeee

But as I write in Qbytearray json?
And why this doesn't it work:

    /*QString imageString = QString::fromLocal8Bit(bArray);
    bArray2 = QByteArray::fromHex(imageString.toLocal8Bit());*/
    QString imageString = QString::fromLatin1(bArray);
    bArray2 = QByteArray::fromHex(imageString.toLatin1());
    if (bArray != bArray2) {qDebug()<<"bArray != bArray2";}
    else {"bArray == bArray2";}

debug return: bArray != bArray2

SGaist

At one point you are encoding in base64 and the other you are decoding from hex. These are two completely different things.

Also, QString does a conversion to UTF16. So again, the fact that JSON contains text doesn't mean that you need to use QString.

Mikeeeeee

But as a json, you can write a qbytearray? And how to write the qbytearray to a string and then again in qbytearray?

SGaist

Take a look at QJsonDocument, no QString there.

Mikeeeeee

But I can't write qbytearray to json. And how to write the qbytearray to a string and then again in qbytearray?

SGaist

You have to understand that it's the QByteArray that will contain the JSON data.

Again, read the documentation of QJsonDocument, the related classes and example code.

Mikeeeeee

I read the documentation, it does not specify the ability to add QByteArray to json. So how can I translate QByteArray to Qstring and then to QByteArray?

aha_1980

@mikeeeeee Then you need to read more carefully.

https://doc.qt.io/qt-5/qjsondocument.html#toJson
https://doc.qt.io/qt-5/qjsondocument.html#fromJson

Mikeeeeee

I don't see how it will help to write QByteArray to json. Again, I can write Qstring to json. It remains only to learn how to safely translate from Qstring to QByteArray .

J.Hilk

@mikeeeeee do you actually want a qbytearray as a value to a key inside your son file ?

Mikeeeeee

So it turns QString into a picture to translate

QString AppCore::getImage(QString addressFile)
{
    QImage myImage(addressFile);

    QByteArray bArray;
    QBuffer buffer(&bArray);
    buffer.open(QIODevice::WriteOnly);
    myImage.save(&buffer, "JPEG");

    QString image("data:image/jpg;base64,");
    image.append(QString::fromLatin1(bArray.toBase64().data()));

    QImage testImage(image);
    qDebug()<<"myImage.size() : "<<myImage.size();

    return image;
}