QImageReader could not find the size of images

Grit Clef

Hi,
Using Qt 6.9 on Windows 11 23H2, I found that QImageReader couldn't work properly on some formats, especially in getting the pictures' size info. In particular, I tested bmp, png, jpg formats, and found that QImageReader would only give me the right info of the jpg formatted pictures. For example, the following code:

QImageReader imgReader;
imgReader.setFileName("...");
imgReader.setAutoTransform(true);

if (imgReader.canRead()) {
    QMessageBox::warning(nullptr, "...", QString::number(imgReader.size().width()),
                                     QMessageBox::Close);
}

will tell me the correct width of a jpg image, while only -1 of pictures in other formats.
Surprisingly, if I then read a pixmap from that QImageReader, use the getters from that QPixmap, and I would see all the correct info.

QPixmap pixmap{ QPixmap::fromImageReader(&imgReader) };
QMessageBox::warning(nullptr, "...", QString::number(pixmap.width()),
                                     QMessageBox::Close);

The result above is always right.
Also, I've tested the supportsOption function:

imgReader.supportsOption(QImageIOHandler::Size);

It's always true.
Why is it? It's so strange that I think I may look for help. Thanks.

SGaist

Hi,

I have not checked the code of these readers but from a quick look at the QImagrReader::size documentation they might fall in the category of "needs to be fully read to get that information".

Grit Clef

Qt's built-in image handlers all support this feature

I was just confused by that sentence. Formats like BMP are built in, but their size cannot be determined until they are fully read... Doesn't the BITMAPINFOHEADER contain useful information about its width and height?

SGaist

The BMP handler should give that information unless something wrong happened when reading the file header.

Can you provide a minimal compilable example with a sample images that would allow to check what happens ?

Christian Ehrlicher

And create a bug report and notify me so I can take a look on it after holidays. I would expect that the size is always returned correctly, no matter if we have to read the whole file or not. I know it is working for png and assume it must work for others in QIconloader class so it is important to see if there is something wrong.

Grit Clef

I think I have learnt sth. For handlers like BMP, we cannot do reading operations before calling size(). If I call size() immediately after canRead(), the result is correct; if I have called read() or sth like QPixmap::fromImageReader() before the call to size(), the result is invalid. However for the JPG handler, there's no such restriction.

Christian Ehrlicher

Please create a bug report

Grit Clef

Create at https://bugreports.qt.io/browse/QTBUG-138530