Qt6 reading JPEG encoded as Progressive using more memory (sometimes till out-of-memory) than Baseline

wahabshah

Hey all,

We are using Qt6 on an embedded device (ARM64 imx8) with limited RAM (total about 400MB).

While loading JPEG image (Iink) which is encoded as progressive in an Qt Widget application :-

bailey.jpg: JPEG image data, JFIF standard 1.02, resolution (DPI), density 72x72, segment length 16, progressive, precision 8, 7360x4912, frames 3

using QImageReader::read to read image scaled to 30px x 30px instead of 7360px x4912px :-

 // Reader for JPEG
 QImageReader reader("bailey.jpg");
 reader.setScaledSize(QSize(30, 30)); // directly decode at 30x30

 QImage image30x30;
 if (!reader.read(&image30x30)) {
     qWarning() << "Failed to load:" << reader.errorString();
     return -1;
 }

Even though a scaled down version of image e.g 30px x 30px is asked
- From my understanding that memory used during decompression (using IDCT shrink-on-load from libjpeg (link)) should be :-
```
width × height × channels × bytes per channel = memory (MB)
```
- calculation for above image :-
```
7360 * 4912 * 3 * 1  = 103MB
```
- But since we are asking for a scaled version so libjpeg would scale to 1/8 as its theoratical limit
```
(7360 / 8 ) * (614 / 8) * 3 * 1 = 1.7MB
```
- so expect it to maximum consume 1.7MB and not 103MB
- may be i am wrong about it but this is just little knowledge of how i have it
During the read, it seems like it is loading whole image that it causes out-of-memory for kernel to kill the application eventually.

The default QJpegPlugin linked is libjpeg :-

$ ldd /usr/lib/plugins/imageformats/libqjpeg.so 
...
libjpeg.so.62 => /usr/lib/libjpeg.so.62 (0x0000ffffbd350000)

But when same image was converted from progressive to baseline (Iink) then it did use some memory but not till out-of-memory

bailey.jpg: JPEG image data, JFIF standard 1.02, resolution (DPI), density 72x72, segment length 16, baseline, precision 8, 7360x4912, components 3

Therefore the question is :-
- does QJpegPlugin and eventually libjpeg can they handle progressive images efficiently or is it a bug or limitation of library itself?
- or should we convert JPEG to baselines as it seems to respect the downscaling and not load whole image into memory ?

Christian Ehrlicher

Please provide a small jpeg and create a bug report. I can only think that libjpeg does either something wrong here or the Qt plugin does not handle a specific combination of parameters correctly.

wahabshah

I added the links to the image in the description above but adding under again :-
- https://gist.github.com/wahabshah/a36cd8524e098a58e3909cc5c3592436#jpeg-progressive
- https://gist.github.com/wahabshah/a36cd8524e098a58e3909cc5c3592436#jpeg-baseline
yeah sure i added that as a bug to be checked by the Qt Team :-
- https://bugreports.qt.io/browse/QTBUG-140124