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) / (1024 * 1024) = memory (MB)
        

  • calculation for above image :-

    7360 * 4912 * 3 * 1 / (1024*1024) = 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  / (1024* 1024) = 1.7MB
        

  • so expect it to maximum consume 1.7MB and not 103MB
  • then using some resampling algorithm like Lanczos filter from 920px * 614px, it is converted to 30px * 30px
  • 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
      

• May be not directly related to Magickimage also had some issue with libjpeg :- link
• 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 ?