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modifying jpeg images from my Nikon D5500 with Microsoft Photo Viewer 6.1 (Windows 7), I am experiencing strange effects. I would like to understand if I am degrading the images or not.

  • When I rate an image with stars, the file size gets about 10% smaller (like from 10 MB to 9 MB), yet pixel number is identical, and if I compare the modified image with the original there are no differences (https://online-image-comparison.com). Furthermore, if I copy the modified image on a SD card and try to view it on the camera, the image looks blurred ... unless I zoom. What could be going on?

  • When I rotate an image (6000x4000) by 90 deg (4000x6000), the file size gets about 10% smaller, yet pixel number is identical, and if I compare the modified image with the original there are no differences (https://online-image-comparison.com). Furthermore, if I copy the modified image on a SD card and try to view it on the camera, I get an error message and cannot see the image at all. What could be going on?

Also note that the properties (Exif) of the original and modified image are identical. So what is that 10% of memory that Photo Viewer is getting rid of, apparently without loss of image quality or metadata?

highalpha
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2 Answers2

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It is possible that Windows re-encodes the picture as interlaced/progressive JPEG(*), more suitable for the web. These are usually slightly smaller than their "baseline" equivalents. Although quite old (20 years now) the "progressive" variant is often not supported by embedded software, such as LCD photoframes and... cameras, which would explain why the picture is not displayable in your camera. Pictures editors (Gimp et al.) usually can save as plain JPEG (even if "progressive" is often the default).

Several tools can report the baseline/progressive encoding option:

  • exiftool -EncodingProcess
  • the file command in Unix/Linux
  • identify -verbose in the ImageMagick toolbox (search "Interlace" in the output)

(*) in that format a low-res version (1 in 4 scan lines, then 1 in 2) of the complete image is available without needing to read the whole file, this was very useful in the 90s when you were downloading such images at modem speeds.

xenoid
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  • Yes, added some possible tools to check this. – xenoid Sep 06 '18 at 23:35
  • You notice that when you accidentally rotate something in windows photo viewer while browsing an sd card ... image becomes instantly unreadable eg to several sony cameras. – rackandboneman Sep 07 '18 at 21:10
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Windows Photo Viewer rotations are lossless when the image dimensions are a multiple of 8 or 16 (depending on whether chroma subsampling is used). See Are “Windows Photo Viewer” rotations lossless?

The change in file size may be caused by alterations to the image preview thumbnail or lossless compression optimizations. The thumbnail, at least, seems likely to have changed given your description of problems viewing the image on camera.

To verify that the Exif has survived intact, use exiftool -G -a to compare. You will also be able to see whether anything else has changed, such as the encoding process, from baseline to progressive. Xenoid discusses progressive encoding further.

xiota
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  • Personally... I've been burned too many times to trust MS software to preserve my data. – xiota Sep 06 '18 at 22:09
  • What do you mean that you have been burned? – highalpha Sep 06 '18 at 22:25
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    I bet it's eating makernotes and other metadata which it does not understand. – mattdm Sep 06 '18 at 22:28
  • @highalpha It's hard to recall specific examples because I haven't used Windows much for about a decade. (Only when needed for firmware updates.) A pertinent example is older versions of Windows Photo Viewer did not rotate losslessly. Old MS software often did stuff the "naive" way (the way non-computer-scientists would do it). Then they would inconsistently update behind the scenes to do it the "right" way. Because it's all proprietary, no one really knows what it's doing. An example of this is how Windows security has basically been hacked on to an insecure base. – xiota Sep 06 '18 at 22:50
  • @mattdm 1 MB of makernotes? – osullic Sep 07 '18 at 07:00
  • @mattdm I might bet it's eating the entire preview thumbnail as well, that would explain the blurriness caused by viewing it on the camera's LCD to scaling errors. – Michael C Sep 07 '18 at 18:45
  • Yeah; it's true that 1mb would be quite large for metadata alone. – mattdm Sep 07 '18 at 18:47