XnView Software

...but no loss of quality!? What is happening there?


After turning a picture 90 degrees and saved it, a pic which had 325KB before now has about 99KB.

But if zooming in in both pictures till seeing the pixels, no difference is seen !?!

What's the reason and what's happening there?

Greetings, Henrik

There seems to be two seperate rotate functions: one is lossless, one is
not. <shift +l> launches the non-lossless one. Lossless transformations
are found under its own button and arrow. The non-lossless rotate will
save according to JPG quality settings (differences in this user setting and
the original's unknown setting can account for a big difference in file size).
Lossless rotate produces a very small size change with both this and other
programs I've seen.

Hello !
- I made just a test with a JPG 371.5 KB - True colours - 150,665 colours.
- Rotation 90° with the “Lossless…” function; I get :
206.4 KB - 24 BPP - 142,887 colours.

- Whether you come back to the original layout of the pic (via a reverse 90° handling), nothing more changes, you keep the same size than after the first rotation…
- The difference of size is so important… And the processing isn't really quite "lossless", since colours are lost.
- In the practice, this is not very visible, with regard to the human eye possibilities…

Kind regards,
Claude
Clo

My tests:

JPEG lossless transformations button - rotate 90° right

1st image - original image size 462.80 KB, after rotate 466.11 KB

2nd image - original image size 132.81 KB, after rotate 137.67 KB

3rd image - original image size 40.04 KB, after rotate 39.96 KB

There is also an option "Trim image (Remove unused pixels)" in JPEG lossless transformations dialog box, but there's no change of file size if it's enabled or disabled (previous images).

4th image - original image size 7.05 KB, after rotate, "Trim image" option disabled 7.11 KB, "Trim image" option enabled 6.82 KB - but dimensions changed from 255x157 to 152x255

5th image - original image size 14.50 KB, after rotate, "Trim image" option disabled 14.07 KB, "Trim image" option enabled 13.88 KB - but dimensions changed from 300x289 to 288x300

The Toolbar button seems to always use Trim/remove unused
pixels even when its unchecked using the other menu screen.
It does not look like anything is being lost.

668x668x24.jpg original_______ 106.20KB 147,769 colors
Rotated 90 with menu notrim___ 106.71KB 148,195 colors
Rotated back with menu notrim_ 106.20KB 147,769 colors
Rotated 90 with toolbar button__ 102.63KB 143,674 colors
Rotated back wi toolbar button__102.16KB 143,864 colors
Rotated 90 with menu trim_____ 102.63KB 143,674 colors
Rotated back with menu trim___ 102.16KB 143,864 colors
Rotated 90 with Other Prg_____ 103.68KB 143,674 colors
Rotated back with Other Prg___ 103.04KB 143,846 colors

Henrik wrote:...but no loss of quality!? What is happening there?

After turning a picture 90 degrees and saved it, a pic which had 325KB before now has about 99KB.

That's quite a difference in file size which really needs an explanation. You're sure you've used lossless rotation?
Just to make sure: Rotating an image and then saving with JPG highest quality 100% is not JPG lossless rotation.

Henrik wrote:But if zooming in in both pictures till seeing the pixels, no difference is seen !?!

Some pictures (JPG) compress very nicely and it's still very hard to tell which was the original.

Henrik wrote:What's the reason and what's happening there?

For all I know, the JPG lossless rotation does not touch the image data at all and just changes the orientation. For all I know such a difference in file does not occur with JPG lossless rotation (see figures above - thanks to Dreamer and lawndart).
One more possibility is that there was EXIF data and/or a thumbnail (preview) in the original image file which has been removed during conversion. But still this would not make a difference of 221 kB.

So no good explanation from my side.

Note: For lossy compression and file size the following topics might be interesting:
- Resizing image results in larger file
- Smaller image has same file size
- JPG LosslessTransformations