Rotating images in Windows

[Garbz]

Do you mean how, or what programs? There are a lot of programs out there that recognise the rotate flag in ExIF. ACDSee, Photoshop, Picasa just to name a few.

If you mean how, your camera sets a flag in the image to rotate it clockwise or counterclockwise, but saves the image in the standard landscape orientation. Software then should read and adjust when opening.

 

[Lipoly]

(Post resurrection alert)

FYI: I just did a test in Windows 7, opening an image and rotating it 45 deg 16 times and then used Beyond Compare to compare with the original and they were identical (in addition to them having the same file size). I also did a 400% zoom side-by-side comparison and say no differences.

It looks like win 7 uses meta data to do rotations in the built-in picture viewer.

 

[Garbz]

(Post resurrection alert)

FYI: I just did a test in Windows 7, opening an image and rotating it 45 deg 16 times and then used Beyond Compare to compare with the original and they were identical (in addition to them having the same file size). I also did a 400% zoom side-by-side comparison and say no differences.

It looks like win 7 uses meta data to do rotations in the built-in picture viewer.

False test. The biggest difference between Windows 7 picture viewer and the earlier versions is that the file is only saved when Windows picture viewer is closed.

Quick test open a file and rotate it once clockwise, and then close it again. My original was 0.97MB with ExIF orientation data "Upper Left". After I rotated it clockwise and closed Windows 7 picture viewer the result is a file that is 330KB with ExIF orientation data still "Upper Left", and now the ExIF resolution disagrees with the actual file resolution too since that wasn't re-written.

So Windows 7 Picture Viewer only sucks a little less than it's predecessor, but what is quite good is that it is output colour managed and will recognise not only the working colour space of the image, but also the output space of the monitor.

 

[Lipoly]

I closed it after every rotation...I'm not seeing what you are seeing. There was no change in the picture after 16 rotations (closing and re-saving after each).

 

[Garbz]

Sure you're working with JPEGs?
Also try right clicking on an image and clicking "rotate clockwise" and see if the result is the same.
One more thing you can do to confirm what's going on is check the ExIF data. See if the orientation flag changes.

I just tried something else to see if another suspicion of mine was right and copied the image, rotated it once, then made another copy of it and rotated it 4 more times to see if the act of a complete rotation after a single window rotation produces the same file as just the single rotation. Result is nope:

Original: 955KB
1 CW rotation: 288KB
3 further CW rotations: 286KB

The JPEG algorithm also doesn't allow for reproducing the original file. It's simply not possible to open a JPEG and save it and have the data come out exactly as the original did. Even photoshop if you repeat this about 20 times you start getting severe image corruption. (edit apparently some clever algorithms can, photoshop does not use this).

Either your software is doing something strange (not saving) or your computer is magic

 

[Lipoly]

Yes, I'm working w/JPEGs.

I did some more testing.

1. I took an old photo from a 2.1MP Olympus camera (619KB original). After about 10 rotation/saves, it had obvious artifacting.

2. Took a pic from my D5000 (4433KB) and rotated it 32 times (4426KB) and could see zero difference in the photos...absolutely no artifacting, and no loss of detail. I studied each side-by-side at extreme zoom...no noticeable difference.

So, I'm wrong about Win7 using exif data. But, is there a chance that JPEG "fine" is lossless? I'm thinking no, but can't explain what I'm seeing.

It does seem that JPEG can be rotated in 90deg increments (and cropped) w/o re-encoding the image and w/o using EXIF : http://en.wikipedia.org/wiki/JPEG#Lossless_editinghttp://en.wikipedia.org/wiki/JPEG#Lossless_editing...maybe maybe that is what is going on (doesn't explain why one degrades, while another doesn't).

 

[Garbz]

Even if it were lossless the JPEG data would be different so your file comparing would have shown it. Also you said the size changed after you rotated it.

Can you do me a favour, open your D5000 image in a different app and save it with the best compression you can, quality 12 in photoshop if you have it. Then note the filesize before and after rotation?

Actually with the new version of photoshop out it's worth re-visiting the test to see if pohtoshop will still corrupt the image after multiple re-saves. I did that on CS2 as far as I remember.