Rotating Images (1 Viewer)

Does rotating a JPEG image recompress it and therefore loses quality?
Is it better to save it as a TIFF file first?

JP

yes and yes

A good point but I can't think of any technical reason why it should - once the digital file exists its data can be read, copied or manipulated I presume in any way necessary without further loss or degradation. You could check the file size before and after to find out if there is any variation, I suppose. I adjust many of my shots at least slightly and have never noticed any degradation - have you?

If you rotate the image you have altered it and when you save it in jpg it will chuck out some info which will degrade the picture - perhaps not a lot but it's something you can't put back later so it's always best to save the jpg straight from the camera and work on a copy.

I once took a series of shots in upright format and rotated them straight from the camera before saving and the file sizes were reduced to 800-900Kbs instead of the usual approx. 1.5Mb

Adey Baker said:

I once took a series of shots in upright format and rotated them straight from the camera before saving and the file sizes were reduced to 800-900Kbs instead of the usual approx. 1.5Mb

Click to expand...

There is normally an extra option when you save jpegs if you 'save as'. Normally a button 'advanced' or similar, whereby you choose the compression ratio; default is often 80% or something.

This may vary according to the software used.

Andy.

I just rotated a picture using PS and the new file size was larger. I would be surprised at any noticeable degradation. PS is used in industry and I can't see them accepting degradation from merely rotating an image. Once you have it as jpeg you have already lost some original image data - but, as has been discussed in another thread, of what consequence this is in practice is, for many people not important.

scampo said:

I just rotated a picture using PS and the new file size was larger. I would be surprised at any noticeable degradation. PS is used in industry and I can't see them accepting degradation from merely rotating an image. Once you have it as jpeg you have already lost some original image data - but, as has been discussed in another thread, of what consequence this is in practice is, for many people not important.

Click to expand...

First, always save the original JPEG and don't overwrite or alter it EVER! Then you always have the original data. Save alterations with names showing the file as a variation of the original.

I agree with Scampo about the degradation of JPEG. Although it is true that resaving an altered JPEG once again in JPEG format will cause the image to be recompressed and will cause a loss of some data, my experience when using the highest quality levels in Photoshop is that any change is not noticeable except through the use of extraordinarily precise comparison techniques.

A 5MP "fine" JPEG image from my CP5000 is typically 1.5MB as saved by the camera. Photoshop at the highest quality typically saves it as a 3.5-4MB JPEG file. A TIFF file using no compression scheme would be around 14MB. With LZW compression and TIFF, an image I just tested was 9MB.

I'm quite happy with the quality of Photoshop's best JPEG compression setting and believe (based on blind tests by myself and others) that the differences are visually imperceptible - even when viewed at 200-300% magnification in Photoshop. I consider the savings of 6-11MB per altered image to be well worth the extremely minor data loss.

Of course, using a lossless 24 bit format will guarantee no data loss due to the format. The question is whether you want to use three to four times the drive space.

I encourage anyone interested to test this on their own and decide for themselves. I suspect people will remain divided on this issue for quite a while though.

Some jpeg FAQs here:
http://graphicssoft.about.com/b/a/012182.htm
Doing ten or more arbitrary rotations on the same picture in your picture editor (not sane behaviour) might not give an outcome you would definitely call degradation of your picture; though you might think it looks a bit crooked. You would get noticeable blurring with Microgrfx Picture Publisher mind you:
http://www.path.unimelb.edu.au/~dersch/interpolator/interpolator.html
And if you absolutely insist on lossless archiving of your processed pictures and want to save a bit of space, there's always PNG or jpeg2000.
(o)<

As I said in another thread, I believe that the artefacts introduced by the very act of digitising an image are "degradation" of a kind. What the camera's sensor and Photoshop do, I should think, is make sure that their final product is acceptable to the human eye.

Many people regularly use image sharpening techniques, for example. Of course, this is image "degradation" when looked at logically, but it is image improvement when considered practically. Compromise is all, I suppose?

There are some programs, which claim to be able to make a "lossless rotation" to the original image from the camera/card. Eg these:
* ACDSee http://www.acdsystems.com/English/index.htm
* irfanview
* Jpegcrop http://sylvana.net/jpegcrop/

The last one does even "lossless cropping".
As a Mac user I have no personal experience on any of them but feel free to try.

Ilkka

Erm - that software upgrade was in 1998 - a long time ago in computer terms. I think PhotoShop or PS Elements might be a touch ahead of that, don't you?

scampo said:

Erm - that software upgrade was in 1998 - a long time ago in computer terms. I think PhotoShop or PS Elements might be a touch ahead of that, don't you?

Click to expand...

Maybe a long time but don't assume that all software has been 'patched' since

http://www.dpreview.com/news/0112/01121402nikonxpwarnings.asp

Andy.

Clearly the developers of EXIF should have tried harder!

(-:

jpoyner said:

Does rotating a JPEG image recompress it and therefore loses quality?
Is it better to save it as a TIFF file first?

JP

Click to expand...

Going back to the original question, the answer is actually, No and No.

Rotating the image does not recompress the image though it may cause the image data to be interpolated. This will depend on the degree of image rotation. 90 degree interval rotations are a special case and interpolation is not necessary and is in fact a slower process than the simpler transposition use for 90 degree intervals. Many programs (I suspect all or nearly all) take advantage of this fact and do not interpolate when images are rotated at 90 degree intervals.

Saving the image as a TIFF "first" brings no benefit. Saving it out as a TIFF afterwards might bring a very small benefit since the TIFF format is lossless.

So, if you open a JPEG, rotate it 90 degrees, save it as a TIFF, re-open the TIFF and rotate the image back 90 degrees, you will have an image identical to the original. At least this is the case in Photoshop.

As pointed out in the FAQ posted by Norm Jackson (http://graphicssoft.about.com/b/a/012182.htm), if the image is the correct pixel dimension and the software is appropriately designed, it is possible to save a 90 degree interval rotated image in JPEG with no loss as well.

If you rotate the image some amount other than a 90 degree interval (an arbitrary angle) such as 12.5 degrees, the image data will be interpolated and the original data will be altered.

Variants of the issue of the significance of JPEG and recompression loss question come up frequently. I finally decided to put together a simple page showing how small this loss really is when using Photoshop's highest quality setting. I hope that some will find it useful.

http://www.jayandwanda.com/Jpegtest/JPEGtest.html