JPG > Crop > Save > Degrade?

NS
Posted By
Nehmo Sergheyev
May 12, 2004
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945
Replies
4
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Closed
In graphics programs like PhotoShop (I’m using v 8 cs), the general advice is to interim-save in the app’s native file format while working on a image. If you repeatedly save in JPG, the image degrades with each save. I understand that saving in PNG doesn’t degrade the image since it’s lossless, but you may lose something important to your work, like layers.

But what if you start with a JPG and then just crop it? Does the save at maximum quality just save the exact data from the cropped part, or does it resample and save, thus degrading?

What about other apps? MS Office picture manager and Paint Shop Pro? What happens when you crop and save in those?

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Hecate
May 12, 2004
On Tue, 11 May 2004 19:48:53 -0500, "Nehmo Sergheyev" wrote:

In graphics programs like PhotoShop (I’m using v 8 cs), the general advice is to interim-save in the app’s native file format while working on a image. If you repeatedly save in JPG, the image degrades with each save. I understand that saving in PNG doesn’t degrade the image since it’s lossless, but you may lose something important to your work, like layers.

But what if you start with a JPG and then just crop it? Does the save at maximum quality just save the exact data from the cropped part, or does it resample and save, thus degrading?

What about other apps? MS Office picture manager and Paint Shop Pro? What happens when you crop and save in those?

It doesn’t matter what app you use. Anytime you do anything other than just open a jpg i.e. anytime you actually do something to the file, and then resave it, image quality is degraded. Maximum quality doesn’t mean retention of all pixels, it just means the best you can get given that the image will be downsampled.

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Hecate

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XT
xalinai_Two
May 12, 2004
On Tue, 11 May 2004 19:48:53 -0500, "Nehmo Sergheyev" wrote:

In graphics programs like PhotoShop (I’m using v 8 cs), the general advice is to interim-save in the app’s native file format while working on a image. If you repeatedly save in JPG, the image degrades with each save. I understand that saving in PNG doesn’t degrade the image since it’s lossless, but you may lose something important to your work, like layers.

But what if you start with a JPG and then just crop it? Does the save at maximum quality just save the exact data from the cropped part, or does it resample and save, thus degrading?

What about other apps? MS Office picture manager and Paint Shop Pro? What happens when you crop and save in those?

Whenever cropping requires the image to be decoded into a pixelmap and re-encoded in JPG-blocks there is a chance of degradation.

If the cropping occurs without decoding along the borders of existing 8×8 pixel JPG-blocks ("Lossless cropping") there is no degradation beyond the existing image condition.

Michael
NS
Nehmo Sergheyev
May 12, 2004
– Kris Zaklika –
Take a look at Guido Vollbeding’s Jpegcrop (http://jpegclub.org/)

– Nehmo –
According to the site, ‘lossless’ cropping of JPEG images’ if you use (free) jpegcrop for Windows.

I’ve been opening my photos in MS Office Picture Manager (It opens quickly), where I do the initial crop. I’m now thinking that’s a mistake (if it resamples than saves). I should initially open with jpegcrop. Then later when I want to edit, in PSP or PS.

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* Nehmo Sergheyev *
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T
tacitr
May 12, 2004
But what if you start with a JPG and then just crop it? Does the save at maximum quality just save the exact data from the cropped part, or does it resample and save, thus degrading?

It degrades.

It’s very important that you use the correct terminology, though. JPEG cxompression does not "resample" the image. "Resample" means "change the number
of pixels." That is not what happens with lossy JPEG compression.

What about other apps? MS Office picture manager and Paint Shop Pro? What happens when you crop and save in those?

The image degrades.

There are programs specifically written to do lossless crops. These programs force you to crop in 8-pixel increments–the block size of the JPEG compression. If you want to crop 6 pixels off the image, then you must re-copress it, which causes it to degrade.

Photoshop, Picture It, et. al. do not offer lossless cropping.

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