Crop JPEG without decreasing quality

Hi there,

I am looking for a way to remove the white stripes around my scanned
photographs. Cropping a file is not difficult, but when I use XnView for
example the file size increases after saving at almost 100%.

Why is this? I know that saving jpegs over and over, reduces the quality of
the picture, but how can the file size actually increase?

And even more important: How can I crop the jpeg without losing quality? If
this is not possible: How can I crop the jpeg without making the file size
increase?

Thanks for any hints,
Nico

Nicolas Mittelmaier wrote:

[Snip Cropping a JPEG and saving at 100%]

Why is this? I know that saving jpegs over and over, reduces the
quality of the picture, but how can the file size actually
increase?

Because it wasn't saved at 100% before.

And even more important: How can I crop the jpeg without losing
quality?

There's a list of such programs at
http://jpegclub.org/losslessapps.html

They all support lossless rotation and some support lossless cropping.

There's a list of such programs at
http://jpegclub.org/losslessapps.html

That's great, thanks.

Because it wasn't saved at 100% before.

Does this mean, that I can actually "upscale" the compression of a saved
jpeg ??? A jpeg with 75% quality can be saved to 100% of the original again?
I thought that using 100% on a 75% quality-jpeg would nearly (!) result in
the "full" 75% image again...

Thanks,
Nico

"Nicolas Mittelmaier" apparently said:

Hi there,

I am looking for a way to remove the white stripes around my scanned
photographs. Cropping a file is not difficult, but when I use XnView for
example the file size increases after saving at almost 100%.

Why is this? I know that saving jpegs over and over, reduces the quality of
the picture, but how can the file size actually increase?

It was (previously) saved with more compression, hence a smaller
file size. By increasing the quality level, you allow less
compression, so the file is larger.

And even more important: How can I crop the jpeg without losing quality? If
this is not possible: How can I crop the jpeg without making the file size
increase?

You can't do anything to a jpeg without losing quality. Jpeg is by
definition a "lossy" compression mode; every time you resave the
file, you lose more quality. I guess using 100% setting might be
an exception, but the point is that the jpeg algorithm works by
throwing away information.

If this is not possible: How can I crop the jpeg without
making the file size increase?

You're asking for a free lunch. If you want quality, you must
expect larger files. If small files are your goal, expect to lose
quality of the images. To maintain the integrity of your images,
save your original scans as TIFFs and only use JPG as the
last save before posting or emailing the image.

Nicolas Mittelmaier wrote:

Does this mean, that I can actually "upscale" the compression of a
saved jpeg ??? A jpeg with 75% quality can be saved to 100% of the
original again?

No, that's technically impossible.

Sorry about the confusion, I see that my explanation wasn't really an
explanation. I'll try again with an example:

1) We have the original raw bits produced by your scanning. Let's say
that there is 1000x1000pixels in 24bit color. The uncompressed size is
3MB.

2) You compress this to a 75% JPEG using XnView. The compressed size is
150KB.

3) You open the JPEG, crop a bit and save as 100% JPEG. The size jumps
to 400KB.

Now, one reasoning would be "There's only 150KB's worth of data left
after step 2. When I take something away (crop), there should be even
less". It does make sense for GIFs, PNGs, TIFF with LZW or other
formats that compresses losslessly, but JPEG is different.

When a JPEG is decompressed, the result is a 24bit image (unless you're
using grayscale JPEGs, but let's forget that for now). Some information
was lost at step 2, but that process isn't as simple as just reducing
colors.

For your program it's simple: It just opened an image and now has
1000x1000pixels at 24bit to work with. When you react step 3 it just
saves that image as if it were any 1000x1000pixel 24bit image.


Hmm... Not sure if that helped. I recommend reading the JPEG FAQ
instead (see below).

I thought that using 100% on a 75% quality-jpeg
would nearly (!) result in the "full" 75% image again...

That understanding is quite correct.


The "100%" is quite a source of confusion. It isn't 100%, it's just the
end of the scale for the program you're using. It might as well state
"Highest quality this program provides".

It certainly isn't 100% of the original image, unless the program
supports lossless JPEGs. Those programs that do that tend to state so
quite clearly. Lossless JPEGs aren't really that usefull, as the
support for such beasts are not very widespread.

I recommend taking a look at
http://www.faqs.org/faqs/jpeg-faq/part1/preamble.html
if you want more information on JPEGs.

Hmm... Not sure if that helped. I recommend reading the JPEG FAQ
instead (see below).

I think I got it. Thank you very much for your help. I will also take a look
at the faq.

I just discovered that my hp scanning software can also scan the photos as a
tiff. Although the file size is quite big, this will be the best solution. I
will burn them on a cd then.
When I finished any kind of editing I will use the "series convert" function
of xnview to get the jpegs for a website.

Thanks,
Nico