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Images and Privacy
#1
JPEG images have a wealth of unnecessary information. I wouldn't say, that this information usually reveals the identity of the creator, but it is possible. In theory, a jpeg can have your full name, address, bank account, social security number, blood group and sexual preferences, among other, pretty much anything that someone would like to add. It could even contain a joke about privacy maniacs, even in French. But mormally, it rather contains information about the image processor, camera, size, date and the like.

Here a quick method to see (most of???) this info, if you have windows xp, should work on newer windows too:

Right-click an image, select "properties" and there "file properties". The text fields shown here, are usually empty. Now click "more" or whatever button you have to show more. This will show you a lot of information, most of which is not necessary to display the image properly.

If you like to remove all useless information, there is a small program available here:

http://www.sentex.net/~mwandel/jhead/

The website also explains, which information can be displayed, altered or removed on jpegs by this very program.

The program runs in a dos window and perhaps you need some explanation. Follow these steps for a simple start on windows XP:

- download the program "jhead"
- in explorer (the file explorer from windows), go to "C:\documents and settings\<username>, where <username> has to be replaced by your logon name for windows.
- create a new folder there, give it a short name, as you will have to type it soon - "a" will do perfectly.
- Copy jhead.exe (the program, you just downloaded) to this new folder.
- Copy all images you want to process or check to this folder too.
(do yourself a favor, and really copy, don't move the files. You can delete the originals later, if you like, but you can't retrieve any information that you are going to remove in the next steps).
- Push "ctrl+esc", then click on "run", enter "cmd" into the text field there, push enter. This will open a dos window at "c:\documents and settings\<username>".
- from now on, forget about your mouse, you have to type all commands here.
- type "cd a" and push enter. This assumes, that the new folder, you have created a few steps earlier, is called "a".
- type "jhead -purejpg *.jpg" and push enter, to remove all header data from all jpeg files in this folder. This step is irreversable, but that doesn't matter, since you are working on copies only (you didn't move the files, did you?). The jpegs are automatically updated in this new folder, the originals in their original places aren't touched.

If you want to change a single file, but have many files in the new folder, simply replace "*.jpg" by the complete file name. The asterisk "*" stands for "anything", so any file ending with ".jpg" will be altered if you use "*.jpg". There are more options, feel free to study the dos section in windows help.

Now you can look at the file information again, and you will see, that only some basic and vital information remains: width, height, vertical and horizontal resolution, colour depth and number of images (apparently, a jpeg file could contain more than one image).

For those, familiar with dos and dos programs, here a nifty command:
"jhead -h > jhead.txt
This will write the complete documentation of jhead into a text file "jhead.txt" and you can read this in notepad or any other text editor.
And you can add "> filename" to any command in dos, and all output will be written to the file "filename".

By the way, removing the information and the internal thumbnail will reduce the file size insignificantly.

PS: dos in this context has nothing in common with DOS-Attacks, which stand for "denial of service attacks" and are aimed to make a computer inoperational. Instead, dos here stands for "disk operating system" and is a more basic way of operating a computer, than windows. Still a product from Microsoft though.

PPS: If you have any questions about using the program, ask them here. In case you don#t get an answer within one or two days, feel free to send me a private message, I might not check this thread every day.

PPPS: DISCLAIMER
I am not the author of this program. It seems free from virus, trojans and any other malicious functions and it seems to work as described on the website and in the help section. I you have specific questions about the program, contact the author directly.
I checked all instructions given in this post and they worked well on my computer. However, path names can be different on your computer and in theory there could be reasons why these procedures won't work on your computer or even do damage. This is your risk. But it doesn't mean, that using the program really involves any risk, it rather means, that even after thorough tests, I can't exclude the possibility of surprising effects.
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#2
Also, an easy way to remove all the irrelevant information is to convert it to a format that cannot possibly contain such information, like BMP, and then back to JPG, in such steps that do not keep a cache of the lost information (such as saving the BMP in Paint, closing, opening the BMP and saving as JPG). This method works on any operating system that has some tool that can load&save image files in BMP and JPG formats.
Aside from BMP, you can also use PPM / PNM. Don't use GIF or PCX, for it will reduce the image to a paletted version with possibly significant loss of quality.
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#3
I usually use Adobe PhotoShop feature "Save for web". It strips pretty much everything. But, let me check, just in case ...
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#4
Yes, no info left in my published self-bondage photos ;-)
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#5
I made an interesting discovery. Download both pictures below and check their tags. On the first one, I added lots of silly stuff, from the second one, I stripped everything before uploading:

vintage.jpg thumbnail   

light.jpg thumbnail   

Hmmm, that is even more interesting! Last time, I uploaded a photo, all tags were gone, this time, they stayed.

By the way, I can confirm that photoshop's "save for web" writes no information to the image, it only puts the basic size and resolution stuff. But on that 3d-image (blog) of that Japanese virtual girl, it added a stamp (Photoshop 7).
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