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SoCoder -> Link Home -> Useful Apps

 
HoboBen
Created : 18 February 2010
 

Was it 'shopped? Image Error Level Analyser



https://errorlevelanalysis.com/permalink/26d2e6a/
Error level analysis can help you work out if a photo has been digitally altered. Photos in a jpeg file format actually lose quality each time they are resaved. We can take advantage of this to try and work out if an image has been digitally manipulated.

 

Comments
Thursday, 18 February 2010, 03:41
Jayenkai
 		"It works by resaving an image at a known quality, and comparing that to the original image. As a jpeg image is resaved over and over again, its image quality decreases. When we resave an image and compare it to the original, we can guess just how many times the image has been resaved. If an image has not been manipulated, all parts of the image should have been saved an equal amount of times. If parts of the image are from different source files, they may have been saved a number of different times, and thus they will stand out as a different colour in the ELA test."

If you save as .png the whole time, this isn't going to work, is it?
Thursday, 18 February 2010, 04:22
HoboBen
 		Photos will have been saved as JPG to begin with by the camera, and also saving photos as PNGs is going to take up a lot of space. Even if you save it as a PNG, it's been a JPG at least once so the information will be there.
Thursday, 18 February 2010, 04:52
Sticky
 		You can strip out all the metadata, and saving as JPEG does have a quality setting so if you just make sure you save with 100% quality then there shouldn't be any reduction, hence no noticeable difference in this test.
Thursday, 18 February 2010, 07:12
flying_cucco
 		Resaving a jpg amplifies the artifacts from the blocks used for compression. Scaling the image or applying smoothing should invalidate the results by removing those artifacts or unaligning them in progressive saves.
Thursday, 18 February 2010, 12:08
JL235
HoboBen Photos will have been saved as JPG to begin with by the camera, and also saving photos as PNGs is going to take up a lot of space.
For camera images, probably, but for images with large sections of colour PNGs are far more efficient then JPEGs.

Sticky You can strip out all the metadata, and saving as JPEG does have a quality setting so if you just make sure you save with 100% quality then there shouldn't be any reduction, hence no noticeable difference in this test.
Not true. Even 100% looses quality.

flying_cucco Resaving a jpg amplifies the artifacts from the blocks used for compression. Scaling the image or applying smoothing should invalidate the results by removing those artifacts or unaligning them in progressive saves.
Scaling and smoothing will obscure, but not remove the arftifacts.
Thursday, 18 February 2010, 13:21
Jayenkai
 		A Professional photographer would take shots in RAW format. And doing all your image editing in the editor's own layered lossless format makes perfect sense. I don't get why this is expected to work.
Nice tech, shame that other tech makes it flawed.
Thursday, 18 February 2010, 13:51
Phoenix
A Professional photographer would take shots in RAW format

Emphasis being on "professional." Non-fancy cameras (such as mine!) will save images as JPEGs, so it isn't completely useless.
Thursday, 18 February 2010, 14:21
flying_cucco
DiablosDevil Scaling and smoothing will obscure, but not remove the arftifacts.

Hmm. I have played about with it a bit, and you are right. Much better is to do the opposite and add noise. Nice and even now, just the red screen showing variance in error level (expected as jpg renders reds poorly).
Monday, 22 February 2010, 13:34
CodersRule
Emphasis being on "professional." Non-fancy cameras (such as mine!) will save images as JPEGs, so it isn't completely useless.


You don't need to have an amazing camera to save raw. My Canon Rebel can, so that's all I ever use... XD