How can I process an image to remove a watermark within my iPhone application?

Question

I want to remove watermark from a picture within my iPhone / iPad application. Is there any kind of image processing I can perform within this application to do this?

Answer 1

Can't be done, sorry.

The watermarked image were originally two images (the base and the watermark), which were merged together to form the result. The problem here is that the most common image formats (such as JPG, PNG, or GIF) have no concept of layers - so that the base would be one layer, and the watermark another: the result is just one layer, onto which both were redrawn. This is somewhat similar to a physical painting: if you paint one image on a paper using watercolors, and then another over the same spot, their colors will mix and you won't be able to tell which parts belong to one or the other, as they'd become a single image.

This is similar with the computer image formats: there is only one "layer", which for every pixel encodes exactly one color that is there - only the current color exists, and the image doesn't keep track what was on that pixel before.

Now, the information is irreversibly lost from the result - in other words, it is not possible to recover the base knowing just the result (or the result and watermark) - BTW, that's exactly the point of watermarking.

I have borrowed the image sprites of StackOverflow for a demonstration; the actual images used are not unique, the technique would work just as well with any images. This was the watermark I used:

And this is the result image, after merging with the base:

Now, even though we have the exact watermark image used, there's no way to recover what was underneath that star in the original image. Through image processing operations, we could almost remove the star from the result, but there's not enough data to tell us what used to be underneath: - that information got erased in the merge at the beginning.

We could guess what used to be there, but then we're not doing recovery any more, we're interpreting the image and guessing what possibly could have been there - and that's pretty hard, even for a human; computers are really bad at that. This is the original image, before I watermarked it - I bet you were expecting something slightly different, no?