Street Photography Magazine

No-one wants to purchase images that

contain visible watermarks. When you

purchase an image from a stock agency like

Fotolia, the image you receive will not have a

watermark and is simple to copy and use

illegally. This is the point at which invisible

watermarks come into play.

Hidden Labels

Wikipedia defines digital watermarks as

invisible markings made in carrier media such

as digital images. The visible watermarks

described above are created digitally, but can

be read using analog techniques. In other

words, a digital image that contains a visible

watermark can be analyzed without the use of

digital technology, and does not conform to

the generally accepted definition of a digital

watermark. In everyday situations, visible

watermarks are often incorrectly described as

digital watermarks.

It goes without saying that an invisible

watermark in a digital image shouldn’t be

immediately obvious to the viewer. As a

rule, the creation and reading of a digital

watermark requires the use of dedicated

software. Digital watermarks are generally

classified as either ‘fragile’ or ‘robust’, but both

types use steganographic methods to bind

the watermark with the medium it is designed

to protect. Copyright and camera metadata

for example, are fragile and simple to remove,

while invisible watermarks are robust and

much more difficult to manipulate.

Fragile Watermarks –

a Seal for Your Image File

Fragile watermarks are useful if you want to

prevent unwanted changes to an image file.

Even the slightest manipulation of a file

destroys the watermark, making the

manipulation traceable. Fragile digital

watermarks are thus equivalent to a seal in the

analog world. A seal on an envelope doesn’t

prevent the postal service from opening it, but

does prove to the recipient that the letter has

been tampered with. Fragile watermarks are

therefore a great device for checking and

documenting data integrity, but do not offer

direct data protection.

Robust Watermarks –

Hidden Proof of Ownership

The only thing that robust and fragile

watermarks have in common is that they are

invisible. Unlike fragile watermarks, robust

watermarks are designed to resist the

application of even the most abrasive

image manipulation techniques. Ideally, they

will survive format conversions and the

application of common image processing

steps, and even deliberate attacks. The basic

aim is to embed the watermark in such a way

that removing it will destroy the data that it is

part of. If successful, the protection process

makes a stolen image useless to a thief.

Uses for Invisible Watermarks

The wide range of potential uses for invisible

watermarks goes way beyond simple

detection of digital image theft, even as far as

tracing the paths taken by illegally copied

images. For example, if you attach images

with invisible watermarks to a press release,

you can quite easily perform a media

response analysis using crawler software.

Watermarks embedded in product photos

can also be used to analyze sales and

competition on the Web, and can help to

identify gray market importers who use a

manufacturer’s own images for advertising

purposes. Most manufacturers also prohibit

the use of official images for second-hand

sales – on eBay, for example – and watermarks

can help to stem this kind of usage.

Invisible Watermarks | Overview

Digimarc watermarks are not immediately visible ...

... and you can only see which pixels have been altered if you massively

increase the contrast of the difference image created by subtracting the

original image from the watermarked version