Street Photography Magazine

Invisible Watermarks | Tool Test

also more robust than the older technology

(now called ‘Classic’), but doesn’t function

with grayscale images, so Digimarc now offers

both. By default, the software watermarks RGB

images using the Chroma process, while

grayscale images still require the older

software.

The only way to switch between Chroma

and Classic modes is using the Ctrl+Alt+E

shortcut, which isn’t immediately obvious.

During the embedding process, the software

displays the watermark that is being applied,

but you still have to guess whether you are

looking at a Classic or a Chroma watermark

when it comes to reading one that has already

been applied. We tested both types and

found them to be equally robust. Most of

our attempts to remove the watermarks

from our high-resolution sample image were

ineffective, regardless of whether we tried

rotating, scaling or any one of a number of

potential hacks. Chroma is more resistant to

heavy image size reduction than Classic, but

a watermark created using the Chroma

process can still be completely removed by

simply converting the image that contains it

to grayscale.

All in all, we were pleasantly surprised by

the robustness and overall performance of the

Digimarc system. The inability of the Chroma

process to deal with grayscale images raises

the question, “Is it a bug or a feature?”, and

you will have to decide for yourself whether

this is a serious loophole, especially in view

of the fact that Classic mode is still resistant

to black-and-white conversion. Price-wise,

Digimarc is aimed directly at the commercial

end of the market, which makes its lack of

16-bit support all the more surprising.

A well-made digital watermark will be

invisible to the naked eye. However, it is

relatively simple to use Photoshopto

make the differences caused by the

watermark visible. Note that without

access to the original, unmarked image or

reader software from the watermark

manufacturer, it will be impossible to

detect a high-quality invisible watermark.

To make a watermark visible using

Photoshop:

1. Open the source image and the
   watermarked imagein a tiled view.


2. Copy the source image using the
   Image > Duplicate command and save
   the new ‘difference’ image using an
   appropriate name.
   3. Copy the watermarked image to the
   clipboard using Select > All followed by
   the Edit > Copy command.
   4. Create a new layer in the difference
   image using Layer > New Layer.
   5. Insert the contents of the clipboard into
   the new layer using Edit > Paste.
   6. Select Difference in the blend mode
   drop-down in the Layers panel. If the
   image contains a high-quality watermark
   it should appear completely black - i.e.,
   the changes the watermark has made to
   the original image data are invisible.
   7. Merge the two layers using the
   Layer > Merge Visible command.
   8. Open the Image > Adjustments > Levels
   dialog, and shift the white slider under
   the figure 255 to the left until it meets
   the black tone curve. The watermark will
   become visible once you have
   performed this radical adjustment.

Performing these steps should make the

differences between the source and

watermarked image data clearly visible in

the new ‘difference’ image. This test helps

you to judge how a watermark affects your

image data, and to determine which parts

of the image might suffer from unwanted

artifacts as a result.

Quick Tip: How to Make Digital Watermarks Visible

DIGITAL WATERMARK TOOL TEST

Program Digimarc Chroma 4 Digimarc Classic 4 SignMyImage 4.0

Manufacturer Digimarc Advanced Photo Tools

URL http://www.digimarc.com http://www.adptools.com

Minimum price

(incl. 1 year crawler subscription)

US$98 US$32

Test: Embedding Watermarks

JPEG yes yes yes

PNG yes yes yes

GIF no no no

TIFF 8-bit color yes yes yes

TIFF 16-bit color no no no

TIFF 32-bit color no no no

TIFF 8-bit B&W no yes no

TIFF 16-bit B&W no no yes

TIFF 32-bit B&W no no no

Test: Bit Depth Converions

8-bit RGB to 256 colors yes yes no

8-bit RGB to grayscale no yes no

8-bit RGB to 1-bit B&W no yes no

Test: Image Editing

Distortion correction yes yes no

Luminance noise filtering yes yes no

Color noise filtering yes yes no

Unsharp mask yes yes yes

Cropping from 4928 x3264 to

512 x512 pixels

yes yes no

Cropping from 4928 x3264 to

256 x256 pixels

yes yes no

Cropping from 4928 x3264 to

128 x128 pixels

yes yes no

Test: Scaling

Reduce to 91.5 % yes yes yes

Reduce to 50 % yes yes yes

Reduce to 25 % yes no no

Reduce to 11.75 % yes no no

Reduce to 10 % no no no

Test: Rotate

0.3 degrees yes yes no

1 degree yes yes no

3 degrees yes yes no

3 degrees yes yes no