Get the Best out of SureSign - A Guide Get the Best out of SureSign - A Guide
During their life cycle, digital images are routinely cropped, resized, manipulated and saved using lossy compression schemes. All of these processes are inherently destructive to the image data and, consequently, to any embedded fingerprint data. However, SureSign fingerprints are considerably tolerant of image manipulation processes, and they can be detected in as little as 10% of the original fingerprinted image data (if the image sample is large enough - see Image Size below).
To help you get the most from using SureSign technology, please follow the guidelines set out below.
When to Apply a SureSign Fingerprint
Generally, a SureSign fingerprint should be applied after ALL editing and manipulation of the image has taken place and just before an image file passes out of your control (i.e. before image files are saved to a CD-ROM master file, uploaded to a Web site or delivered to a client via ISDN, etc.).
Excessive resizing makes it more difficult to detect a fingerprint in an image, so a fingerprint embedded in a 50MB file intended for print reproduction is unlikely to be readable if the image is resized down to 100K. A growing trend amongst content providers and media asset managers is to deliver a 'data package' of multiple resolution image files, each fingerprinted at the appropriate size - in this way, you get maximum effectiveness from the fingerprint.
Fingerprints in Resized Images
SureSign fingerprints are designed to be detectable in images that have been resized within a range of approximately 50-170% of their original size (although the range varies according to the information content). If you think that a fingerprinted image has been resized, but no fingerprint can be detected, resize the image as close to its original size as possible (if known) and then re-examine it for a fingerprint.
Lossy compression schemes such as JPEG (JFIF) discard varying amounts of image data, including fingerprint data. The amount of data loss is directly proportional to the level of compression selected, with the higher compression ratios losing greater amounts of image and fingerprint data. Successively decompressing and recompressing an image, i.e. opening a JPEG file and resaving it in a JPEG format, results in a degraded image and may render an embedded fingerprint undetectable. Therefore, if image quality and preservation of a fingerprint are more important than file size, consider using an uncompressed file format, such as TIFF.
If you need to save images in a compressed file format, use the SureSign fingerprinting tools as the final step before saving and, ideally, use a compression ratio of 10:1 or less (often designated as 'high' or 'excellent' quality) to preserve maximum image quality and the robustness of the fingerprint.
The SureSign fingerprinting algorithm operates principally on the luminance component, so SureSign fingerprinting tools work as effectively with greyscale as with colour images. Where greyscale image files have been converted from fingerprinted colour images (by discarding the colour information) the embedded fingerprint data is usually unaffected.
To fingerprint GIF images (for a web site, for example), avoid dithering when you convert to indexed colour as this can make a fingerprint more difficult to detect.
To search for a fingerprint on a GIF image, first convert the image to RGB. If the SureSign software can't find a fingerprint, try applying one or more 'Sharpen' filters and then retry the SureSign Detector. If you think the image has been dithered, try applying a 'Blur' filter before the 'Sharpen' filter and then run the SureSign Detector. It is possible to detect a fingerprint by repeatedly applying 'Sharpen' filters (as many as six or seven times) where dithering, indexed colour conversion and other changes have taken place to an image after it has been fingerprinted.
SureSign detection software is designed to decode fingerprints if the image is within approximately 1.1 degree of its original orientation or the image is at 90, 180 or 270 degrees of its original orientation. For images rotated outside these limits, return the image close to its original orientation BEFORE running the SureSign detector tools.
Recovering Fingerprints from Printed Media
One of the most remarkable characteristics of the SureSign technology is its ability to detect and recover fingerprint data embedded in images that have been reproduced in print media (hardcopy).
To detect a fingerprint on a printed image, scan the print sample using a good quality flatbed scanner with a spatial resolution of 300 ppi or higher - single pass scanners are preferable. Before scanning, align the image as squarely as possible on the scanner bed and make sure that it is held completely flat during the scan. If you use a graphics application, such as Adobe Photoshop, you can apply the SureSign Detector tool to all or part of the acquired scan. If the image has come from a book, magazine, etc. it will be screened, so you may have to apply a descreening filter if a fingerprint is not found first time.
If you are trying to recover fingerprints from printed material, the general rule is that the fingerprint data is easier to read from:
Tests have proved that it is possible to recover fingerprints from newsprint, but this is technically more demanding due to its lower quality print reproduction.
We advise you to keep records of SureSign usage as proof that your images have been fingerprinted - this is useful if copyright and contractual issues arise in the future. Holders of pursuit policies (such as "CoPS" from New Mexico Software), which provide legal assistance in the event of copyright infringements, may be required to keep these records as a matter of course.
Most of the SureSign applications allow you to record details (in a text format log file) of the image files they have processed. You should archive these log files regularly.
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