Student Speak: Encoding digital images

Advances in computer software are making it easier and easier to alter photos. Such changes can be a cause for concern when applied to sensitive information such as medical scans, fingerprints, photographs with the intention of deceiving.

Researchers have thus devised a way to encode digital images to detect fraud. The new technique embeds a Computer Generated Hologram (CGH) into an image.

Traditional holograms were created by recording the interference pattern generated by a reference beam and by another beam reflected from the object being imaged.

CGH simplifies the process because it does not require physical interference between two beams. The technique is based on the simulation of the pattern of light waves that are recorded when a real hologram is made.

The hologram - usually a simple word or picture - hides in the ‘noise', which is the random pixel-to-pixel variations that one's eye usually glosses over.

When the CGH is thus added to an image it replaces the image ‘noise' that has been filtered out beforehand.

The CGH information is in the same high-frequency band as the noise, so it is invisible to the human eye when added to the photograph.

To view the CGH, a numerical code is needed to reconstruct it from the image file. In this way CGH acts like an invisible ‘watermark' - even the slightest change to the original picture will destroy the hologram.

This watermark could be used by defence agencies that want to protect satellite images from manipulation or by media organisations to verify a picture's authenticity.

To prevent someone from doctoring an image and then correcting the CGH, the watermark is encrypted.

Therefore, only those who have the secret ‘key' can actually extract the hologram from the image to see it. Besides, the encryption makes it difficult to detect whether an image has been watermarked or not.

Forgery can then be detected by using a computer to extract the watermark and check it for damage. In a forged picture, it can even identify the object or section which has been tampered with.

Besides authenticating photos, the system could protect fingerprint records and medical scans, facilitating the storage and transfer of this information for court cases and other legal proceedings.

The writer is an electronics/ communication engineering graduate from Ajman University of Science and Technology Network