From color to gray and back

What happens when you send your latest color presentation by fax to a colleague? He gets a black-and-white copy, and all your beautiful graphics have suddenly lost a large part of their information value. But now, researchers at Xerox Corporation have found a way to recover the original colors from a print made on a black-and-white printer, fax or copier. Instead of converting colors to different shades of gray, the Xerox researchers have developed a method to change each color into a different texture or pattern in an image. Theoretically, these algorithms could be integrated within the software of a black-and-white printer, but it will certainly take a while before such a method is adopted by Xerox's competitors.

Here is the introduction of the document from Xerox.

When a colored document is faxed on a black-and-white machine, is the color gone for good?

Not necessarily, according to Karen M. Braun, a Xerox Corporation imaging scientist and co-developer of the first way to encode documents so that the colors of the original image can be recovered from a print made on a black-and-white printer, fax or copier.

Karen Braun worked with Ricardo L. deQueiroz, from the Universidade de Brasilia in Brazil, to develop a new method to convert colors before a transmission by fax.

When a color image is copied, printed or faxed on a black-and-white device, the colors are converted to shades of gray. Two different colors with the same luminance - or perceived brightness - may "map" to the same shade of gray, making it impossible to interpret the information the colors carry. When that happens on graphics like pie charts or bar charts, two colors will look the same and the chart loses its information value.

While trying to figure out how to retain the information conveyed in color graphs and pictures, the researchers looked for new ways to represent color images in black-and-white. Their method turns each color into a microscopically different texture or pattern in the gray parts of an image. It makes it easy to identify colors with similar luminance value, making the pictures more pleasing and the graphs more useful.

So how this technology will be used for?

In practice, the part of the algorithms that code the colors could be integrated within the software of a black-and-white printer so colors could be transformed to textured grays. The decoding part of the algorithms could be part of a multifunction system's scanner, recovering the original colors so the document could be switched back to vivid color for display or print.

The research work has been recently described at the IS&T/SID's Color Imaging Conference (CIC), which was held on November 7-11 in Scottsdale, Arizona. The researchers discussed a paper named "Color to Gray and Back: Color Embedding Into Textured Gray Images."

Here is a link to the abstract of this paper which provides more details about the method.

We have developed a reversible method to convert color graphics and pictures to grayscale images. The method is based on mapping colors to low-visibility high-frequency textures that are applied onto the gray image. More specifically, the image is textured by carrying a sub-band (wavelet) transform and replacing band-pass sub-bands by the chrominance signals. The low-pass sub-band is the same as that of the luminance signal.

Alone, this algorithm can be used to convert color images to grayscale in a way that adds texture to increase discernability between regions of similar luminance but different color. However, the technique is also easily reversible.

After receiving a monochrome textured image, the decoder can identify the textures and recover the color information. The decoder performs a wavelet transform on the received gray image and recovers the chrominance channels from the appropriate wavelet bands. This allows color images to be archived in grayscale (useful for proceedings and other documents not printed in color) and converted back to color digitally.

For more information about this technology, Xerox has filed the patent #20050069197 which was published on March 31, 2005. Here is a direct link to this patent application. If the link is broken, go to the United States Patent and Trademark Office web site, click on the "Qick Search" link on the right part on the screen, and enter "20050069197" as your query on the next screen (without the quotes).

Will this technology be integrated into commercial products anytime soon? Xerox doesn't say anything about this. But I suspect that other manufacturers would also have to use this technology before it could be widely deployed. So I doubt that you could turn your black-and-white fax into a pretty colored hardcopy before a while.

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