Why a Copier Is Not a Printer

The HP CopyJet Color Printer-Copier

Although the 2-in-1 HP CopyJet printer-copier uses the same print engine for both printing and copying, they are in reality two separate and distinct functions that receive their input from very different sources. Understanding how copying is different from printing may help to explain why the output of the CopyJet looks different for each function.

When working as a printer, the CopyJet receives text, graphics and images from a computer as individual digital objects to be placed on a page. Each type of object is recognizable by the printer and can be treated differently using HP's ColorSmart technology to achieve optimal output quality. When working as a copier, however, the CopyJet, like all digital copiers, uses its flatbed scanner to digitize all the text, graphics and images on the original into a single raster image. The scanning process produces a digital representation of the original that may vary significantly in some respects. Also, the scanning process itself cannot distinguish between text, graphics or images, making it much more difficult to optimize the output.

The scanning process, by its very nature, creates a digital approximation of the original, and is influenced by the original's color content, the mixture of text, graphics and images, the type of ink used to print it, the paper it's printed on, and the printing process used to create it. Each of these can affect the appearance of the copy to varying degrees.

Metamerism -- The appearance of color depends on the characteristic response of the detector -- the eye or the scanner -- and the light used for illumination. The human eye's sensitivity to individual colors and the light used the view an original are not the same as those of a scanner, and so the same colors will appear differently to each. The effect of seeing colors differently under different viewing conditions is called metamerism.

People are often not aware of metamerism because the brain tends to compensate for it, but sometimes it is quite apparent. Clothing that seems to match well in the store may not match outdoors. A red car may suddenly appear white under sodium vapor street lamps. Color photographs taken in tungsten or fluorescent light with "outdoor" film take on an unusual cast.

Because a scanner sees colors differently from the human eye, a copier must compensate for this difference to achieve accurate color output. This would be simpler to do if the copier's print engine could reproduce all the colors the scanner could see, but this is not the case.

Gamut Mapping -- The scanner can sense many more colors than the print engine can produce. If the scanner sees a color not available from the print engine, an available color must be substituted for it. Determining the difference between what the scanner can sense and what the copier can print is pure science. Determining which colors to substitute is much more of an art.

People expect to see certain colors reproduced accurately, such as skin tones, the sky, vegetables and other "memory" colors. They also have an inherent sense of what a "good" red looks like, for example. While accuracy of color reproduction is important, HP research has shown that people consistently prefer a slightly different, brighter color over a perfectly matched, but less vivid color. In other words, a convenience copier like the CopyJet should produce color copies that are pleasing to the eye.

The CopyJet maintains accurate color whenever possible, preserving important memory colors while favoring more vibrant renditions of other colors. When a color must be changed, it is done in the most intelligent way. The CopyJet also compensates for light colors that tend to produce grainy output. User controls for red, green, blue and overall color brightness, as well as emphasis on light colors, can be set for special requirements.

Black and White Text -- A color scanner senses the original source being copied as a mesh of individual pixels of varying hues, saturation and intensity. When copying areas of black on white text, especially small characters, the scanner sees pixels solidly over a character as dark and pixels solidly over the background as light. The pixels straddling the boundary, covering part of the character and part of the background, become various shades of gray, and often take on a hue due to variations during the scanning process. As a result, the scanner sees dark, gray-green, fuzzy characters with a colored fringe around them on a light yellow background instead of the crisp, black characters on a pure white background the human eye sees.

HP's Text Enhancement technology addresses a major constraint imposed by the scanning process by identifying areas of text and treating them separately from the rest of the page to maintain the sharpness, crispness and blackness of the original. Most copiers make no attempt to do this, and black and white text copied on most other color copiers can be less than satisfactory.

Text Enhancement technology uses HP-developed statistical techniques and algorithms to identify and separate areas of text from the rest of the page, and then applies special processing to enhance these areas. The enhanced text is then recombined with the rest of the page so that the CopyJet will reprint it the way the human eye expects to see it, improving the overall appearance of the copy.

Moiré Patterns -- Sometimes an interaction between the scanner and the printed original can cause the appearance of stripes and dot patterns on the copy. These moiré patterns tend to occur most often when the resolution of the printed original is close to (or an even multiple of) the scanner's resolution, and are most pronounced on lightly colored areas.

Common to all copiers, moiré patterns may be very noticeable when copying a 300 dot-per-inch original that was printed on the CopyJet, another inkjet printer or on a laser printer. Zooming in or out slightly when making a copy can sometimes compensate for this phenomenon.

Metamerism, gamut mapping, black and white text and moiré patterns affect all copiers, and depending on how they are dealt with, can impact the overall quality of the copier output to a greater or lesser degree. Every copier maker is faced with a similar set of challenges to achieving optimal output. Each must decide how best to treat the single scanned image to compensate for the scanning process and for the differences between the scanner, the output device and the human eye. And each will approach these challenges differently.

Printers, on the other hand, have a much easier time dealing with the individual objects on a page. As a result, the output from the HP CopyJet will often look different when printing and copying even though both functions use the same inkjet print engine for their output.