1. Field of the Invention
This invention relates to improvements in the encoding scheme of gray pixels. More particularly, this invention relates to gray pixel encoding schemes which are used in combination with a halftoning imaging that produces gray level halftoned images.
2. Description of Related Art
Xerographic printers use halftoning processes to generate digital images from originals. The image is formed by using pixels which are completely black, completely white or different shades of gray. These are represented as explicit gray pixels. When viewed from a distance, the pixels blend together to form the image.
In traditional binary xerographic printers, halftone methods use binary systems such that the laser has only two laser intensity levels: ON (black) and OFF (white). Another method is disclosed in U.S. Pat. No. 4,868,587 (Loce et al.), which uses two gray levels: the black and the white level for each pixel. The different levels of gray are based on the intensity of the laser light. Therefore, black has a higher light intensity than either of the gray levels. This method is an improvement over using only black and white pixels. The disadvantage of the system is that the control of the graying levels is difficult and the information is not easily transferable to other printers.
The quality of a binary xerographic printer is based on two important features: the halftone frequency, which is the number of halftone cells per linear inch; and the number of distinguishable gray steps. A typical number of halftone cells for good quality image is between 100 to 200 cells per inch. Usually around nine pixels are used per halftone cell. The maximum number of gray steps is limited to the number of pixels per halftone cell plus one. Therefore, a 3.times.3 halftone cell has about 10 output gray steps.
In the prior art, a halftone cell was divided into pixels which were turned either on or off to form reflectance modulation level. From a distance, the human eye detects a reflectance of about 0.5%, which would be interpreted as a gray level. In a good quality printer, the number of distinguishable gray steps should be around 100.
Halftone images are commonly used in printed materials to reproduce continuous tones using printers that are binary in nature. Conventional digital halftoning processes grow halftone dots by activating individual pixels within a halftone cell. The pixels are typically at printer resolution or at some small integral subdivision thereof. The printer resolution is broken down to the number of spots which are used. For example, a 400 SPI printer has a spot size of 1/400th of an inch. Pixels are used as data within the spot boundary.