Facsimile and static presentation processing – Static presentation processing – Attribute control
Reexamination Certificate
1997-05-28
2003-04-29
Moore, David (Department: 2624)
Facsimile and static presentation processing
Static presentation processing
Attribute control
C358S520000, C358S522000
Reexamination Certificate
active
06556311
ABSTRACT:
FIELD OF THE INVENTION
This invention relates in general to imaging systems and print resolution enhancement and, more particularly, to resolution enhancement for color printers.
BACKGROUND OF THE INVENTION
Electrophotographic processes for producing a permanent image on media are well known and commonly used. In general, a common process includes: (1) charging a photoreceptor such as a roller or continuous belt bearing a photoconductive material; (2) exposing the charged area to a light image to produce an electrostatic charge on the area in the shape of the image; (3) presenting developer particles (toner) to the photoreceptor surface bearing the image so that the particles are transferred to the surface in the shape of the image; (4) transferring the particles in the shape of the image from the photoreceptor to the media; (5) fusing or fixing the particles in the shape of the image to the media; and (6) cleaning or restoring the photoreceptor for the next printing cycle. Many image forming apparatus, such as laser printers, copy machines, and facsimile machines, utilize this well known electrophotographic printing process.
In laser printers, an image is typically rendered to form an image bitmap (or bit pattern) for subsequent transfer to the print engine for hardcopy output. The image bitmap is also referred to as a picture element (“pixel”) raster image and is stored either as a binary image bitmap or as a multi-bit per pixel image bitmap. In the rendering process (i.e., forming the bitmap), graphic elements, such as continuous lines (line art) and text character outlines are converted to pixel patterns that approximate the source image shape. Continuous tone data, such as photographic data (both color and gray value images) are also converted to pixel patterns that approximate the source continuous tone image data. However, to effectively portray the original source image for continuous tone data, each pixel of the source image must be represented by multiple bits which define either a color or a gray level. For example, when a multi-bit configuration of 8 bits per pixel is employed, 256 gray levels can be represented by the digital pixel values. In color images, 24 bits are typically used, with 8 bits for each of the color components, i.e., Red, Green, Blue (RGB), etc. Hereafter, it is to be understood that when the term “gray” is used, it applies to both color and black/white images and, when applied to a color image, relates to the luminance of the color.
Once a raster page buffer array (image) bitmap is generated from a source image, whether the image is text, line art, vector graphics or continuous tone data, the desired output image is created by causing a laser to be modulated in accordance with the bit pattern stored in the image page buffer array bitmap. The modulated laser beam is scanned across a charged surface of a photosensitive drum in a succession of raster scan lines. Each scan line is divided into the pixel areas dictated by the resolution of the bitmap and the pitch of the laser scan. The modulated laser beam causes some pixel areas to be exposed to a light pulse and some not, thus causing a pattern of overlapping dots on each scan line. Where a pixel area (dot) is illuminated, the photosensitive drum is discharged, so that when it is subsequently toned, the toner adheres to the discharged areas and is repelled by the still charged areas. The toner that is adhered to the discharged areas is then transferred to paper and fixed in a known manner.
In general, the fidelity of the output image relative to the source data is directly related to the resolution of pixels (dots) in the output image. Arbitrary analog images cannot be exactly reproduced by a bitmap raster unless an infinite resolution is used. For example, as a result of the images's pixel configuration, image edges that are either not parallel to the raster scan direction or not perpendicular to it appear stepped. This is especially noted in text and line art.
Various techniques have been developed to improve the quality of the output image of a raster bitmap. These enhancement techniques include: edge smoothing, fine line broadening, anti-aliasing (to smooth jagged edges), and increasing the resolution of the laser printer. These enhancing techniques typically modify (modulate) the signals to the laser to produce smaller dots that are usually offset from the pixel center, or in other words, to produce multi-level dots. However, most of the enhancing techniques operate on the data after it has already been rendered into a raster bit map, and hence after the fine detail has already been lost. Thus, most enhancing techniques employ interpolation methods upon the bitmap data to “best” recreate the original image. Furthermore, most enhancing techniques attempt to increase the virtual resolution of the image beyond the actual resolution of the print engine.
Although the prior art has attempted in a variety of ways to overcome the stepped appearance of pixel image edges for text and line art, an example of one of the more widely known techniques is described in U.S. Pat. No. 4,847,641 to Tung, the disclosure of which is incorporated in full herein by reference. Tung discloses a character generator that produces a bitmap of image data and inputs that bitmap into a first-in first-out (FIFO) data buffer. A fixed subset of the buffer stored bits forms a sampling window through which a selected block of the bitmap image data may be viewed (for example, a 9×9 block of pixels with the edge pixels truncated). The sampling window contains a center bit cell which changes on each shift of the image bits through the FIFO buffer. As the serialized data is shifted, the sampling window views successive bit patterns formed by pixels located at the window's center bit cell and its surrounding neighbor bit cells. Each bit pattern formed by the center bit and its neighboring bits is compared in a matching network with prestored templates. If a match occurs, indicating that the center bit resides at an image edge and that the pixel it represents can be altered so as to improve the image's resolution, a modulation signal is generated that causes the laser beam to alter the center pixel configuration. In general, the center pixel is made smaller than a standard unmodified bitmap pixel and is possibly moved within the confines of the pixel cell. The pixel size alteration is carried out by modulating the laser contained in the “laser print engine” of the laser printer. The system taught by Tung is now generally referred to as Resolution Enhancement Technology (RET) and enables substantially improved image resolutions to be achieved for text and line art over actual print engine resolution capability. It is the goal of RET to reduce the undesirable visual effects caused by printing continuous curves or slanted lines with discrete dots.
A significant drawback of conventional resolution enhancement technology is that it is not suitable for color imaging. Specifically, RET is applicable only to binary image bitmaps. On the other hand, color imaging utilizes multi-bit per pixel data to create the desired color image and, as such, is not typically processed through conventional RET.
For color imaging, many multi-bit per pixel color space schemes are known in the art. For example, RGB (Red, Green and Blue), CMYK (Cyan, Magenta, Yellow and Black), YCC (Y, Cb, Cr), YIQ and YES each describe different color models and represent just a few conventional examples. These models are typically described with a coordinate system, but each model may represent the color data differently. For example, in the RGB system, the lightness or darkness (i.e., luminance, or measure of how bright the color appears) of a pixel is contained in a combination of all three of the signals R, G, and B. However, in other systems (including YCC, YIQ and YES), the “Y” component describes luminance and the other components (i.e., CC, IQ and ES) each describe chroma (i.e., hue and saturation). In all of these models, each component is generall
Benear Richard H.
Nottingham James R.
Hewlett-Packard Development Co. L.P.
Moore David
Simmons Lane R.
Tran Douglas
LandOfFree
Luminance-based color resolution enhancement does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Luminance-based color resolution enhancement, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Luminance-based color resolution enhancement will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3016952