Image analysis – Image compression or coding – Including details of decompression
Reexamination Certificate
1998-05-12
2001-01-16
Mancuso, Joseph (Department: 2723)
Image analysis
Image compression or coding
Including details of decompression
Reexamination Certificate
active
06175653
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention generally relates to processing compressed digital images. More particularly, this invention relates to methods and apparatus which combine rotation with decompression of compressed images for devices with asymmetric resolution. The rotation and scaling are built into the decompressing process to avoid spending time and effort in scaling and rotating the decompressed image.
2. Description of Related Art
Data compression is required in data handling processes, where too much data is present for practical applications using the data. Commonly, compression is used in communication links to reduce the transmission time or required bandwidth. Similarly, compression is preferred in image storage systems, including digital printers and copiers, where “pages” of a document to be printed are stored temporarily in precollation memory. The amount of media space on which the image data is stored can be substantially reduced with compression. Generally speaking, scanned images, i.e., electronic representations of hard copy documents, are often large, and thus make desirable candidates for compression.
The image compression standard disseminated by the Joint Photographic Experts Group (JPEG) committee is a compression technique which reduces data redundancies based on pixel-to-pixel correlations. Generally, an image does not change very much on a pixel-to-pixel basis and therefore has what is known as “natural spatial correlation.” In natural scenes, correlation is generalized, but not exact. Noise makes each pixel somewhat different from its neighbors.
Many output devices provide an output image having different spatial resolutions in pixels per inch (ppi) along the horizontal and vertical orientation. One example of such a device can be a printer employing 400 ppi×600 ppi resolution, i.e., the printer prints 400 ppi in the vertical direction and 600 ppi along the horizontal direction. One typical problem encountered when processing images directed for those devices arises when it is necessary to change the orientation of the image. In such an instance, in order to maintain the same aspect ratio while changing the image's orientation, the amount of image pixels in each direction has to be changed accordingly to cope with the change in resolution. For example, a rotation by 90° or an image transposition will change the orientation of the image. Examples of applications are those that require the image to be changed from “portrait” mode to “landscape” mode for printing, either to adjust the paper output requirements or by any aesthetic motivation.
FIG. 1
shows an example of an image being printed on a 400 ppi×600 ppi printer occupying a square region of 1 in.×1 in. If the image is simply rotated, maintaining the number of pixels, it would occupy a region of 2/3 in.×3/2 in. and be distorted. Thus, a spatial scaling operation is necessary to “stretch” the image in one direction and to “shrink” the image in the other direction to maintain the image's aspect ratio. Alternatively,
FIG. 2
shows a sequence to obtain the same output by pre-scaling the image before rotation.
SUMMARY OF THE INVENTION
In a conventional method to perform rotation for devices with asymmetric resolutions, an image source provides the compressed image data to the system. The image source can be an input device such as a camera or scanner, a transmission channel or a storage device. The compressed image data is input to a decompression unit that reconstructs the image. The uncompressed image is fed into the image rotation unit, whose output is sent to a scaling unit to correct the size and aspect ratio. The image output by the scaling unit is sent to an output image sink, which can be a storage device, a transmission line, or a display device such as a printer or monitor. The disadvantage of this method is that scaling and rotation operations are performed on the decompressed image. Color documents typically contain tens of millions of pixels, such that even simple operations on those image can be expensive and time consuming.
The system and method of this invention improve the performance of the rotation process, by reducing its complexity.
This invention is directed to reducing the computation effort spent in decompressing and rotating a compressed image. The image targeted to be printed in a device possesses asymmetric resolution, i.e. different resolution between the vertical and horizontal orientations. In this context, rotation is assumed to be an operation which rotates the image by +90° or −90° or even may represent image transposition. The operations can also be combined with image mirroring in the vertical or horizontal direction.
This invention provides an apparatus and method for converting, for example, a first image with unequal resolution into a second image with unequal resolution image while performing an image rotation. The conversion takes place in the JPEG decompression module. Image rotation is accomplished by modifying the signs of specific DCT coefficients before decompression and by re-arranging blocks. The scaling operation may precede or follow the rotation, as illustrated in FIG.
1
and
FIG. 2
, and is performed by modifying the inverse DCT operation. If the image was compressed using forward DCT of size M×M, scaling can be accomplished by replacing the M×M inverse DCT by other sizes. Unlike simple scaling processes, the scaling for rotation purposes often requires resolution changes by rational factors which do not fit in the DCT framework. This invention provides a method to scale the image by general rational factors by applying a set of inverse DCT matrices of different sizes.
In one preferred embodiment, rotation and scaling can be accomplished in the DCT domain by processing each block of M×M DCT coefficients of a compressed image. The DCT coefficients are modified to achieve intra-block rotation and are grouped into a group of blocks. The blocks in a group of blocks undergo an inverse DCT operation utilizing DCT operators of different sizes, while the resulting pixel blocks are arranged in a rotated macroblock. Rotated macroblocks are the components of the final rotated image.
By avoiding the very expensive process of scaling and rotating the decompressed image, this invention reduces considerably the computation necessary to rotate an image for an asymmetric device. Using this method, the rotated image has excellent quality, and artifacts caused by the different block sizes are generally avoided.
This technique can be implemented, for example, by modifying basic JPEG decompression engines, where image rotation is performed on the compressed image data (compressed units). Accordingly, the technique can be applied to any number of systems, including digital printers and copiers, that need to provide a rotated image. An apparatus implementing this invention can include data or image processing systems capable of compressing images. The techniques and systems described herein are advantageous because they are efficient and result in decreased time spent in scaling the decompressed image.
These and other features and advantages of this invention are described in or are apparent from the following detailed description of the preferred embodiments.
REFERENCES:
patent: 5020115 (1991-05-01), Black
patent: 5297217 (1994-03-01), Hamilton et al.
patent: 5867612 (1999-02-01), Robson
patent: 5911007 (1999-06-01), Miyake
patent: 6002810 (1999-12-01), Wakisawa et al.
Cooperrider F. E.
Mancuso Joseph
Oliff & Berridg,e PLC
Xerox Corporation
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