Image analysis – Image compression or coding – Adaptive coding
Reexamination Certificate
1998-12-07
2001-07-03
Couso, Jose L. (Department: 2724)
Image analysis
Image compression or coding
Adaptive coding
C382S253000, C382S248000, C382S232000
Reexamination Certificate
active
06256421
ABSTRACT:
The present invention is directed to a method and apparatus for transmitting data without performing conventional data compression. More specifically, the invention accomplishes image compression by analyzing the content of an image and transmitting data that is closely matched to that which would be produced if conventional data compression was allowed to take place.
BACKGROUND OF THE INVENTION
The transmission of electronic data via facsimile machines and similar devices has become quite common. Efforts to transmit significantly larger volumes of this data within a substantially shortened period of time are constantly being made. This is true not only to allow data to be sent from one location to another at faster speeds and to cause less inconvenience to the user, but to enable more complex data to be transmitted between the same locations without drastically increasing the required transmission time. For example the facsimile transmission time for a detailed halftoned image will be many times more than that of a simple sheet of black text on a white page when using the same fax machine. By the same token, fax transmission of a color image will require an even greater amount of time than its greatly detailed halftoned counterpart. It is desirable to be able to transmit documents that contain these types of data—as well as others—within a short period of time.
Without any form of data reduction, transmission of color image data files via facsimile would require extensive resources—very fast modems and/or large buffers—and would still take a great deal of time. This would cause transmission of these large data files to become very expensive and therefore, impractical. Instead, the transmission of color image data via fax is typically accomplished using some form of data compression prior to transmission.
The JPEG (Joint Photographic Experts Group) standard provides a well known method of compressing electronic data. JPEG uses the discrete cosine transform (DCT) to map space data into spatial frequency domain data. Simply put, the first step in JPEG compression is to transform an 8×8 block of pixels into a set of 8×8 coefficients using the DCT. The DCT with the lowest frequency is referred to as the DC coefficient (DCC), and the remaining coefficients are AC coefficients (ACCs). The DCC and ACCs are quantized—divided by an integer referred to as the “step size” and rounded to the nearest whole number. The losses that occur during JPEG compression typically occur during the quantization step. The magnitude of this loss is obviously dependent upon the step size selected and the resulting amount of roundoff required to perform quantization.
Next, the quantized coefficients are arranged in a one dimensional vector by following a selected path (i.e. zigzag) through the 8×8 block of quantized coefficients. The DCC is typically the first value in the vector. Ordinary JPEG compression typically includes replacing the quantized DCC with the difference of its actual value minus the DCC of the previous block, to provide a differential DCC. Finally, the vector is encoded into a bit stream through a sequence of Run Length Counting (RLC) operations, combined with Variable Length Codes (VLC) to produce a compressed data stream.
Fax transmission of color image data is often accomplished by scanning the image at the sending fax to generate digital color image data, subjecting this digital color image data to JPEG compression and then transmitting the compressed digital color image data over telephone lines to the receiving fax. Since color image data is so complex, high compression ratios must usually be applied in order to complete transmission of a JPEG compressed file within an acceptable time frame. High compression ratios lead to more data loss, typically at the higher end of the frequency range. Further, the imaging devices typically included with fax machines in the lower end of the market usually include thermal ink-jet printers and would likely use error diffusion halftoning techniques. The halftoning that occurs when using a thermal ink jet printer results in an additional loss of high frequency data. Thus, much of the detail in the original image that is preserved and transmitted will never actually be viewed by the ultimate user.
The “sending” portion of fax transmission includes scanning the original image, generating a corresponding digital image, and reducing the data using any one of a number of techniques, one of which is JPEG compression described above. Once these steps are completed, the compressed data is transmitted serially to the receiving fax in a bit stream. The length of the bit stream used to describe the image is inversely proportional to the amount of compression that has been applied. Thus, if the compression ratio is large the length of the bit stream used to describe the image will be very short, resulting in a substantial reduction in the transmission time for the data stream.
With this in mind, successful fax transmission requires a proper correspondence between the compression ratio being applied to the image and the clock speed of CPU of the sending fax. In other words, if the compression ratio is smaller than necessary for a given CPU speed the data will have to wait to be transmitted, and an appropriately sized buffer will be required. On the other hand, if the compression ratio is high relative to the CPU speed the data will be compressed so much that the modem will become idle waiting for the CPU to complete image processing for the few bits that remain after compression. Since modems are typically configured to detect a large lapse in data transmission as the end of transmission, this large gap typically causes them to disconnect.
Thus, it is advantageous to continue the stream of data from the sending fax to the receiving fax, and eliminate gaps in the data stream. One way to do this is obviously to implement a faster JPEG compressor which can keep the data moving through the modem even if a high compression ratio is used. However, this solution results in significant cost increases and is often impractical. Thus, it is advantageous to provide a continuous stream of data during transmission of a color facsimile by transmitting stored data that emulates that produced by JPEG compression. This eliminates the need for actually performing JPEG compression, which as indicated above, can be a relatively time consuming task when large volumes of data are being processed.
In the present invention, blocks of data similar to those which would be produced by JPEG are stored in advance. The scanned image is then analyzed to assess its content, and the blocks that are similar to those that would be produced during JPEG compression of the image being processed are identified. The identified blocks are then retrieved from storage and transmitted to the receiving device instead of actual JPEG compressed original data. This means that full JPEG compression will not have to take place, and the amount of image processing time can be dramatically reduced.
All pixels, and therefore blocks of pixels, are defined using a certain number of bits. In an image processing operation known as vector quantization (VQ), a block of X×Y pixels is mapped to a single “codeword” which is defined using a smaller number of bits than the number required by the original block. Hierarchical Vector Quantization (HVQ) is used in the present invention to analyze the scanned image and select the most closely matching data block. HVQ block matching searches are performed two samples at a time. Thus, look up tables (LUTs) can be used directly to perform HVQ in two or more levels. In the first level, two image pixels are mapped to one codeword, reducing the number of samples by a factor of 2. In the next level, the process is repeated to map pairs of codewords to single codewords. As the process continues, the resulting codewords are mapped to larger and larger amounts of data. The codeword to which each pixel block is ultimately mapped is that associated with image data tha
Couso Jose L.
Do Anh Hong
Dudley Mark Z.
Waites Michelle W.
Xerox Corporation
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