Decompression of standard ADCT-compressed images

• US 5,379,122 A
• Filed: 10/02/1992
• Issued: 01/03/1995
• Est. Priority Date: 10/02/1992
• Status: Expired due to Term

• Current Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Original Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Inventors: Eschbach, Reiner
• Primary Examiner(s): Coles, Sr., Edward L.
• Assistant Examiner(s): NGUYEN, ANH-VINH THI
• Application Number:
  US07/956,128
• Time in Patent Office: 823 Days
• Field of Search: 358/426, 358/433, 358/427, 358/133, 358/141, 370/100, 348/384, 348/405
• US Class Current: 382/248
• CPC Class Codes: H04N 19/124 Quantisation
• CPC Class Codes: H04N 19/126 Details of normalisation or weighting functions, e.g. normalisation matrices or variable uniform quantisers
• CPC Class Codes: H04N 19/154 Measured or subjectively estimated visual quality after decoding, e.g. measurement of distortion use of rate-distortion criteria H04N19/147
• CPC Class Codes: H04N 19/527 Global motion vector estimation
• CPC Class Codes: H04N 19/60 using transform coding
• CPC Class Codes: H04N 19/86 involving reduction of coding artifacts, e.g. of blockiness

Abstract

A method of decompressing a document image compressed with JPEG ADCT or the like method of compression includes: a) receiving encoded quantized transform coefficient blocks for the original image; b) removing any lossless encoding of the quantized transform coefficient blocks for the original image; c) multiplying each quantized transform coefficient in a block by a corresponding quantizing value from the quantization table to obtain a block of received transform coefficients; d) recovering the image by applying an inverse transform operation to the received transform coefficients; e) reducing high frequency noise appearing in the recovered image as a result of the lossy quantization process, while preserving edges, whereby the appearance of the recovered image is rendered more visually appealing; f) changing the filtered recovered image into blocks of new transform coefficients by the forward transform coding operation using the frequency space transform compression operation; g) comparing each block of new transform coefficients to a corresponding block of received transform coefficients and the selected quantization table, to determine whether the filtered recovered image is derivable from the original image; and h) upon the determination transferring the filtered recovered image to an output buffer.

Claims

1. A method of improving the appearance of a decompressed document image while maintaining fidelity with an original document image from which it is derived, wherein for compression, an original document image is divided into blocks of pixels, said blocks of pixels are changed into blocks of transform coefficients by a forward transform coding operation using a frequency space transform operation, said transform coefficients subsequently quantized with a lossy quantization process in which each transform coefficient is quantized according to a quantizing value from a quantization table and the result is used as a quantized transform coefficient, the method including the decompression steps of:

a) receiving said quantized transform coefficient blocks for said original image;

b) dequantizing the transform coefficients in a block according to a corresponding quantizing value from the quantization table to obtain a block of received transform coefficients;

c) recovering the image by applying an inverse transform operation to the received transform coefficients;

d) with a selected non-linear filter, reducing high frequency noise appearing in the recovered image as a result of the lossy quantization process, while preserving edges, whereby the appearance of the recovered image is rendered more visually appealing;

e) changing the filtered recovered image into blocks of new transform coefficients by the forward transform coding operation using the frequency space transform compression operation;

f) comparing each block of new transform coefficients to a corresponding block of received transform coefficients and the selected quantization table, to determine whether the filtered recovered image is derivable from the original image; and

g) upon said determination transferring the filtered recovered image to an output buffer.

2. A method as described in claim 1, wherein step f) includes the additional steps ofdetermining that the block of new transform coefficients is not derivable from the original image;

and

2) altering individual new transform coefficients, so that a block of altered new transform coefficients is derivable from the original image;

3) recovering the image from the blocks of altered new transform coefficients.

3. A method as described in claim 1, wherein step f) includes the additional steps ofdetermining that the block of new transform coefficients is not derivable from the original image;

and

2) altering individual new transform coefficients, so that a block of altered new transform coefficients is derivable from the original image;

3) recovering the image from the blocks of altered new transform coefficients;

4) repeating steps d-f iteratively until a selected condition occurs.

4. A method as described in claim 1, wherein the forward transform coding operation using the frequency space transform operation is a discrete cosine transform.

5. A method as described in claim 1, wherein for compression, the quantized transform coefficients are further encoded with a lossless encoding method, and the additional step of removing any lossless encoding is provided prior to step b).

6. A method of improving the appearance of a decompressed document image while maintaining fidelity with an original document image from which it is derived, wherein for compression, an original document image is divided into blocks of pixels, said blocks of pixels are changed into blocks of transform coefficients by a forward transform coding operation using a frequency space transform operation, said transform coefficients subsequently quantized with a lossy quantization process in which each transform coefficient is quantized according to a quantizing value from a quantization table and the result is used as a quantized transform coefficient, and said blocks of quantized transform coefficients are encoded with a lossless encoding method, the method including the decompression steps of:

a) receiving said encoded quantized transform coefficient blocks for said original image;

b) removing any lossless encoding of the quantized transform coefficient blocks for said original image;

c) dequantizing the quantized transform coefficients in a block according to a quantization table to obtain a block of received transform coefficients;

d) recovering the image by applying an inverse transform operation to the received transform coefficients;

e) with a selected filter, reducing high frequency noise appearing in the recovered image as a result of the lossy quantization process, while preserving edges, whereby the appearance of the recovered image is rendered more visually appealing;

f) changing the filtered recovered image into blocks of new transform coefficients by the forward transform coding operation using the frequency space transform compression operation;

g) comparing each block of new transform coefficients to a corresponding block of received transform coefficients and the selected quantization table, to determine whether the filtered recovered image is derivable from the original image; and

h) upon said determination transferring the filtered recovered image to an output buffer.

7. A method as described in claim 6, wherein step g) includes the additional steps of:

1) determining that the block of new transform coefficients is not derivable from the original image; and

2) altering individual new transform coefficients, so that a block of altered new transform coefficients is derivable from the original image;

3) recovering the image from the blocks of altered new transform coefficients.

8. A method as described in claim 6, wherein step g) includes the additional steps of:

1) determining that the block of new transform coefficients is not derivable from the original image; and

2) altering individual new transform coefficients, so that a block of altered new transform coefficients is derivable from the original image;

3) recovering the image from the blocks of altered new transform coefficients;

4) repeating steps e-g iteratively until a selected condition occurs.

9. A method as described in claim 6, wherein the forward transform coding operation using the frequency space transform operation is a discrete cosine transform.

10. A method of improving the appearance of a decompressed document image while maintaining Fidelity with an original document image from which it is derived, wherein for compression, an original document image is divided into blocks of pixels, said blocks of pixels are changed into blocks of transform coefficients by a forward transform coding operation using a frequency space transform operation, said transform coefficients subsequently quantized with a lossy quantization process in which each transform coefficient is divided by a quantizing value from a quantization table and the integer portion of a result is used as a quantized transform coefficient, and said blocks of quantized transform coefficients are encoded with a lossless encoding method, the method including the decompression steps of:

a) receiving said encoded quantized transform coefficient blocks for said original image;

b) removing any lossless encoding of the quantized transform coefficient blocks for said original image;

c) multiplying each quantized transform coefficient in a block by a corresponding quantizing value from the quantization table to obtain a block of received transform coefficients;

d) recovering the image by applying an inverse transform operation to the received transform coefficients;

e) with a selected filter, reducing high frequency noise appearing in the recovered image as a result of the lossy quantization process, while preserving edges, whereby the appearance of the recovered image is rendered more visually appealing;

f) changing the filtered recovered image into blocks of new transform coefficients by the forward transform coding operation using the frequency space transform compression operation;

g) comparing each block of new transform coefficients to a corresponding block of received transform coefficients and the selected quantization table, to determine whether the filtered recovered image is derivable from the original image; and

h) upon said determination transferring the filtered recovered image to an output buffer.

11. A method as described in claim 10, wherein step g) includes the additional steps of:

1) determining that the block of new transform coefficients is not derivable from the original image; and

2) altering individual new transform coefficients, so that a block of altered new transform coefficients is derivable from the original image;

3) recovering the image from the blocks of altered new transform coefficients.

12. A method as described in claim 10, wherein step g) includes the additional steps of:

1) determining that the block of new transform coefficients is not derivable from the original image; and

2) altering individual new transform coefficients, so that a block of altered new transform coefficients is derivable from the original image;

3) recovering the image from the blocks of altered new transform coefficients;

4) repeating steps e-g iteratively until a selected condition occurs.

13. A method as described in claim 10, wherein the forward transform coding operation using the frequency space transform operation is a discrete cosine transform.