Text and image sharpening of JPEG compressed images in the frequency domain

• US 5,850,484 A
• Filed: 09/30/1997
• Issued: 12/15/1998
• Est. Priority Date: 03/27/1995
• Status: Expired due to Fees

• Current Assignee: Hewlett-Packard Company (HP Inc.)
• Original Assignee: Hewlett-Packard Company (HP Inc.)
• Inventors: Bhaskaran, Vasudev, Beretta, Giordano, Konstantinides, Konstantinos
• Primary Examiner(s): Couso, Jose L.
• Assistant Examiner(s): Bella, Matthew C.
• Application Number:
  US08/940,695
• Time in Patent Office: 441 Days
• Field of Search: 382/298, 382/233, 382/251, 382/244, 382/232, 382/253, 382/250, 382/274, 382/252, 382/238, 382/236, 382/166, 382/280, 382/270, 358/427, 358/426, 358/432, 358/261.3, 358/448, 358/261.1, 358/433, 358/261.2, 358/430, 358/458, 348/404, 348/432, 348/405, 348/433, 348/403, 348/391, 348/384, 348/422, 348/393, 348/430, 348/394, 348/409, 348/395, 348/390
• US Class Current: 382/250
• 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 19/60 using transform coding

Abstract

The text and image enhancing technique according to the invention is integrated into the decoding or inverse quantization step that is necessarily required by the JPEG standard. The invention integrates the two by using two different quantization tables: a first quantization table (Q_E) for use in quantizing the image data during the compression step and a second quantization table used during the decode or inverse quantization during the decompression process. The second quantization table Q_D is related to the first quantization table according to a predetermined function of the energy in a reference image and the energy in a scanned image. The energy of the reference image lost during the scanning process, as represented by the energy in the scanned image, is restored during the decompression process by appropriately scaling the second quantization table according to the predetermined function. The difference between the two tables, in particular the ratio of the two tables, determines the amount of image enhancing that is done in the two steps. By integrating the image enhancing and inverse quantization steps the method does not require any additional computations than already required for the compression and decompression processes.

Claims

1. A machine for transmitting color images comprising:

means for rendering a color image into color source image data representing the color image;

a compression engine that compresses the source image data into compressed image data, the engine including;

a transforming means for converting the source image data into transformed image data,

means for storing a first multi-element quantization table (Q_E);

a quantizer means for converting the transformed image data into quantized image data in accordance with elements contained in the first quantization table,

means for storing a second multi-element quantization table (Q_D) related to but nonidentical to the first quantization table Q_E,

an entropy encoder means for transforming the quantized image data into the encoded image data using an entropy table;

means for encapsulating the encoded image data, the second quantization table, and the entropy table to form an encapsulated data file; and

means for transmitting the encapsulated data file.

2. A machine for transmitting color images according to claim 1 further comprising a scaler for scaling the first quantization table (Q_E) in accordance with a predetermined function to form the second quantization table (Q_D).

3. A machine for transmitting color images according to claim 1 wherein the second quantization table is related to the first quantization table in accordance with a predetermined function of the energy in a reference image and the energy in a scanned image.

4. A machine for transmitting color images according to claim 3 wherein the predetermined function is given by the following expression:

space="preserve" listing-type="equation">Q.sub.D =S×

Q.sub.E,

where S is a scaling matrix having each element S k,l! formed according to the following expression;

space="preserve" listing-type="equation">S k,l!.sup.2 =V* k,l!/V.sub.Y k,l!,

where V* is a variance matrix of the reference image and V_Y is a variance matrix of the scanned image.

5. A machine for transmitting color images according to claim 1 wherein the means for rendering a color image into color source image data representing the color image includes:

a color scanner;

means for gamma correcting the color source image data; and

means for transforming the color source image data from a first color space to a second color space.

6. A machine for transmitting color images according to claim 1 wherein the means for rendering a color image into color source image data representing the color image includes computer software operable on a computer.

7. A machine for transmitting color images according to claim 1 further comprising:

means for receiving an encapsulated data packet;

means for decoding the encapsulated data packet to extract a header, received image data, a received quantization table (Q_R), and a received entropy table;

a decompression engine that decompresses the received image data into color source image data using the received quantization table and the received entropy table; and

means for reproducing a color image from the color source image data.

8. A machine for transmitting color images according to claim 7 further comprising a scaler for scaling the received quantization table into a scaled quantization table (Q_S), which is used by the decompression engine in lieu of the received quantization table.

9. A machine for transmitting color images according to claim 8 wherein the scaled quantization table is related to the received quantization table in accordance with a predetermined function of the energy in a reference image and the energy in a scanned image.

10. A machine for transmitting color images according to claim 9 wherein the predetermined function is given by the following expression:

space="preserve" listing-type="equation">Q.sub.S =S×

Q.sub.R,

where S is a scaling matrix having each element S k,l! formed according to the following expression;

space="preserve" listing-type="equation">S k,l!.sup.2 =V* k,l!/V.sub.Y k,l!,

where V* is a variance matrix of the reference image and V_Y is a variance matrix of the scanned image.

11. A machine for transmitting color images according to claim 10 further comprising means for selectively scaling the received quantization table responsive to the header.

12. A machine for transmitting color images according to claim 7 wherein the means for reproducing a color image from the color source image data includes computer software operable on a computer for displaying the color image on a computer display associated with the computer.

13. A machine for transmitting color images according to claim 1 wherein the transforming means includes means for transforming the source image data into transformed image data using the discrete cosine transform (DCT).

14. A method of compressing and transmitting images which produces decompressed images having improved text and image quality, the method comprising:

compressing a source image into compressed image data using a first quantization table (Q_E);

forming a second quantization table (Q_D), wherein the second quantization table is related to the first quantization table in accordance with a predetermined function of the energy in a reference image and the energy in a scanned image, such that the second quantization table (Q_D) is configured to compensate for image degradation caused by a scanner;

transmitting the compressed image data; and

decompressing the compressed image data using the second quantization table Q_D.

15. A method of compressing and transmitting images which produces decompressed images having improved text and image quality according to claim 14 wherein the step forming a second quantization table includes scaling the first quantization in accordance with the predetermined function.

17. A method of compressing and transmitting images which produces decompressed images having improved text and image quality according to claim 15 wherein the step scaling the first quantization in accordance with the predetermined function is performed subsequent to the transmitting step.

18. A method of compressing and transmitting images which produces decompressed images having improved text and image quality according to claim 15 wherein the step forming a second quantization table includes forming a second quantization table (Q_D), where the second quantization table is related to the first quantization table according to the following expression:

space="preserve" listing-type="equation">Q.sub.D =S×

Q.sub.E,

where S is a scaling matrix having each element S k,l! formed according to the following expression;

space="preserve" listing-type="equation">S k,l!.sup.2 =V* k,l!/V.sub.Y k,l!,

where V* is a variance matrix of the reference image and V_Y is a variance matrix of the scanned image.

20. A method of compressing and transmitting images having improved text and image quality according to claim 14 wherein the step of forming a second quantization table (Q_D) includes:

selecting a target image; and

rendering the target image into an image file.

21. A method of compressing and transmitting images having improved text and image quality according to claim 20 wherein the step of selecting a target image includes selecting a target image having critical image elements that are critical to the quality of the image.

22. A method of compressing and transmitting images having improved text and image quality according to claim 20 wherein the step of selecting a target image includes selecting a target image having text.

23. A method of compressing and transmitting images having improved text and image quality according to claim 20 wherein the step of selecting a target image includes selecting a target image having text with a serif font.

16. A method of compressing and transmitting images which produces decompressed images having improved text and image quality, the method comprising:

compressing a source image into compressed image data using a first quantization table (Q_E);

forming a second quantization table (Q_D), wherein the second quantization table is related to the first quantization table in accordance with a predetermined function of the energy in a reference image and the energy in a scanned image;

transmitting the compressed image data; and

decompressing the compressed image data using the second quantization table Q_D ;

wherein the step of forming a second quantization table includes scaling the first quantization table in accordance with the predetermined function prior to the transmitting step.

19. A method of compressing and transmitting images which produces decompressed images having improved text and image quality, the method comprising:

compressing a source image into compressed image data using a first quantization table (Q_E);

forming a second quantization table (Q_D), wherein the second quantization table is related to the first quantization table in accordance with a predetermined function of the energy in a reference image and the energy in a scanned image;

transmitting the compressed image data;

decompressing the compressed image data using the second quantization table Q_D ;

encapsulating the second quantization table Q_D with the compressed image data to form an encapsulated data file; and

transmitting the encapsulated data file.

24. A method of forming a quantization table for use in compressing images in accordance with the JPEG compression standard, the method comprising:

selecting a first quantization table (Q_E); and

forming a second quantization table (Q_D), where the second quantization table is related to the first quantization table according to the following expression;

space="preserve" listing-type="equation">Q.sub.D =S×

Q.sub.E,

where S is a scaling matrix having each element S k,l! formed according to the following expression;

space="preserve" listing-type="equation">S k,l!.sup.2 =V* k,l!/V.sub.Y k,l!,

where V* is a variance matrix of a reference image and V_Y is a variance matrix of a scanned image.

25. A method of forming a quantization table according to claim 24 wherein the step of forming a second quantization table (Q_D) includes:

selecting the reference image;

rendering the reference image into reference image data; and

computing the variance matrix V* for the reference image data.

26. A method of forming a quantization table according to claim 25 wherein the step of forming a second quantization table (Q_D) includes:

scanning the scanned image to produce scanned image data; and

computing the variance matrix V_Y for the scanned image data.

27. A method of forming a quantization table according to claim 26 wherein the step of scanning the scanned image to produce scanned image data includes scanning the reference image.

28. A method of improving text and image quality of compressed images that are compressed using the JPEG compression standard, the method comprising:

selecting a reference image;

determining the energy content of the reference image;

selecting a scanned image;

determining the energy content of the scanned image;

selecting a first quantization table (Q_E);

scaling the first quantization table (Q_E) to form a second quantization table (Q_D) according to the ratio of the energy in the reference image to the energy content of the scanned image, such that the second quantization table (Q_D) is configured to compensate for image degradation caused by a scanner;

compressing a source image in accordance with the JPEG standard using the first quantization table (Q_E); and

decompressing the source image in accordance with the JPEG standard using the second quantization table Q_D whereby the decompressed image has improved image quality.

29. A method of improving text and image quality according to claim 28 wherein the step of determining the energy content of the reference image includes determining a variance matrix (V*) of the reference image.

30. A method of improving text and image quality according to claim 29 wherein the step of determining the energy content of the scanned image includes determining a variance matrix (V_Y) of the scanned image.

31. A method of improving text and image quality according to claim 30 wherein the step of scaling the first quantization table (Q_E) to form a second quantization table (Q_D) according to the ratio of the energy in the reference image to the energy content of the scanned image includes:

determining a scaling matrix (S), where each element S k,l! being formed according to the following expression;

space="preserve" listing-type="equation">S k,l!.sup.2 =V* k,l!/V.sub.Y k,l!,

where V* k,l! is a corresponding element in the variance matrix V* and V_Y k,l! is the corresponding element in the variance matrix V_Y ; and

scaling the first quantization table (Q_E) to form a second quantization table (Q_D) according to the following expression;

space="preserve" listing-type="equation">Q.sub.D =S×

Q.sub.E.

33. A method of improving text and image quality according to claim 28 further comprising:

selecting a plurality of reference images;

determining the energy content of each reference image;

averaging the energy content of the plurality of reference images; and

scaling the first quantization table (Q_E) to form a second quantization table (Q_D) according to the ratio of the average energy in the reference images to the energy content of the scanned image.

34. A method of improving text and image quality according to claim 33 wherein the step of selecting a plurality of reference images includes selecting a plurality of reference images each reference image having a different graphical content.

35. A method of improving text and image quality according to claim 28 further comprising:

selecting a plurality of scanned images;

determining the energy content of each scanned image;

averaging the energy content of the plurality of scanned images; and

scaling the first quantization table (Q_E) to form a second quantization table (Q_D) according to the ratio of the energy in the reference image to the average energy content of the scanned images.

32. A method of improving text and image quality of compressed images that are compressed using the JPEG compression standard, the method comprising:

selecting a reference image;

determining the energy content of the reference image;

selecting a scanned image;

determining the energy content of the scanned image;

selecting a first quantization table (Q_E);

scaling the first quantization table (Q_E) to form a second quantization table (Q_D) according to the ratio of the energy in the reference image to the energy content of the scanned image;

compressing a source image in accordance with the JPEG standard using the first quantization table (Q_E);

decompressing the source image in accordance with the JPEG standard using the second quantization table Q_D whereby the decompressed image has improved image quality;

encapsulating the second quantization table (Q_D) with the compressed image to form a JPEG file; and

transmitting the JPEG file over a limited bandwidth channel.