Variable compression encoding of digitized images

• US 6,144,772 A
• Filed: 01/29/1998
• Issued: 11/07/2000
• Est. Priority Date: 01/29/1998
• Status: Expired due to Term

• Current Assignee: Canon Kabushiki Kaisha (Canon Inc.)
• Original Assignee: Canon Kabushiki Kaisha (Canon Inc.)
• Inventors: Manickam, Olagappan, Garland, Harry T.
• Primary Examiner(s): Lee, Thomas D.
• Assistant Examiner(s): Chen, Wenpeng
• Application Number:
  US09/015,591
• Time in Patent Office: 1,013 Days
• Field of Search: 382/232, 382/128, 382/130, 382/131, 382/132, 382/240, 382/239, 382/244, 382/250, 382/256, 382/270, 382/272, 382/299, 378/901, 378/37
• US Class Current: 382/239
• CPC Class Codes: G06T 9/007 Transform coding, e.g. discrete cosine transform

Abstract

Regions of a source image (900) are encoded at different levels of image quality. A plurality of image quality levels are defined (1010). In addition, the source image (900) is divided into one or more regions (912, 914, 916) and one of the image quality levels is assigned to each region (1012). Next, the entire source image (900) is encoded, preferably with JPEG encoding, at the lowest image quality level Q₁, assigned to the image (1016). Then, the encoded source image (900) is decoded by a matching decoding process into a reference frame (1018). If the next highest image quality level Q₂ is lossy, then the reference frame and source image are scaled according to the image quality level Q₂ (1020). Then, differences between the source image and the reference frame are determined for all lossy regions assigned an image quality level greater than or equal to the image quality level Q₂, and stored in a differential frame (1024). The differential frame is encoded according to the image quality level Q₂ (1026), stored with the reference frame, then decoded to become the new reference frame (1028). These steps are repeated until the highest quality level assigned to a region is encoded. If the image quality level is lossless, then the reference frame is scaled to the resolution of the source image (1030), and the differential components are losslessy encoded (1038). When the variably encoded image is decoded, each of the selected regions (912, 914, 916) has the image quality defined by the assigned image quality level.

Claims

1. A method for variably encoding a source image, such method comprising the steps of:

defining a plurality of image quality levels each specifying a quality at which an image is encoded;

assigning, to each of a plurality of regions of the source image, one of the plurality of image quality levels; and

iteratively encoding, for each assigned image quality level and in increasing order of image quality level, each region of the source image having the assigned image quality level and those regions assigned a lossy image quality level at least as great as the assigned image quality level to produce an encoded image.

2. The method of claim 1, wherein the step of defining a plurality of image quality levels comprises the substeps of:

determining a resolution to which the image is scaled during encoding; and

determining an amount of frequency information in the image that is lost during encoding.

3. The method of claim 2, wherein the determined resolution scales the image by a power of two.

4. The method of claim 2, wherein no image frequency information is lost during encoding.

5. The method of claim 1, wherein the assigning step comprises the substeps of:

displaying the source image;

defining a border for each of the plurality of regions on the displayed source image; and

assigning, to each of the plurality of regions defined by a border, one of the plurality of image quality levels.

6. The method of claim 5, wherein the encoding step operates on blocks of the source image and further comprises the substep of:

adjusting the defined borders to include blocks of the source image through which the borders pass.

7. The method of claim 1, wherein the step of iteratively encoding each region of the source image having the assigned image quality level comprises the substeps of:

scaling the source image by a scale factor determined from the image quality level at which the region is being encoded; and

computing differential components for those regions assigned a lossy image quality level at least as great as the region being encoded to produce a differential frame.

8. The method of claim 7, further comprising the step of:

lossy compressing the differential frame, wherein an amount of information loss is determined from the image quality level.

9. The method of claim 5, further comprising the steps of:

decoding the single encoded image to a decoded image;

displaying the decoded image; and

superimposing, over the displayed decoded image, graphical data indicating the defined borders.

10. The method of claim 9, wherein the graphical data indicates the image quality level assigned to each of the plurality of regions.

11. The method of claim 1, wherein computer instructions for performing the method steps are stored on a computer-readable medium.

16. The method of claim 10, wherein computer instructions for performing the method steps are stored on a computer-readable medium.

12. A method for variably encoding a digitized source image, such method comprising the steps of:

selecting at least one region of the source image, wherein the at least one selected region is less than the entire source image;

encoding the source image at a first image quality level to produce a reference image;

assigning, to each selected region of the source image, a lossy image quality level higher than the first image quality level; and

for each assigned image quality level, and in increasing order of assigned image quality level, refining the reference image by iteratively encoding the selected regions having the assigned lossy image quality level and those regions assigned a lossy image quality level at least as great as the assigned image quality level, to produce an encoded image.

13. The method of claim 12, wherein the encoding step encodes blocks of the source image, the refining step encodes blocks of the source image, and the selecting step further comprises the substeps of:

determining borders of the at least one selected region of the source image; and

adjusting the borders to include the blocks through which the borders pass.

14. The method of claim 12, wherein the refining step comprises the substeps of:

generating differential components from the reference image and the selected regions to produce a differential image; and

storing the differential image with the reference image in a variably encoded image.

15. The method of claim 14, wherein the storing step comprises the substep of:

encoding the differential image and the reference image with JPEG encoding.

17. A computer-readable medium having computer program instructions recorded thereon for variably encoding a source image stored in a memory of a computer system, the instructions comprising:

instructions for defining a plurality of image quality levels each specifying a quality at which an image is encoded;

instructions for assigning, to each of a plurality of regions of the source image in the memory, one of the plurality of image quality levels; and

instructions for iteratively encoding, for each assigned image quality level and in increasing order of image quality level, each region of the source image having the assigned image quality level and those regions assigned a lossy image quality level at least as great as the assigned image quality level to produce an encoded image.