Apparatus and accompanying method for converting a bit mapped monochromatic image to a grey scale image using table look up operations

• US 5,029,107 A
• Filed: 03/31/1989
• Issued: 07/02/1991
• Est. Priority Date: 03/31/1989
• Status: Expired due to Fees

• Current Assignee: International Business Machines Corporation
• Original Assignee: International Business Corporation (Kleangas Energy Technologies, Inc.)
• Inventors: Lee, Jack C.
• Primary Examiner(s): Harkcom, Gary V.
• Assistant Examiner(s): Nguyen, Phu K.
• Application Number:
  US07/331,485
• Time in Patent Office: 823 Days
• Field of Search: 364/518, 364/519, 364/521, 364/522, 340/747, 340/750, 340/721, 340/724, 340/728, 340/734, 382/46, 382/44
• US Class Current: 345/698
• CPC Class Codes: G06T 3/4023 based on decimating pixels or lines of pixels; based on inserting pixels or lines of pixels
• CPC Class Codes: H04N 1/40075 Descreening, i.e. converting a halftone signal into a corresponding continuous-tone signal; Rescreening, i.e. combined descreening and halftoning

Abstract

Apparatus and an accompanying method for converting a relatively high resolution halftone bit-mapped monochromatic document, such as illustratively a halftone separation which exists in a CDPF print file, into a relatively low resolution continuous tone grey scale document which, when the latter is applied to a display screen of a video monitor having appropriate grey scale capability, would provide a readable displayed page, i.e. a "preview", that approximately depicts how the bit-mapped document would appear when printed.

Claims

1. In a system that provides a high resolution bit-mapped pattern formed of single bit pels, a method of generating a reduced resolution grey scale "preview" image from said bit-mapped pattern, wherein the "preview" image substantially depicts how the bit-mapped pattern would appear when printed at said high resolution, said method comprising the steps of:

forming successive groups of a pre-defined number of vertically contiguous blocks of pels from said bit-mapped pattern, such that said groups are arranged horizontally across said pattern and form one of a plurality of contiguous horizontal strips of said bit-mapped pattern, wherein each block has a pre-determined number of pels that exist within said bit-mapped pattern;

determining a corresponding multi-bit grey scale value, in response to the number of pels that have a common binary value and are situated in each one of said blocks in said one strip, so as to produce a corresponding number of horizontal lines of successive grey-scale values in said "preview" image such that each of said grey scale values corresponds to a different one of said blocks and said grey scale values occur in the same order as said blocks corresponding thereto, wherein said determining step comprising the steps of;

successively applying each pre-defined cluster of vertically adjacent pels that is situated within each column of vertically contiguous pels situated within said one block as input to a pre-defined look-up table to determine an output count for said column, wherein said output count represents the number of pels in said column that have said common binary value;

accumulating the output count for every successive column in said block to provide the grey scale value corresponding to said one block; and

repeating said forming and determining steps for every successive one of said horizontal strips occurring in said bit-mapped pattern in order to vertically sequence through said bit-mapped pattern and generate corresponding lines of grey scale output values for each successive one of said strips so as to generate the "preview" image.

2. The method in claim 1 wherein said pre-defined look-up table comprises a plurality of separate pre-defined look-up tables, and said successively applying step comprises the steps of:

applying said each pre-defined cluster of vertically adjacent pels as input to a different corresponding one of said plurality of separate pre-defined look-up tables, and additively combining output values produced by said plurality of pre-defined look-up tables in order to produce said output count.

3. The method in claim 2 wherein said bit-mapped pattern is a bit-mapped halftoned document.

4. The method in claim 3 wherein said bit-mapped halftoned pattern is encoded as a print file in CDPF format.

5. The method in claim 4 further comprised of the steps of:

reading a pre-defined number of vertically contiguous and horizontally oriented sections of eight-bit vertically aligned pel scans from said print file, wherein said sections form a corresponding one of said strips in said pattern;

storing each one of said sections in a corresponding one of a plurality of input buffers;

pointing to one of said scans stored within each of said input buffers, all of said one scans being vertically contiguous in said one strip;

forming individual clusters of different successively occurring pels that collectively form all of said one scans, such that said individual clusters can be successively grouped to collectively form a pre-defined number of columns of vertically contiguous pels equal to the number of horizontal lines of successive grey-scale values in said strip of said "preview" image;

applying at least the pels that comprise each of said individual clusters to a different corresponding one of said plurality of separate pre-defined look-up tables so as to provide a count of the number of pels in said each individual cluster that have said common binary value; and

summing the counts produced by various ones of said plurality of separate pre-defined look-up tables to form corresponding ones of said output counts.

6. The method in claim 5 wherein each of the separate pre-defined look-up tables provides a count of the number of pels in a corresponding one of said individual clusters of pels that are darkened.

7. The method in claim 6 further comprising the steps of:

determining the associated grey scale value for each block in a group of vertically contiguous blocks in one of said strips;

pointing to a column in an output buffer;

loading the associated grey scale value obtained for each one of said blocks in said group into a separate corresponding location in said pointed column in the output buffer; and

incrementing the output pointer to point to a next successive column in said output buffer.

8. The method in claim 7 wherein each of the blocks is square shaped and the corresponding number of lines of grey-scale values produces for any one of said strips in said bit-mapped pattern is given by the expression 8*M/n, where, for a chosen value of n equalling a number of pels situated in each direction of one of said blocks, M is the number of vertical eight-bit bytes existing in said one strip and is the least number of eight-bit bytes whose bits are divisible by n.

9. The method in claim 8 wherein said bit mapped pattern is a bit-mapped halftoned separation.

10. In a system that provides a high resolution bit-mapped pattern formed of single bit pels, apparatus for generating a reduced resolution grey scale "preview" image from said bit-mapped pattern wherein the "preview" image substantially depicts how the bit-mapped pattern would appear when printed at said high resolution, said apparatus comprising:

means for forming successive groups of a pre-defined number of vertically contiguous blocks of pels from said bit-mapped pattern, such that said groups are arranged horizontally across said pattern and form one of a plurality of contiguous horizontal strips of said bit-mapped pattern, wherein each block has a pre-determined number of pels that exist within said bit-mapped pattern;

means for determining a corresponding multi-bit grey scale value, in response to the number of pels that have a common binary value and are situated in each one of said blocks in said one strip, so as to produce a corresponding number of horizontal lines of successive grey-scale values in said "preview" image such that each of said grey scale values corresponds to a different one of said blocks and said grey scale values occur in the same order as said blocks corresponding thereto, wherein said determining means comprises;

means for successively applying each pre-defined cluster of vertically adjacent pels that is situated within each column of vertically contiguous pels situated within said one block as input to a pre-defined look-up table to determine an output count for said column, wherein said output count represents the number of pels in said column that have said common binary value;

means for accumulating the output count for every successive column in said block to provide the grey scale value corresponding to said one block; and

means for causing said forming and determining means to vertically sequence through said bit-mapped pattern for every successive one of said horizontal strips occurring in said bit-mapped pattern in order to generate corresponding lines of grey scale output values for each successive one of said strips and thereby generate the "preview" image.

11. The apparatus in claim 10 wherein said pre-defined look-up table comprises a plurality of separate pre-defined look-up tables, and said successively applying means comprises:

means for applying said each pre-defined cluster of vertically adjacent pels as input to a different corresponding one of said plurality of separate pre-defined look-up tables, and means for additively combining output values produced by said plurality of pre-defined look-up tables in order to produce said output count.

12. The apparatus in claim 11 wherein said bit-mapped pattern is a bit-mapped halftoned document.

13. The apparatus in claim 12 wherein said bit-mapped halftoned pattern is encoded as a print file in CDPF format.

14. The apparatus in claim 13 further comprising:

means for reading a pre-defined number of vertically contiguous and horizontally oriented sections of eight-bit vertically aligned pel scans from said print file, wherein said sections form a corresponding one of said strips in said pattern;

a corresponding one of a plurality of input buffers for storing each one of said sections;

means for pointing to one of said scans stored within each of said input buffers, all of said one scans being vertically contiguous in said one strip;

means for forming individual clusters of different successively occurring pels that collectively form all of said one scans, such that said individual clusters can be successively grouped to collectively form a pre-defined number of columns of vertically contiguous pels equal to the number of horizontal lines of successive grey-scale values in said strip of said "preview" image;

means for applying at least the pels that comprise each of said individual clusters to a different corresponding one of said plurality of separate pre-defined look-up tables so as to provide a count of the number of pels in said each individual cluster that have said common binary value; and

means for summing the counts produced by various ones of said plurality of separate pre-defined look-up tables to form corresponding ones of said output counts.

15. The apparatus in claim 14 wherein each of the separate pre-defined look-up tables provides a count of the number of pels in a corresponding one of said individual clusters of pels that are darkened.

16. The apparatus in claim 15 further comprising:

means for determining the associated grey scale value for each block in a group of vertically contiguous blocks in one of said strips;

an output buffer;

means for pointing to a column in the output buffer;

means for loading the associated grey scale value obtained for each one of said blocks in said group into a separate corresponding location in said pointed column in the output buffer; and

means for incrementing the output pointer to point to a next successive column in said output buffer.

17. The apparatus in claim 16 wherein each of said blocks is square shaped and the corresponding number of lines of grey-scale values produced for any one of said strips in said bit-mapped pattern is given by the expression 8*M/n, where, for a chosen value of n equalling a number of pels situated in each direction of one of said blocks, M is the number of vertical eight-bit bytes existing in said one strip and is the least number of eight-bit bytes whose bits are divisible by n.

18. The apparatus in claim 17 wherein said bit mapped pattern is a bit-mapped halftoned separation.