Methods and decompression units for decompressing image data compressed using pattern-based compression

1. A method of decompressing a selected sub-block of image element values from a compressed block of image element values, the method comprising:

identifying, from the compressed block of image element values, a pattern of a plurality of patterns of image element values relevant to the selected sub-block;

identifying, from the compressed block of image element values, an image element value unit relevant to the selected sub-block, the image element value unit comprising a set of one or more image element values relevant to the selected sub-block; and

generating the selected sub-block from the pattern and the set of one or more image element values relevant to the selected sub-block.

2. The method of claim 1 , wherein:

the compressed block of image element values is divided into a plurality of sub-blocks;

the compressed block of image element values comprises information, for each sub-block of the plurality of sub-blocks, identifying an encoding format of that sub-block; and

the pattern relevant to the selected sub-block is identified from the information identifying the encoding format of the selected sub-block.

3. The method of claim 2 , wherein:

the compressed block of image element values comprises, for each sub-block of the plurality of sub-blocks, an image element value unit that comprises a set of one or more image element values associated with that sub-block; and

the image element value unit relevant to the selected sub-block is the image element value unit for the selected sub-block.

4. The method of claim 2 , wherein:

the information identifying the encoding format of a sub-block identifies (i) one pattern of the plurality of patterns or (ii) another sub-block that matches that sub-block; and

identifying, from the compressed block of image element values, the pattern of the plurality of patterns relevant to the selected sub-block comprises:

if the information identifying the encoding format of the selected sub-block identifies one pattern, identifying that pattern as the pattern relevant to the selected sub-block; and

if the information identifying the encoding format of the selected sub-block identifies another sub-block that matches the selected sub-block, identifying a pattern associated with the matching sub-block as the pattern relevant to the selected sub-block.

5. The method of claim 4 , wherein the information identifying the encoding format of a sub-block is a value that identifies (i) one pattern of the plurality of patterns; or (ii) another sub-block that matches that sub-block.

6. The method of claim 4 , wherein

the compressed block of image element values comprises, for each sub-block whose corresponding information identifying the encoding format of the sub-block identifies one pattern, an image element value unit that comprises a set of one or more image element values of that sub-block that form the identified pattern; and

the image element value unit relevant to the selected sub-block is the image element value unit associated with the selected sub-block or the image element value unit associated with the sub-block that matches the selected sub-block.

7. The method of claim 6 , wherein:

the plurality of sub-blocks are ordered;

the image element value units are stored in the compressed block of image element values in the order of their corresponding sub-blocks; and

the method further comprises identifying a location of the image element value unit relevant to the selected sub-block in the compressed block of image element values based on the information identifying the encoding format of each of the sub-blocks preceding the selected sub-block in the order.

8. The method of claim 7 , wherein identifying the location of the image element value unit relevant to the selected sub-block in the compressed block of image element values comprises identifying a number of image element values preceding the image element value unit relevant to the selected sub-block in the compressed block of image element values.

9. The method of claim 8 , wherein:

the information identifying the encoding format of one or more sub-blocks identifies a pattern of the plurality of patterns;

each pattern is formed by a number of image element values; and

identifying the number of image element values preceding the image element value unit relevant to the selected sub-block comprises summing the number of image element values forming the patterns identified by the information identifying the encoding format of each of the sub-blocks preceding the selected sub-block in the order.

10. The method of claim 6 , wherein the image element value units are stored in a body of the compressed block of image element values.

11. The method of claim 2 , wherein the information identifying the encoding format of a sub-block comprises a fixed-length encoding format field.

12. The method of claim 1 , wherein each pattern of the plurality of patterns defines a number of image element values and a location of those image element values in a sub-block.

13. The method of claim 1 , wherein each image element value is a colour value.

14. The method of claim 1 , wherein each image element value is a compressed value representing a colour value.

15. The method of claim 1 , wherein the image element values are image element values generated by a rasterization process on a graphics processing unit.

16. The method of claim 1 , wherein each sub-block comprises an N×M block of image element values wherein N and M are integers greater than or equal to one.

17. The method of claim 1 , further comprising outputting the generated sub-block.

18. A decompression unit to decompress a selected sub-block of image element values from a compressed block of image element values, the decompression unit comprising:

a pattern identification unit configured to identify, from the compressed block of image element values, a pattern of a plurality of patterns of image element values relevant to the selected sub-block;

an image element value identification unit configured to identify, from the compressed block of image element values, an image element value unit relevant to the selected sub-block, the image element value unit comprising a set of one or more image element values relevant to the selected sub-block; and

a sub-block reconstruction unit configured to generate the selected sub-block from the pattern and the set of one or more image element values relevant to the selected sub-block.

19. A non-transitory computer readable storage medium having stored thereon computer readable instructions that, when executed at a computer system, cause the computer system to decompress a selected sub-block of image element values from a compressed block of image element values as set forth in claim 1 .

20. A non-transitory computer readable storage medium having stored thereon a computer readable dataset description of the decompression unit as set forth in claim 18 that, when processed in an integrated circuit manufacturing system, causes the integrated circuit manufacturing system to manufacture an integrated circuit embodying said decompression unit.