Image data compression

A compression unit configured to perform lossy compression on a plurality of blocks of image data in accordance with a multi-level difference table, wherein the blocks of image data relate to respective channels of multi-channel image data, wherein each block of image data comprises a plurality of image element values, and wherein each level of the multi-level difference table comprises a plurality of entries, the compression unit being configured to compress the block of image data for each of the different channels of the multi-channel image data independently by:

• origin value determination logic determining an origin value for the block of image data;
• level determination logic determining a level within the multi-level difference table for the block of image data;
• entry selection logic selecting, for each image element value in the block of image data, one of the entries at the determined level within the multi-level difference table;
• and
• compressed block formation logic forming a compressed block of data for the block of image data, the compressed block of data comprising:
• (i) data representing the determined origin value, (ii) an indication of the determined level, and (iii) for each image element value in the block of image data, an indication of the selected entry for that image element value;
• wherein said determining a level within the multi-level difference table for the block of image data comprises:
• determining a maximum difference between the determined origin value and any one of the image element values in the block of image data;
• and
• selecting, from the multi-level difference table, the level whose largest entry most closely represents the determined maximum difference.

A non-transitory computer readable storage medium having stored thereon a computer readable dataset description of a compression unit that, when processed in an integrated circuit manufacturing system, causes the integrated circuit manufacturing system to manufacture an integrated circuit embodying the compression unit, wherein the compression unit is configured to perform lossy compression on a plurality of blocks of image data in accordance with a multi-level difference table, wherein the blocks of image data relate to respective channels of multi-channel image data, wherein each block of image data comprises a plurality of image element values, and wherein each level of the multi-level difference table comprises a plurality of entries, the compression unit being configured to compress the block of image data for each of the different channels of the multi-channel image data independently by:

• origin value determination logic determining an origin value for the block of image data;
• level determination logic determining a level within the multi-level difference table for the block of image data;
• entry selection logic selecting, for each image element value in the block of image data, one of the entries at the determined level within the multi-level difference table;
• and
• compressed block formation logic forming a compressed block of data for the block of image data, the compressed block of data comprising:
• (i) data representing the determined origin value, (ii) an indication of the determined level, and (iii) for each image element value in the block of image data, an indication of the selected entry for that image element value;
• wherein said determining a level within the multi-level difference table for the block of image data comprises:
• determining a maximum difference between the determined origin value and any one of the image element values in the block of image data, and
• selecting, from the multi-level difference table, the level whose largest entry most closely represents the determined maximum difference.