Compressed MIP map decoding method and decoder

A method of sampling compressed image data, the compressed image data comprising interleaved blocks of data encoding a first image and blocks of data encoding differences between the first image and a second image, wherein the second image is twice the width and the height of the first image, the method comprising:

• identifying, in a fetch unit, four neighbouring pixels based on coordinates of a sample position;
• fetching, by the fetch unit, encoded data from the compressed image data for each of the four pixels, the fetched encoded data comprising, for each pixel, an encoded block of the first image and a sub-block from an encoded block of the differences between the first and second images, along with information on how to predict the second image from the first;
• decoding, in a first decoder, the fetched encoded blocks of the first image;
• decoding, in a difference decoder, the fetched encoded sub-blocks of the differences between the first and second images;
• outputting, from the difference decoder, a difference quad and a prediction value for each of the four pixels;
• and
• generating, in a filter sub-unit, a reconstruction of the image at the sample position using the decoded blocks of the first image, difference quads and prediction values.

An integrated circuit manufacturing system comprising:

• a non-transitory computer readable storage medium having stored thereon a computer readable description of an integrated circuit that describes a decoder unit;
• a layout processing system configured to process the integrated circuit description so as to generate a circuit layout description of an integrated circuit embodying the decoder unit;
• and
• an integrated circuit generation system configured to manufacture the decoder unit according to the circuit layout description,
• wherein the decoder unit comprises:
• a fetch unit arranged to identify four neighbouring pixels based on coordinates of a sample position and to fetch encoded data from compressed image data for each of the four pixels, the compressed image data comprising interleaved blocks of data encoding a first image and blocks of data encoding differences between the first image and a second image, wherein the second image is twice the width and the height of the first image and the fetched encoded data comprising, for each pixel, an encoded block of the first image and a sub-block from an encoded block of the differences between the first and second images, along with information on how to predict the second image from the first;
• a first decoder arranged to decode the fetched encoded blocks of the first image;
• a difference decoder arranged to decode the fetched encoded sub-blocks of the differences between the first and second images and output a difference quad and a prediction value for each of the four pixels;
• and
• a filter sub-unit arranged to generate a reconstruction of the image at the sample position using the decoded blocks of the first image, difference quads and prediction values.