Digital image processing

A non-transitory computer program product having stored thereon computer executable instructions that, when executed by a processor, cause a computing system to perform a method of introducing dither to reduce banding artefacts in a digital image, the banding artefacts resulting from an operation applied to the digital image to reduce the pixel bit depth, the bit depth reduction produces a reduced bit depth version of the digital image which results in pixel information loss, the method comprising, prior to encoding the digital image:

• applying a mask to the digital image, the mask indicating a pattern for selective adjustment of the pixel values in the reduced bit depth version of the digital image;
• and
• for each pixel in the digital image:
• determining the pixel's location in the mask;
• if the mask pattern indicates selective adjustment then:
• determining a pixel adjustment factor from the pixel information lost during the bit depth reduction;
• applying the pixel adjustment factor to the reduced bit depth version of the pixel;
• wherein the mask pattern indicates no selective adjustment for at least one pixel and indicates applying selective adjustment to at least one pixel in five in the horizontal direction, and at least one pixel in five in the vertical direction of the digital image.

A data processing apparatus comprising:

• a processor;
• and
• non-transitory computer readable memory having stored thereon computer executable instructions that, when executed by a processor, cause the data processing apparatus to perform the following:
• apply a mask to a digital image, the mask indicating a pattern for selective adjustment of pixel values in a reduced bit depth version of the digital image;
• and
• for each pixel in the digital image:
• determining the pixel's location in the mask;
• if the mask pattern indicates selective adjustment then:
• determining a pixel adjustment factor from the pixel information lost during the bit depth reduction;
• applying the pixel adjustment factor to the reduced bit depth version of the pixel;
• wherein the mask pattern indicates no selective adjustment for at least one pixel and indicates applying selective adjustment to at least one pixel in five in the horizontal direction, and at least one pixel in five in the vertical direction of the digital image.