A computer implemented image processing method comprising: obtaining microscope image data defining a microscope slide image of a haematoxylin and eosin stained tissue sample, wherein the microscope slide image data comprises a plurality of image chunks and each chunk comprises image pixel data representing a plurality of image pixels; storing, in a first computer memory location, chunk colour data comprising, for each image chunk, three colour component data elements representing a colour of the each image chunk in three colour components; calculating, from the chunk colour data, two largest contributions to the variations of the chunk colour data wherein each of the largest contributions comprise three colour component data elements representing the each contribution in said three colour components, and storing the two largest contributions in a second computer memory location; calculating, for each image chunk, a combination of the two largest contributions that approximates a representation of the three colour component data elements of the each image chunk, thereby to provide a plurality of approximated image chunks, and storing the approximated image chunks in a third computer memory location; determining, from the approximated image chunks: (i) a first item of approximated image chunk data, vMin, that is most similar to a first one of the two largest contributions, and (ii) a second item of the approximated image chunk data, vMax, that is most similar to a second one of the two largest contributions, wherein the first item of approximated image chunk data and the second item of approximated image chunk data together provide a mapping transformation; and storing the mapping transformation in a fourth computer memory location; for each image pixel, obtaining image pixel data comprising three colour component data elements of the each image pixel, and transforming the colour component data elements of the each image pixel using the mapping transformation to calculate normalised image pixel data for the each image pixel; obtaining, from the third computer memory location, the approximated image chunks, and comparing the approximated image chunks with an allowed colour range to identify the presence of artefact in the microscope image; and characterized in that in the event that an artefact is identified, at least one of the image pixel data and the chunk colour data is compared with artefact colour reference data to identify an artefact region of the microscope image that matches the artefact colour reference data. obtaining microscope image data defining a microscope slide image of a haematoxylin and eosin stained tissue sample, wherein the microscope slide image data comprises a plurality of image chunks and each chunk comprises image pixel data representing a plurality of image pixels; storing, in a first computer memory location, chunk colour data comprising, for each image chunk, three colour component data elements representing a colour of the each image chunk in three colour components; calculating, from the chunk colour data, two largest contributions to the variations of the chunk colour data wherein each of the largest contributions comprise three colour component data elements representing the each contribution in said three colour components, and storing the two largest contributions in a second computer memory location; calculating, for each image chunk, a combination of the two largest contributions that approximates a representation of the three colour component data elements of the each image chunk, thereby to provide a plurality of approximated image chunks, and storing the approximated image chunks in a third computer memory location; determining, from the approximated image chunks: (i) a first item of approximated image chunk data, vMin, that is most similar to a first one of the two largest contributions, and (ii) a second item of the approximated image chunk data, vMax, that is most similar to a second one of the two largest contributions, (i) a first item of approximated image chunk data, vMin, that is most similar to a first one of the two largest contributions, and (ii) a second item of the approximated image chunk data, vMax, that is most similar to a second one of the two largest contributions, wherein the first item of approximated image chunk data and the second item of approximated image chunk data together provide a mapping transformation; and storing the mapping transformation in a fourth computer memory location; for each image pixel, obtaining image pixel data comprising three colour component data elements of the each image pixel, and transforming the colour component data elements of the each image pixel using the mapping transformation to calculate normalised image pixel data for the each image pixel; obtaining, from the third computer memory location, the approximated image chunks, and comparing the approximated image chunks with an allowed colour range to identify the presence of artefact in the microscope image; and characterized in that in the event that an artefact is identified, at least one of the image pixel data and the chunk colour data is compared with artefact colour reference data to identify an artefact region of the microscope image that matches the artefact colour reference data.