1. A method of generating a vector description of a colour gradient in an image, said method comprising the steps of: receiving the image comprising an array of pixels, each pixel having one or more associated colour values, the image having at least one gradient area in which at least one of the colour values varies between neighbouring pixels; generating, in each gradient area, a path comprising pixels having substantially the same colour; determining one or more control points defining a line substantially perpendicular to the path; calculating representative colour values at points along the line; and generating a piecewise-linear colour function from the representative colour values, wherein the one or more control points and the piecewise-linear colour function form said vector description of the colour gradient in the gradient area.