Patent US 6,809,730

Three dimensional computer graphic images are shaded by first supplying data which defines the objects to be inserted into the image, this data including depth data relating to the objects. A depth store (4) includes data for objects already inserted into the image and a depth comparison unit (6) compares the depth of an object to be inserted with the currently stored depth data for a particular pixel. The result of this is used to determine which of a number of depth operations is to be applied to the depth store (4). A blending value is selected for each pixel for combining the object with the image in dependence on the selected depth operation. A combining unit then combines the object with the image in proportions dependent upon the selecting blending value.