The object of the present invention is to appropriately determine an attribute of a synthesized pixel at the time of synthesizing processing. An attribute of a pixel, synthesized based on a transmissivity of a source pixel, is determined in the following manner. A threshold value T is obtained from attribute designation data (S603), the transmissivity is compared with the threshold value T (S604), and if the transmissivity is larger than the threshold value T, an attribute of a destination pixel is determined as the attribute of the synthesized pixel (S605), while if the transmissivity is smaller than the threshold value T, an attribute of a source pixel is determined as the attribute of the synthesized pixel (S606).