In an apparatus for composing image data of a main picture and image data
of a sub-picture by a chroma key process, an attribute of an image
contained in the sub-picture is detected, a partial area of the
sub-picture is set as a chroma key mask area in accordance with the
attribute detection result, and composing the image data of the main
picture and the image data of the sub-picturee by executing the chroma key
process for an area other than the chroma key mask area without executing
the chroma key process for the chroma key mask area in the sub-picture.
