1. An apparatus for video data processing in digital video decoding, which produces new video data according to an aspect ratio, main picture data, and sub-picture unit (SPU) data, the apparatus comprising: an SPU-decoding and color-determining device for performing sub-picture decoding and color determination on the SPU data, and producing decoded SPU data; a mixer, coupled to the SPU-decoding and color-determining device, for mixing the main picture data and the decoded SPU data, and outputting combined video data; a buffer, coupled to the mixer, for storing the combined video data outputted from the mixer; and an interpolator, for receiving the combined video data from the mixer and reading previously combined video data from the buffer, and generating the new video data by performing an interpolation operation according to the aspect ratio, the combined video data, and the previously combined video data.