When a high resolution image is downsampled, subsampling is the way a low resolution image represents the original high resolution one. Pixels to be merged should decide what color to be painted using, for example, average downsampling.
When a low resolution image is upsampled, it is required that new pixels be generated. Once the original image is subdivided, new pixels can be generated by referencing neighboring pixels and interpolating them.
 카츠라 요스케 , 요츠쿠라 타츠오 , 마크 살바티, 테크니컬 아티스트를 위한 최고의 교과서: 내공 있는 CG 아티스트로 이끌어주는 체계적인 입문서, 2014