The term "upsample" is used in digital audio and video to mean increasing the sampling resolution of a sound or an image. More generally the term can be used to mean increasing the size of a set of numbers by some factor, keeping the original numbers and using interpolation to determine the values of the new intermediate numbers.
Any desired color is produced by blending three primary colors of light: red, green and blue, each with its own intensity. The 4 planes of the matrix in the jit.matrix object are considered the alpha, red, green, blue, planes of the color of each pixel. The planes are numbered starting from 0: alpha=0, red=1, green=2, blue=3. Normally the data in a 4-plane image matrix is of type char, representing numbers from 0 to 255.