1. A three dimensional sensing method, for performing a depth image operation upon a virtual first pending image and a real second pending image, and a resolution of the first pending image and a resolution of the second image are respectively M×N, the method comprising: projecting a projection pattern to an object, wherein the projecting device serves as a virtual image acquiring device, and an inherent image of the projection pattern serves as a virtual first pending image; sensing the projection pattern projected on the object to generate a real second pending image; performing a resolution scaling on the first pending image and the second pending image to respectively generate a first scaled image and a second scaled image, and a resolution of the first scaled image and a resolution of the second scaled image are respectively m×n, where m<M, n<N, and M, N, m, n are positive integers performing a full-scene three dimensional measurement on the first scaled image and the second scaled image to obtain a full-scene depth image; setting a first region of interest (ROI) and a second ROI of at least one image acquiring device respectively by analyzing the full-scene depth image, wherein the at least one image acquiring device generates the first and the second pending images; and restoring a resolution of the first ROI and the second ROI of the full-scene depth image according to a scaling factor of the resolution scaling, so as to obtain a first ROI image and a second ROI image respectively, and performing a partial-scene three dimensional measurement on the first ROI image and the second ROI image to obtain a partial-scene depth image.