Structure of noise

As discussed above, each atomic wi∈Wwi​∈W has its distortion contribution at the spatial region kiki​ on the rendered image. Since the rendering process contains multiple options to improve the quality of the output (noise reduction, anti-aliasing, etc.), it is severe if the region kiki​(s) fall into perceptually insignificant regions of the image because the deliberate distortions raised by wiwi​(s) would be eliminated by the rendering enhancement. Hence, distortions are preferred to be placed in human perceptually significant regions. However, to keep the compromise between the fidelity of the rendered frames and the robustness of noise verification, the strength of the distortion of each wiwi​ must be tuned so that its deviation from locally enclosed regions is within a predetermined bound.