Image invariant is generally meaning the geometric invariant

which is an important theory to improve the adaptability of computer vision system.But image gray is often degraded

such as noise

illumination change and motion blur.Geometric invariant can not fit the requirements of object recognition in practice.Image gray degradation and geometric transform are treated as image degradation

and image degradation invariant is proposed

and the construction of degradation invariant is analyzed.The degradation invariance of several classical geometric invariants is analyzed.The simulation experiments show that SIFT is a good degradation invariant and can be directly applied to image matching of unknown degradation.