1. An image processing apparatus, comprising; a memory that stores a plurality of instructions; a processor that couples to the memory and causes the image processing apparatus to: decompose luminance of an input image into an illumination component and a reflectance component; compress the illumination component; extend the reflectance component; combine a compressed illumination component and an extended reflectance component, to obtain a combined luminance; and adjust the combined luminance to obtain a processed image with an adjusted luminance, whose contrast is enhanced; wherein when decomposing luminance of an input image into an illumination component and a reflectance component, the processor further causes the image processing apparatus to: perform edge-preserving filtering on the luminance of the input image, to obtain the illumination component; and determine the reflectance component according to the luminance of the input image and the illumination component; wherein when performing edge-preserving filtering on the luminance of the input image, the processor further causes the image processing apparatus to perform the edge-preserving filtering by using weighted least squares (WLS); wherein formula (4) is used for adjusting the combined luminance: Y out ′ ⁡ ( x ) = y m mean ⁡ ( Y out ) * Y out ⁡ ( x ) ; ( 4 ) where, Y out (x) denotes combined luminance, Y′ out (x) denotes an adjusted luminance, mean(Y out ) denotes an average value of the combined luminance, and y m denotes a predefined target luminance.