1. An image processor configured to perform denoising on an input image to generate an output image, the image processor comprising: a transform circuit configured to transform first image data of a spatial region extracted from the input image in a unit of a predetermined block into second image data of a frequency region including a plurality of frequency components; a quantization circuit configured to perform quantization on the second image data to generate third image data; an arithmetic circuit configured to calculate a per-block activity evaluation value based on the first image data; and a quantization value setting circuit configured to set a per-block quantization value for quantization based on the per-block activity evaluation value.