An image signal processor includes a shared circuit, a first circuit, and a second circuit. The shared circuit processes input image data. The first circuit provides a first algorithm for processing the input image data and is used by the shared circuit to process the input image data at a first operation mode in a first shooting environment. The second circuit provides a second algorithm for processing the input image data and is used by the shared circuit to process the input image data at a second operation mode different from the first operation mode in a second shooting environment different from the first shooting environment.