An image processing apparatus comprises a central processing unit (CPU) and a graphics processing unit. The CPU comprises a first table specification, a second table specification, an inverse discrete cosine transformation (IDCT) module, a color space converting module and an entropy decoder. The entropy decoder retrieves and processes input image data as a two-dimensional quantized image according to the first table specification. The graphics processing unit performs a dequantizing operation on the input image data according to the second table specification. The IDCT module performs an IDCT operation on the dequantized input image data. The color space converting module performs a color space conversion on the input image data and outputs the converted image data.