1. A method for processing transform blocks according to quantization matrices in a video coding system, the method comprising: receiving, via a receiving circuit, one or more initial quantization matrices having first sizes; deriving, using a circuit, one or more derived quantization matrices having second sizes from one of said one or more initial quantization matrices, wherein the second size is different than the first size; receiving, via a receiving circuit, a transform block having a block size, where the transform block is associated with a picture, and wherein the block size is equal to one of the first sizes or one of the second sizes; processing, using a processing circuit, the transform block according to respective one of said one or more initial quantization matrices if the block size is equal to said one of the first sizes; and processing, using a processing circuit, the transform block according to respective one of said one or more derived quantization matrices if the block size is equal to said one of the second sizes; wherein said one or more initial quantization matrices comprise one or more square initial quantization matrices, wherein said one or more derived quantization matrices comprise one or more non-square, rectangular transform block sizes.