A method for inverse discrete cosine transformation (IDCT) in video coding is provided that includes receiving a transform block, identifying a region of non-zero transform coefficients in the transform block using a group significance map corresponding to the transform block, wherein any transform coefficients not in the region have a value of zero, applying a one-dimensional (1D) IDCT to the region of non-zero transform coefficients in a first direction to generate an interim results block, wherein 1D IDCT computations are not performed on transform coefficients outside the region, and applying a 1D IDCT to the interim results block in a second direction to generate a residual block.