An artifact in a discrete cosine transform based decoder output may be detected by developing a set of templates. An average intensity within each block in a reconstructed picture is calculated (34). The differences of each pixel value from the average output intensity within each block is determined (36). That difference is then multiplied by one of the templates within each block and the results of the multiplications are summed to obtain a calculated result (38). The calculated result is then compared to a threshold to detect an artifact such as ringing or mosquito noise artifacts (40).