Visibility thresholds for compression-induced image blocking: measurements and models

The present investigation compares performance of two objective video quality metrics in predicting the visual threshold for the detection of blocking impairments associated with MPEG-2 compression. The visibility thresholds for both saturated color and gray-scale targets are measured. The test material consists of image sequences in which either saturated color or gray-scale targets exhibiting blocking are varied in luminance contrast from -44 dB to -5 dB against a constant gray background. Stimulus presentation is by the "method of limits" under International Telecommunications Union (ITU) Rec. 500 conditions. Results find the detection of blocking impairments at Michelson contrast levels between -28 dB and -33 dB. This result is consistent with values reported by other investigators for luminance contrast detection thresholds. A small, but statistically significant difference is found between the detection threshold of saturated color patterns versus luma-only images. The results suggest, however, that blocking impairment detection is controlled mainly by display luminance. Two objective metrics are applied to gray-scale (luminance-only) image sequences, yielding measures of perceptible image blocking for each frame. A relatively simple blocking detector and a more elaborate discrete cosine transform (DCT) error metric correlate well over the contrast range examined. Also, the two measures correlate highly with measured image contrast. Both objective metrics agree closely with visual threshold measurements, yielding threshold predictions of approximately -29 dB.