Subjective Image Quality of Low-Rate and Low-Resolution Video Sequences

摘要

For a variety of QCIF video sequences with bit rates between 30 and 64 kbit/s and frame rates between 5 and 15 fps quality related criteria like "total quality", "quality concerning blocking effects", "information value", and "smoothness of movement" were estimated by human observers on a six grades scale reaching from "1, very good", "2, good", "3, satisfactory", "4, sufficient", "5, imperfect" to "6, poor". Presentation of the sequences was done in succession of reference video (128 kbit/s, 15 fps) and test video. Bit rates higher than 30 kbit/s led to a "total quality" better than "4, sufficient". For the highest bit rate of 64 kbit/s and frame rates of 15 or 7 fps the criterion "total quality" as well as the dynamical criterion "smoothness of motion" are dominated by the static quality of individual coded frames. Both criteria improve with lower frame rates leading to their best estimates for the low frame rate of 7 fps. For medium bit rates of 50 kbit/s and frame rates of 7 or 5 fps we got impairments of "total quality" and "smoothness of movement" with decreasing frame rate. At a frame rate of 5 fps and bit rates of 50 and 30 kbit/s impairments of "total quality" and "smoothness of movement" were obtained for decreasing the bit rate. An appropriate metric for the subjective quality of video sequences was found as a combination of a static and a dynamic component. Mean PSNR averaged over coded frames plus a beneficial contribution from higher frame rates proves to be a good estimate for it.