tailieunhanh - Digital video quality vision models and metrics phần 9

Việc thực hiện dự đoán của số liệu kết quả nhận thức ngăn chặn sự biến dạng (PBDM) được phân tích bằng cách sử dụng dữ liệu từ các thí nghiệm chủ quan trên blockiness. Thứ hai là sự kết hợp của PDM với phân khúc đối tượng. | 144 METRIC EXTENSIONS highest quality over all scenes closely followed by condition 7 MPEG-4 at 2 Mb s . At 1 Mb s the MPEG-4 codec condition 6 outperforms conditions 1 3 and 8. It should be noted that the Intel Indeo Video codecs and the Sorenson Video codec were designed for lower bitrates than the ones used in this test and obviously do not scale well at all as opposed to MPEG-2 and MPEG-4. Comparing Figures a and b reveals that the perceived quality depends much more on the codec and bitrate than on the particular scene content in these experiments. PDM Prediction Performance Before returning to the image appeal attributes let us take a look at the prediction performance of the regular PDM for these sequences. This is of interest for two reasons. First as mentioned before no normalization of the test sequences was carried out in this test. Second the codecs and compression algorithms described above used to create the test sequences and the resulting visual quality of the sequences are very different from the VQEG test conditions cf. Table . The latter rely almost exclusively on MPEG-2 and which are based on very similar compression algorithms blockbased DCT with motion compensation whereas this test adds codecs based on vector quantization the wavelet transform and hybrid methods. One of the advantages of the PDM is that it is independent of the compression method due to its underlying general vision model contrary to specialized artifact metrics cf. section . The scatter plot of perceived quality versus PDM predictions is shown in Figure a . It can be seen that the PDM is able to predict the subjective ratings well for most test sequences. The outliers belong mainly to conditions 1 and 8 the lowest-quality sequences in the test as well as the computergraphics scenes where some of the Windows-based codecs introduced strong color distortions around the text which was rated more severely by the subjects than by the PDM. It .