Các chuỗi kết quả được chia thành các khối không thời gian. Một số các tính năng đo số lượng và định hướng hoạt động trong các khối này sau đó được tính từ gradient độ sáng không gian. Để đo lường sự biến dạng, các tính năng từ các tài liệu tham khảo và trình tự bóp méo được so sánh bằng cách sử dụng một quá trình tương tự như mặt nạ. Số liệu này là một trong những | METRIC EVALUATION a Original b Condition 7 c Condition 20 Figure Original test image and two examples of distorted versions. It consists of distorted versions of a color image of 320 X 400 pixels in size showing the face of a child surrounded by colorful balls see Figure a . To create the test images the original was JPEG-encoded and the coding noise was determined in YUV space by computing the difference between the original and the compressed image. Subsequently the coding noise was scaled by a factor ranging from 1 to 1 in the Y U and V channel separately and was then added back to the original in order to obtain the distorted images. A total of 20 test conditions were defined which are listed in Table and the test series were created by varying the noise intensity Table Coding noise components and signs for all 20 test conditions 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Y - - - - - - - U - - - - - - - V - - - - - - along specific directions in YUV space in this fashion van den Branden Lambrecht and Farrell 1996 . Examples of the resulting distortions are shown in Figures b and c . Subjective Experiments Psychophysical data was collected for two subjects GEM and JEF using a QUEST procedure Watson and Pelli 1983 . In forced-choice experiments the subjects were shown the original image together with two test images STILL IMAGES one of which was the distorted image and the other one the original. Subjects had to identify the distorted image and the percentage of correct answers was recorded for varying noise intensities van den Branden Lambrecht and Farrell 1996 . The responses for two test conditions are shown in Figure . a Condition 7 Noise amplitude a Condition 20 Figure Percentage of correct answers versus noise amplitude and fitted psychometric functions for subjects GEM stars dashed curve and JEF circles solid curve for two test conditions. The dotted horizontal line indicates the detection threshold. METRIC .
