tailieunhanh - DIGITAL IMAGE PROCESSING 4th phần 4

Những hình ảnh dưới bên phải là một phiên bản được mài sắc của hình ảnh ban đầu thu được bằng cách trừ đi một phiên bản biên độ trọng của hình ảnh bị mờ từ một phiên bản trọng của hình ảnh ban đầu. Kỹ thuật chế biến được gọi là mặt nạ không sắc nét. | 142 IMAGE QUANTIZATION Let aU i represent the upper bound of x i and aL i the lower bound. Then each quantization cell has dimension q i au i- aL 2B i Any color with color component x i within the quantization cell will be quantized to the color component value x i . The maximum quantization error along each color coordinate axis is then i x i - x i aU i aL i ----TTT--- -- 2B i 1 FIGURE . Chromaticity shifts resulting from uniform quantization of the smpte_girl_linear color image. REFERENCES 143 Thus the coordinates of the quantized color become x i x i e i subject to the conditions aL i x i av i . It should be observed that the values of x i will always lie within the smallest cube enclosing the color solid for the given color coordinate system. Figure illustrates chromaticity shifts of various colors for quantization in the Rn GNBN and Yuv coordinate systems 12 . Jain and Pratt 12 have investigated the optimal assignment of quantization decision levels for color images in order to minimize the geodesic color distance between an original color and its reconstructed representation. Interestingly enough it was found that quantization of the Rn Gn Bn color coordinates provided better results than for other common color coordinate systems. The primary reason was that all quantization levels were occupied in the Rn Gn Bn system but many levels were unoccupied with the other systems. This consideration seemed to override the metric nonuniformity of the Rn Gn Bn color space. Sharma and Trussell 13 have surveyed color image quantization for reduced memory image displays. REFERENCES 1. P. F. Panter and W. Dite Quantization Distortion in Pulse Code Modulation with Nonuniform Spacing of Levels Proc. IRE 39 1 January 1951 44-48. 2. J. Max Quantizing for Minimum Distortion IRE Trans. Information Theory IT-6 1 March 1960 7-12. 3. V. R. Algazi Useful Approximations to Optimum Quantization IEEE Trans. Communication Technology COM-14 3 June 1966 .