tailieunhanh - Digital video quality vision models and metrics phần 4

80-90% của tất cả các tế bào thần kinh trong bộ não con người được ước tính là có liên quan trong nhận thức trực quan (Young, 1991). Điều này đã được một dấu hiệu của sự phức tạp to lớn của hệ thống thị giác của con người. Các cuộc thảo luận trong chương này là nhất thiết phải giới hạn trong phạm vi và tập trung chủ yếu vào các khía cạnh có liên quan | 44 VIDEO QUALITY Blur manifests itself as a loss of spatial detail and a reduction of edge sharpness. It is due to the suppression of the high-frequency coefficients by coarse quantization see Figure . Color bleeding is the smearing of colors between areas of strongly differing chrominance. It results from the suppression of high-frequency coefficients of the chroma components. Due to chroma subsampling color bleeding extends over an entire macroblock. The DCT basis image effect is prominent when a single DCT coefficient is dominant in a block. At coarse quantization levels this results in an emphasis of the dominant basis image and the reduction of all other basis images see Figure b . Slanted lines often exhibit the staircase effect. It is due to the fact that DCT basis images are best suited to the representation of horizontal and vertical lines whereas lines with other orientations require higher-frequency DCT coefficients for accurate reconstruction. The typically strong quantization of these coefficients causes slanted lines to appear jagged see Figure b . Ringing is fundamentally associated with Gibbs phenomenon and is thus most evident along high-contrast edges in otherwise smooth areas. It is a direct result of quantization leading to high-frequency irregularities in the reconstruction. Ringing occurs with both luminance and chroma components see Figure . False edges are a consequence of the transfer of block-boundary discontinuities due to the blocking effect from reference frames into the predicted frame by motion compensation. Jagged motion can be due to poor performance of the motion estimation. Block-based motion estimation works best when the movement of all pixels in a macroblock is identical. When the residual error of motion prediction is large it is coarsely quantized. Motion estimation is often conducted with the luminance component only yet the same motion vector is used for the chroma components. This can result in chrominance