Abstract-This paper presents a new direct Fourier-based algorithm for performing image-to-image registration to subpixel accuracy, where the image differences are restricted to translations and uniform changes of illumination. The algorithm detects the Fourier components that have become unreliable estimators of shift due to aliasing, and removes them from the shift-estimate computation. In the presence of aliasing, the average precision of the registration is a few hundredths of a pixel.Experimental data presented here show that the new algorithm yields superior registration precision in the presence of aliasing when compared to several earlier methods and has comparable precision to the iterative method of Thévenaz et al. [21].
Abstract-Many image display devices allow only a limited number of colors to be simultaneously displayed. Usually, this set of available colors, called a color palette, may be selected by a user from a wide variety of available colors. Such device restrictions make it particularly difficult to display natural color images since these images usually contain a wide range of colors which must then be quantized by a palette with limited size. This color quantization problem is considered in two parts: the selection of an optimal color palette and the optimal mapping of each pixel of the image to a color from the palette. This paper develops algorithms for the design of hierarchical tree structured color palettes incorporating performance criteria which reflect subjective evaluations of image quality. Tree structured color palettes greatly reduce the computational requirements of the palette design and pixel mapping tasks, while allowing colors to be properly allocated to densely populated areas of the color space. The algorithms produce higher quality displayed images and require less computations than previously proposed methods.Error diffusion techniques are commonly used for displaying images which have been quantized to very few levels. This paper studies problems related to the application of error diffusion techniques to the display of color images. A modified error diffusion technique is proposed for resolving these problems. The new error diffusion technique is shown to be easily implemented using the tree structured color palettes developed earlier.
We present an estimation-theoretic analysis of motion compensation that, when used with fields of block-based motion vectors, leads to the development of overlapped block algorithms with improved compensation accuracy. Overlapped block motion compensation (OBMC) is formulated as a probabilistic linear estimator of pixel intensities given the limited block motion information available to the decoder. Although overlapped techniques have been observed to reduce blocking artifacts in video coding, this analysis establishes for the first time how (and why) OBMC can offer substantial reductions in prediction error as well, even with no change in the encoder's search and no extra side information. Performance can be further enhanced with the use of state variable conditioning in the compensation process. We describe the design of optimized windows for OBMC. We also demonstrate how, with additional encoder complexity, a motion estimation algorithm optimized for OBMC offers further significant gains in compensation accuracy. Overall mean-square prediction improvements in the range of 16 to 40% (0.8 to 2.2 dB) are demonstrated.
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