A general matrix formula is proposed for signal spectral aliasing of various or mutual resolution, the concept of spectral aliasing matrix is introduced, and some general spectral methods for spatial resolution improv...A general matrix formula is proposed for signal spectral aliasing of various or mutual resolution, the concept of spectral aliasing matrix is introduced, and some general spectral methods for spatial resolution improvement from multiframes of undersampled digital images are discussed. A simplified iterative method of parallel row-action projection for spectral de-aliasing is also given. The method can be applied to multiframe images with various spatial resolution,relative displacement, dissimilar point spread function, different image radiance, and additive random noise. Some experiments with a resolution test pattern and an image of vertical fin performed the convergence and the effectiveness of the algorithms.展开更多
Water surface is one of the most important components of landscape scenes. When rendering spacious water surface such as that of the lakes and reservoirs, aliasing and/or moire artifacts frequently occur in the region...Water surface is one of the most important components of landscape scenes. When rendering spacious water surface such as that of the lakes and reservoirs, aliasing and/or moire artifacts frequently occur in the regions far from the viewpoint. This is because water surface consists of stochastic water waves which are usually modeled by periodic bump mapping. The incident rays on the water surface are actually scattered by the bumped waves, and the reflected rays at each sample point are distributed in a solid angle. To get rid of the artifacts of moire pattern, we estimate this solid angle of reflected rays and trace these rays. An image-based accelerating method is adopted so that the contribution of each reflected ray can be quickly obtained without elaborate intersection calculation. We also demonstrate anti-aliased shadows of sunlight and skylight on the water surface. Both the rendered images and animations show excellent effects on the water surface of a reservoir.展开更多
Human face can be rebuilt to a three-dimensional (3 D) digital profile based on an optical 3D sensing system named Composite Fourier-Transform Profilometry (CFTP) where a composite structured light will be used. To st...Human face can be rebuilt to a three-dimensional (3 D) digital profile based on an optical 3D sensing system named Composite Fourier-Transform Profilometry (CFTP) where a composite structured light will be used. To study the sampling effect during the digitization process in practical CFTP, the pectinate function and convolution theorem were introduced to discuss the potential phase errors caused by sampling the composite pattern along two orthogonal directions. The selecting criterions of sampling frequencies are derived and the results indicate that to avoid spectral aliasing, the sampling frequency along the phrase variation direction must be at least four times as the baseband and along the orthogonal direction it must be at least three times as the larger frequency of the two carrier frequencies. The practical experiment of a model face reconstruction verified the theories.展开更多
In this paper we investigate frequency aliasing in spectral method of measuring T wave alter-nans, which may lead a high false positive rate. Microvolt T wave alternans(TWA) has been evaluated as a means of predicting...In this paper we investigate frequency aliasing in spectral method of measuring T wave alter-nans, which may lead a high false positive rate. Microvolt T wave alternans(TWA) has been evaluated as a means of predicting occurrence of ventricular tachyarrhythmia events and its association with the genesis of ventricular ar-rhythmias has been demonstrated. Nowadays, spectral method is one of the most widely used procedures for measurement of microvolt TWA. In our study, based on the sampling theory, the alternans frequency 0.5 cycles/beat, at which the power of the spectrum is used to calculated the Valt and K score (these two parameters indicate the TWA), is equal to the nyquist frequency. Thus this generates frequency aliasing which will make the power at the alternans frequency (P0.5) be two times of the real magnitude of the original spectrum amplitude. With the assump-tion that the noise spectrum follows the normal distribution, in spectral method of measuring T wave alternans, the measuring standard K score>3 to consider the T wave alternans sig-nificant is only with a p<0.133. By change the standard to K score>6 can solve this problem and make the p value to p<0.0027.展开更多
A three-dimensional(3D)global adiabatic spectral primitive equation model has been designed.The main features are as follows. (1)Adoption of spherical harmonics and Tschebyscheff polynomials as the basis functions in ...A three-dimensional(3D)global adiabatic spectral primitive equation model has been designed.The main features are as follows. (1)Adoption of spherical harmonics and Tschebyscheff polynomials as the basis functions in the horizontal and vertical respectively,but the unknowns in the spectral equations are two- dimensional; (2)Inclusion of the tropopause,which may vary with time and space; (3)Suggestion of a spectral method for representing the vertical structure of the atmosphere applicable to the unsmoothed profile case; (4)In consideration of nonlinear vertical aliasing a technique is proposed to avoid it and nonlinear computational instability. Based on real data forecasts up to 48 hours have been performed.The results show that the statistical verifications with the model are superior on the average to those with the T42L9 used operationally before 1995 at NMC of China at the same mean resolution.展开更多
文摘A general matrix formula is proposed for signal spectral aliasing of various or mutual resolution, the concept of spectral aliasing matrix is introduced, and some general spectral methods for spatial resolution improvement from multiframes of undersampled digital images are discussed. A simplified iterative method of parallel row-action projection for spectral de-aliasing is also given. The method can be applied to multiframe images with various spatial resolution,relative displacement, dissimilar point spread function, different image radiance, and additive random noise. Some experiments with a resolution test pattern and an image of vertical fin performed the convergence and the effectiveness of the algorithms.
文摘Water surface is one of the most important components of landscape scenes. When rendering spacious water surface such as that of the lakes and reservoirs, aliasing and/or moire artifacts frequently occur in the regions far from the viewpoint. This is because water surface consists of stochastic water waves which are usually modeled by periodic bump mapping. The incident rays on the water surface are actually scattered by the bumped waves, and the reflected rays at each sample point are distributed in a solid angle. To get rid of the artifacts of moire pattern, we estimate this solid angle of reflected rays and trace these rays. An image-based accelerating method is adopted so that the contribution of each reflected ray can be quickly obtained without elaborate intersection calculation. We also demonstrate anti-aliased shadows of sunlight and skylight on the water surface. Both the rendered images and animations show excellent effects on the water surface of a reservoir.
文摘Human face can be rebuilt to a three-dimensional (3 D) digital profile based on an optical 3D sensing system named Composite Fourier-Transform Profilometry (CFTP) where a composite structured light will be used. To study the sampling effect during the digitization process in practical CFTP, the pectinate function and convolution theorem were introduced to discuss the potential phase errors caused by sampling the composite pattern along two orthogonal directions. The selecting criterions of sampling frequencies are derived and the results indicate that to avoid spectral aliasing, the sampling frequency along the phrase variation direction must be at least four times as the baseband and along the orthogonal direction it must be at least three times as the larger frequency of the two carrier frequencies. The practical experiment of a model face reconstruction verified the theories.
文摘In this paper we investigate frequency aliasing in spectral method of measuring T wave alter-nans, which may lead a high false positive rate. Microvolt T wave alternans(TWA) has been evaluated as a means of predicting occurrence of ventricular tachyarrhythmia events and its association with the genesis of ventricular ar-rhythmias has been demonstrated. Nowadays, spectral method is one of the most widely used procedures for measurement of microvolt TWA. In our study, based on the sampling theory, the alternans frequency 0.5 cycles/beat, at which the power of the spectrum is used to calculated the Valt and K score (these two parameters indicate the TWA), is equal to the nyquist frequency. Thus this generates frequency aliasing which will make the power at the alternans frequency (P0.5) be two times of the real magnitude of the original spectrum amplitude. With the assump-tion that the noise spectrum follows the normal distribution, in spectral method of measuring T wave alternans, the measuring standard K score>3 to consider the T wave alternans sig-nificant is only with a p<0.133. By change the standard to K score>6 can solve this problem and make the p value to p<0.0027.
基金This study is supported by the National Natural Science Foundation of China under the program No.49575268.
文摘A three-dimensional(3D)global adiabatic spectral primitive equation model has been designed.The main features are as follows. (1)Adoption of spherical harmonics and Tschebyscheff polynomials as the basis functions in the horizontal and vertical respectively,but the unknowns in the spectral equations are two- dimensional; (2)Inclusion of the tropopause,which may vary with time and space; (3)Suggestion of a spectral method for representing the vertical structure of the atmosphere applicable to the unsmoothed profile case; (4)In consideration of nonlinear vertical aliasing a technique is proposed to avoid it and nonlinear computational instability. Based on real data forecasts up to 48 hours have been performed.The results show that the statistical verifications with the model are superior on the average to those with the T42L9 used operationally before 1995 at NMC of China at the same mean resolution.
