Engineering topics

Villasenor, John

Publications and source records attributed to Villasenor, John.

5 records

NASA NTRS1996

Perceptually Lossless Wavelet Compression

The Discrete Wavelet Transform (DWT) decomposes an image into bands that vary in spatial frequency and orientation. It is widely used for image compression. Measures of the visibility of DWT quantization errors are required to achieve optimal compression. Uniform quantization of a single band of coefficients results in an artifact that is the sum of a lattice of random amplitude basis functions of the corresponding DWT synthesis filter, which we call DWT uniform quantization noise. We measured visual detection thresholds for samples of DWT uniform quantization noise in Y, Cb, and Cr color channels. The spatial frequency of a wavelet is r 2(exp -1), where r is display visual resolution in pixels/degree, and L is the wavelet level. Amplitude thresholds increase rapidly with spatial frequency. Thresholds also increase from Y to Cr to Cb, and with orientation from low-pass to horizontal/vertical to diagonal. We propose a mathematical model for DWT noise detection thresholds that is a function of level, orientation, and display visual resolution. This allows calculation of a 'perceptually lossless' quantization matrix for which all errors are in theory below the visual threshold. The model may also be used as the basis for adaptive quantization schemes.

Watson, Andrew B.↗

NASA NTRS1995

Visibility of Wavelet Quantization Noise

The Discrete Wavelet Transform (DWT) decomposes an image into bands that vary in spatial frequency and orientation. It is widely used for image compression. Measures of the visibility of DWT quantization errors are required to achieve optimal compression. Uniform quantization of a single band of coefficients results in an artifact that is the sum of a lattice of random amplitude basis functions of the corresponding DWT synthesis filter, which we call DWT uniform quantization noise. We measured visual detection thresholds for samples of DWT uniform quantization noise in Y, Cb, and Cr color channels. The spatial frequency of a wavelet is r 2(exp)-L , where r is display visual resolution in pixels/degree, and L is the wavelet level. Amplitude thresholds increase rapidly with spatial frequency. Thresholds also increase from Y to Cr to Cb, and with orientation from low-pass to horizontal/vertical to diagonal. We describe a mathematical model to predict DWT noise detection thresholds as a function of level, orientation, and display visual resolution. This allows calculation of a "perceptually lossless" quantization matrix for which all errors are in theory below the visual threshold. The model may also be used as the basis for adaptive quantization schemes.

Watson, Andrew B.↗

NASA NTRS1992

Studies of temporal change using radar interferometry

Decorrelation of the radar signals with time, which is indicative of changes in the surface occurring during the period of time spanned by the images, is examined. It is concluded that the decorrelation effects due to thermal noise can be easily evaluated and removed, while those due to slight angular changes between flight tracks are negligible. Spatial baseline and rotation-induced decorrelation can be derived using the Fourier transform of the impulse response intensity, and increases linearly with baseline or rotation in an ideal system. Empirical results confirm that as the baseline increases, the overall correlation decreases due to spatial baseline noise.

Villasenor, John↗

NASA NTRS1992

Topographic mapping from ERS-1 and SEASAT radar interferometry

A radar interferometric technique for topographic mapping of surfaces yields a high resolution, globally consistent approach to generation of digital elevation models. The technique is illustrated with maps generated from SEASAT and European Space Agency Remote Sensing Satellite (ERS-1) data. A SEASAT interferometric image of a forested area which includes some unvegetated lava flows is analyzed. An analysis of errors expected from application of the technique to maps generated from ERS-1 data is presented. An orbital scenario for a global mapping mission is outlined.

Zebker, Howard A.↗

NASA NTRS1992

Temporal decorrelation in repeat pass-radar interferometry

Correlation in pass-to-pass, interferometric radar can be degraded by thermal noise, lack of parallelism between the radar flight tracks, spatial baseline noise, and surficial change. The effects of decorrelation due to thermal noise can be easily evaluated and removed, while those due slight angular changes between flight tracks are negligible for data acquired using near-repeat orbits. Empirical results obtained using images of Death Valley confirm that as the baseline increases, the overall correlation decreases due to spatial baseline noise. It is shown that areas of Cottonball Basin in Death Valley remained unchanged over the three-week period for which data was obtained, while a heavily forested area in Oregon exhibited significant temporal decorrelation. Lava in central Oregon also appeared to decorrelate. The results demonstrate that generation of height maps of heavily vegetated areas using pass-to-pass interferometry is practical, provided that the time between passes is at most several weeks.

Villasenor, John↗