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Sparse Sampling (sparse + sampling)
Selected AbstractsSparsely Precomputing The Light Transport Matrix for Real-Time RenderingCOMPUTER GRAPHICS FORUM, Issue 4 2010Fu-Chung Huang Precomputation-based methods have enabled real-time rendering with natural illumination, all-frequency shadows, and global illumination. However, a major bottleneck is the precomputation time, that can take hours to days. While the final real-time data structures are typically heavily compressed with clustered principal component analysis and/or wavelets, a full light transport matrix still needs to be precomputed for a synthetic scene, often by exhaustive sampling and raytracing. This is expensive and makes rapid prototyping of new scenes prohibitive. In this paper, we show that the precomputation can be made much more efficient by adaptive and sparse sampling of light transport. We first select a small subset of "dense vertices", where we sample the angular dimensions more completely (but still adaptively). The remaining "sparse vertices" require only a few angular samples, isolating features of the light transport. They can then be interpolated from nearby dense vertices using locally low rank approximations. We demonstrate sparse sampling and precomputation 5 × faster than previous methods. [source] Crustal structure of the Newfoundland rifted continental margin from constrained 3-D gravity inversionGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2007J. Kim Welford ABSTRACT The rifting history of the Atlantic continental margin of Newfoundland is very complex and so far has been investigated at the crustal scale primarily with the use of 2-D seismic surveys. While informative, the results generated from these surveys cannot easily be interpreted in a regional sense due to their sparse sampling of the margin. A 3-D gravity inversion of the free air data over the Newfoundland margin allows us to generate a 3-D density anomaly model that can be compared with the seismic results and used to gain insight into regions lacking seismic coverage. Results of the gravity inversion show good correspondence with Moho depths from seismic results. A shallowing of the Moho to 12 km depth is resolved on the shelf at the northern edge of the Grand Banks, in a region poorly sampled by other methods. Comparisons between sediment thickness and crustal thickness show deviations from local isostatic compensation in locations which correlate with faults and rifting trends. Such insights must act as constraints for future palaeoreconstructions of North Atlantic rifting. [source] Cross-spectral analysis of the X-ray variability of Markarian 421MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2002Y. H. Zhang ABSTRACT Using the cross-spectral method, we confirm the existence of the X-ray hard lags discovered with cross-correlation function technique during a large flare of Mrk 421 observed with BeppoSAX. For the 0.1,2 versus 2,10 keV light curves, both methods suggest sub-hour hard lags. In the time domain, the degree of hard lag, i.e. the amplitude of the 3.2,10 keV photons lagging the lower energy ones, tends to increase with the decreasing energy. In the Fourier frequency domain, by investigating the cross-spectra of the 0.1,2/2,10 keV and the 2,3.2/3.2,10 keV pairs of light curves, the flare also shows hard lags at the lowest frequencies. However, with the present data, it is impossible to constrain the dependence of the lags on frequencies even though the detailed simulations demonstrate that the hard lags at the lowest frequencies probed by the flare are not an artefact of sparse sampling, Poisson and red noise. As a possible interpretation, the implication of the hard lags is discussed in the context of the interplay between the (diffusive) acceleration and synchrotron cooling of relativistic electrons responsible for the observed X-ray emission. The energy-dependent hard lags are in agreement with the expectation of an energy-dependent acceleration time-scale. The inferred magnetic field (B, 0.11 G) is consistent with the value inferred from the spectral energy distributions of the source. Future investigations with higher quality data that show whether or not the time-lags are energy-/frequency-dependent will provide a new constraint on the current models of the TeV blazars. [source] Bayesian Prediction of Spatial Count Data Using Generalized Linear Mixed ModelsBIOMETRICS, Issue 2 2002Ole F. Christensen Summary. Spatial weed count data are modeled and predicted using a generalized linear mixed model combined with a Bayesian approach and Markov chain Monte Carlo. Informative priors for a data set with sparse sampling are elicited using a previously collected data set with extensive sampling. Furthermore, we demonstrate that so-called Langevin-Hastings updates are useful for efficient simulation of the posterior distributions, and we discuss computational issues concerning prediction. [source] Population pharmacokinetics of mycophenolic acid in children and young people undergoing blood or marrow and solid organ transplantationBRITISH JOURNAL OF CLINICAL PHARMACOLOGY, Issue 4 2010Lihua Zeng WHAT IS ALREADY KNOWN ABOUT THIS PROJECT? , Mycophenolate mofetil (MMF) is an immunosuppressant drug used for the treatment and prevention of graft vs. host disease in blood or marrow transplantation and acute graft rejection in solid organ transplantation. , Mycophenolic acid (MPA) pharmacokinetics have not been thoroughly studied in paediatric blood or marrow transplant recipients and guidance for optimal dosing of mycophenolic acid in children is lacking. , Mycophenolic acid exhibits considerable inter- and intra-patient pharmacokinetic variability in adults and paediatric transplant recipients. , The AUC of mycophenolic acid over a 12 h dose interval at steady-state is generally agreed to be the most reliable metric associated with the risk of acute rejection. , Population pharmacokinetic analysis can utilize concentration information from both intensive sampling and sparse sampling to provide pharmacokinetic parameter estimates, estimates of inter- individual and intra-individual variability in these parameters and allows patient characteristics explaining inter-individual variability to be quantified. WHAT THIS STUDY ADDS , This study is one of the first investigations in which a population pharmacokinetic modelling approach was applied to assess the pharmacokinetics of both intravenous and oral MMF in children and young people undergoing blood or marrow and solid organ transplantation. , Bodyweight and concomitant ciclosporin were found to influence MPA pharmacokinetics. , This study evaluated current dosing strategies and found that they may be suboptimal for children weighing less than 10 kg. AIMS To characterize the population pharmacokinetics of mycophenolic acid (MPA) and evaluate dose regimens using a simulation approach and accepted therapeutic drug monitoring targets in children and young people undergoing blood or marrow, kidney and liver transplantation. METHODS MPA concentration,time data were collected using an age specific sampling protocol over 12 h. Some patients provided randomly timed but accurately recorded blood samples. Total and unbound MPA were measured by HPLC. NONMEM was employed to analyze MPA pharmacokinetic data. Simulations (n= 1000) were conducted to assess the suitability of the MPA dose regimens to maintain total MPA AUC(0,12 h) within the range 30 and 60 mg l,1 h associated with optimal outcome. RESULTS A two-compartment pharmacokinetic model with first-order elimination best described MPA concentration,time data. Population mean estimates of MPA clearance, inter-compartmental clearance, volumes of distribution in the central and peripheral compartments, absorption rate constant and bioavailability were 6.42 l h,1, 3.74 l h,1, 7.24 l, 16.8 l, 0.39 h,1 and 0.48, respectively. Inclusion of bodyweight and concomitant ciclosporin reduced the inter-individual variability in CL from 54.3% to 31.6%. Children with a bodyweight of 10 kg receiving standard MPA dose regimens achieve an MPA AUC below the target range suggesting they may be at a greater risk of acute rejection. CONCLUSIONS The population pharmacokinetic model for MPA can be used to explore dosing guidelines for safe and effective immunotherapy in children and young people undergoing transplantation. [source] Pharmacokinetic-pharmacodynamic modelling in the early development phase of anti-psychotics: a comparison of the effects of clozapine, S 16924 and S 18327 in the EEG model in ratsBRITISH JOURNAL OF PHARMACOLOGY, Issue 1 2001T J Parker The use of pharmacokinetic/pharmacodynamic (PK/PD) analysis in early compound development was investigated in the rat for two developmental anti-psychotic compounds with clozapine as a positive control. Three plasma samples were collected from each of eight animals according to a pre-defined sampling matrix allowing a total of 12 time points for PK analysis. Quantitative electroencephalography (QEEG), particularly the theta and beta frequencies, was used as a measurement of pharmacological effect. PK/KD modelling of the sparse PK data available relative to a rich set of PD data was achieved using a population approach in NONMEM (IV). Individual PK parameter estimates were incorporated into a PK/PD model. Qualitative EEG changes in rat and human were similar for clozapine, but different for the two developmental compounds, suggesting that changes in these PD parameters may not be specifically related to the anti-psychotic activity. Although no definitive data are available concerning the signal specificity of EEG frequency bands with respect to dopaminergic or serotonergic receptor activity, qualitative and quantitative differences seen in EEG parameters are likely to result from the multiple receptor occupancy for these compounds. The results confirm the value of population PK/PD modelling in conjunction with sparse sampling to enable determination of concentration effect relationships in the pre-clinical development programme of CNS-active drugs. British Journal of Pharmacology (2001) 132, 151,158; doi:10.1038/sj.bjp.0703791 [source] | ||||||||||