Aperçu sur les échanges mathématiques entre la Chine et la France (1880–1949)

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Aper?u sur les échanges mathématiques entre la Chine et la France (1880–1949)

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(Manuscrit re?u le 4 décembre 1997)     

Simulating Human Visual Perception in Nighttime Illumination

This paper presents an image-based algorithm for simulating the visual adaptation of the human visual system to various illuminations,especially in dark nighttime conditions.The human visual system exhibits different characteristics depending on the illumination intensity,with photopic vision in bright conditions,scotopic vision in dark conditions,and mesopic vision between these two.A computational model is designed to simulate multiple features of mesopic vision and scotopic vision,including the chromaticity change,luminance change,and visual acuity loss.The system uses a source image under bright illumination as input.Then assuming that the viewer has already adapted to the new conditions,the color spectrum of the input image is reconstructed to replace the source with modifications of the chromaticity and the luminance of the relighted scene.A bilateral filter is used to simulate the visual acuity loss.The model parameters have clear physical meanings and can be obtained from experimental data to achieve realistic results.The algorithm can be used not only for visual perception simulation,but also as a day-for-night tool to produce realistic nighttime images from daytime images.     

A new hominid from the Upper Miocene of Chad,Central Africa

The search for the earliest fossil evidence of the human lineage has been concentrated in East Africa. Here we report the discovery of six hominid specimens from Chad, central Africa, 2,500 km from the East African Rift Valley. The fossils include a nearly complete cranium and fragmentary lower jaws. The associated fauna suggest the fossils are between 6 and 7 million years old. The fossils display a unique mosaic of primitive and derived characters, and constitute a new genus and species of hominid. The distance from the Rift Valley, and the great antiquity of the fossils, suggest that the earliest members of the hominid clade were more widely distributed than has been thought, and that the divergence between the human and chimpanzee lineages was earlier than indicated by most molecular studies.     

Changes in plant community composition lag behind climate warming in lowland forests

Climate change is driving latitudinal and altitudinal shifts in species distribution worldwide, leading to novel species assemblages. Lags between these biotic responses and contemporary climate changes have been reported for plants and animals. Theoretically, the magnitude of these lags should be greatest in lowland areas, where the velocity of climate change is expected to be much greater than that in highland areas. We compared temperature trends to temperatures reconstructed from plant assemblages (observed in 76,634 surveys) over a 44-year period in France (1965-2008). Here we report that forest plant communities had responded to 0.54 °C of the effective increase of 1.07 °C in highland areas (500-2,600 m above sea level), while they had responded to only 0.02 °C of the 1.11 °C warming trend in lowland areas. There was a larger temperature lag (by 3.1 times) between the climate and plant community composition in lowland forests than in highland forests. The explanation of such disparity lies in the following properties of lowland, as compared to highland, forests: the higher proportion of species with greater ability for local persistence as the climate warms, the reduced opportunity for short-distance escapes, and the greater habitat fragmentation. Although mountains are currently considered to be among the ecosystems most threatened by climate change (owing to mountaintop extinction), the current inertia of plant communities in lowland forests should also be noted, as it could lead to lowland biotic attrition.     

Stabilization of the disk around beta Pictoris by extremely carbon-rich gas

The edge-on disk surrounding the nearby young star beta Pictoris is the archetype of 'debris disks', which are composed of dust and gas produced by collisions between--and evaporation of--planetesimals, analogues of Solar System comets and asteroids. These disks may provide insight into the formation and early evolution of terrestrial planets. Previous work on beta Pic concluded that the disk gas has roughly solar abundances of elements, but this poses a problem because such gas should rapidly be blown away from the star, contrary to observations showing a stable gas disk in keplerian rotation. Here we report the detection of singly and doubly ionized carbon (C II, C III) and neutral atomic oxygen (O I) gas in the beta Pic disk. Carbon is extremely overabundant relative to every other measured element. This appears to solve the problem of the stable gas disk, because the carbon overabundance should keep the gas disk in keplerian rotation. The overabundance may indicate that the gas is produced from material more carbon-rich than expected of Solar System analogues.