Curvature extrema provide significant information about the shape of an image contour, such as a silhouette, and are the basis for the Hoffman-Richards codon representation for shape. This representation based on curvature easily translates into a binary string that will describe the abstract shape of any smooth image curve. The computation of the basic shape primitives requires dealing with two ever-pervasive problems: contour noise and scale. We show how contour noise can be estimated given knowledge of the shape of the filter used to compute curvature from the edge list of the contour. To handle the scale problem, we use an adaptation of Witkin's scale space. Our algorithm differs from Witkin's by using a notion of parts to set criteria for significant structures.
For long periods observers fixated low spatial frequency coloured gratings. Black and white test gratings of the same spatial frequency and orientation as the adapting gratings appeared coloured with the hue complementary to the adapting patterns when the dark test stripes fell on retinal areas previously occupied by the dark adapting stripes; no colour or very weak colour was seen when the test gratings were reversed in phase (contrast reversed). No colour aftereffects were produced with coloured gratings that lacked luminance contrast. The selectivity to the polarity of local luminance contrast can be explained by mechanisms that respond conjointly to colour and luminance contrast. The aftereffects are selective to spatial phase.
This paper uses a quantitative dynamic ecosystem vegetation model to explore the potential impact of warming up to 9-12 degrees C on global ecosystems. The paper does not find evidence of a global collapse in terrestrial ecosystems but there is evidence of substantial changes. Temperate and tropical forests expand and replace boreal forests and forests shift to woodlands and parkland at high temperatures. Net primary productivity and standing forest biomass per hectare rise. These changes will affect dependent animal species. Further research is needed to measure the resulting benefits and damages to market and nonmarket services.
After prolonged fixation of coloured gratings of low spatial frequency, images of the gratings can be elicited up to 90 min thereafter when the colour of a spatially homogeneous test field is suddenly changed. Only adapting gratings with luminance contrast induce clear aftereffects. Control experiments rule out afterimages as an explanation of the aftereffects.
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