'sampling matrix'). Typically, as the subtense of the individual raster lines increases from a value much less than the foveal retinal receptor spacing, the performance first improves, then levels off and finally falls again. That performance improves with increasing size when the raster line subtense is small relative to the retinal receptor spacing is no surprise. Under such conditions the raster modulation is effectively invisible to the eye (due to the blurring by the dioptrics), and so we have basically a manifestation of visual threshold laws with increasing image size. Equally it is not unduly surprising that for very large and obvious rasters the performance first limits and then eventually degrades. Under such viewing conditions eventually all that can be seen is the raster. Both the small size and large size effects are equally true of images discretely sampled in two dimensions. What is, perhaps, surprising - and is of great importance for computer vision studies using presampled images - is that for raster sampled images it is found that the best performance is reached when the spatial pitch of the raster lines subtends around three retinal receptor spacings rather than one. Although no fully objective data are to hand, subjective assessments suggest that this finding is also equally true for images discretely sampled in two dimensions. The implication of these findings is that best performance is achieved when the Laplacian-like differencing mechanisms of the eye are operating on essentially some form of interpolation of the discretely sampled input image. Thus it is both unfair and inefficient (compared to human visual performance) to offer discretely presampled images directly to any Laplacian-like (or other) differencing operator, without first considering some form of preprocessing.
Another feature which can be of importance in presampled imagery is response variation from pixel to pixel. Some form of superimposed pattern is particularly likely to occur on images generated by 2-D discrete sensor arrays (where the individual sensor response almost inevitably produces a form of sampling noise), on some types of thermal images (where inbalance between individual sensors in a 1-D strip of elements scanned mechanically results in characteristic striping in the image), and on some full colour images a variety of registration problems, due to the mechanics of colour video
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