8.1. INTRODUCTION.
It is frequently the case that images which are to be offered to a computer vision system are already prerecorded (e.g. recorded on video tape from thermal imagers, standard commercial video equipment, photographs etc.). This is particularly true of virtually all real world images which are used for research studies into computer vision. These input images are therefore presampled at discrete spatial and temporal intervals, may include patterning due to photographic grain, photodetector noise etc., and in general, incorporate a (usually unknown) small amount of image blur. Now display / observer interface studies for human visual observers have shown a number of important facts regarding perception of discretely sampled images as follows.
A very widely used form of modern image recording and replay is by raster scanning (essentially as used in commercial television). As presented to the observer, such raster scanning will provide an image which is characterised by a modulation structure across the raster lines and an angular subtense of the raster line pitch. The former of these is determined by the internal setting up of the display, whilst the latter is governed by the display size and the observer viewing distance. It must be anticipated that both these factors may have some effect on observer performance. To our knowledge no major studies have been carried out on the effects of detail across the line structure, although the effective modulation depth, which is related to the sharpness of the scanning spot, has been shown to have large subjective effects. What has been researched more objectively is the effect of raster line subtense for a 'typical' form of raster structure having a 1:1 mark/space and a modulation depth of approximately 0.8. When human observers view raster scanned images their thresholds of detection and recognition are very dependent on the relationship between the scale of the raster line pitch and the foveal receptor matrix (which is the human visual system's spatial
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