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Marina Bloj, David Connah, Graham Finlayson; What do we know about how humans choose grey levels for images? . Journal of Vision 2010;10(7):406. doi: https://doi.org/10.1167/10.7.406.
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© ARVO (1962-2015); The Authors (2016-present)
From cave paintings that are more than 32000 years old to the work of current artists using charcoal or pastels humans have used different levels of a single colour (e.g. greyscales) to portray the colourful world that surrounds us. The aim of this study was to investigate how we construct these monochromatic images; do we choose the same relative order of grey levels as other people? How does image content and contrast influence our grey settings? For this purpose, we presented, on a calibrated CRT, 5 simplified cartoon-type colourful images. These images where shown in 4 different versions: two where the content of image was recognisable (one with added black outline around each object, the other without). In the other two versions each pixel in an image was grouped with other pixels of the same colour into rectangular areas in a way that made the content of image abstract while preserving the area taken up by a particular colour. One of these versions had a black outline around each colour area; the other did not. Six naïve, colour normal observers used a digital ‘re-colouring tool’ to create greyscales version of the randomly presented colour images. Preliminary analyses of participants' settings for different version of the images indicate that the addition of black outlines does not seem to affect chosen grey levels. More surprisingly, the change in content from a recognisable scene to abstract rectangles also seems to leave grey levels un-changed. Although each individual's absolute settings were different, the overall ranking was largely preserved across participants for a given image. For our chosen set of test images, the most influential factor driving grey settings was the perceived lightness of the coloured patch, not image content or contrast.
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