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Partly it is because real objects are often made out of a single colour. The camera sees different parts of the object under different lighting conditions. That
by abainbridge 9y ago
Partly it is because real objects are often made out of a single colour. The camera sees different parts of the object under different lighting conditions. That mainly only changes the brightness. Specular highlights also desaturate the base colour, but nothing changes the hue. So all the pixels for that object tend to be aligned on a plane in the colour space plot.
In the example images, the sand in the sand+sky picture is the only thing this is really true for.
- pavel_lishin 9y agoBut I'd expect to see distinct clumps, not a straight plane...
- abainbridge 9y agoImagine you have a flat red piece of card in a photographed scene. If the lighting is just right and the card is matt, you could get all the pixels of the card in the image to be the exact same colour. That would be a single point in the colour space. Now imagine you rolled the card into a cylinder. The result would depend on the lighting, but most likely you'd be able to see some shading across the object, so that there are some darker and lighter pixels. In, the HSV colour space you'd now have a line - all the H and S values for those pixels would be the same, but there would be a range of values for V. Now imagine you replaced the matt card with a shiny piece. You might see a whitish specular highlights where the object reflects a light source or the sky. The added whiteness would have made some pixels have a lower saturation (S). So now you have variation in S and V, but not H. That would form a plane in the HSV plot.
- pavel_lishin 9y agoBut the demos all form planes in RGB... look at them - most are basically thin cylinders.