Thursday, July 16, 2020
Wednesday, July 15, 2020
What is a afterimage?
An afterimage is a type of optical illusion in which an image continues to appear briefly even after exposure to the actual image has ended. There are two major types of afterimages: positive afterimages and negative afterimages. In some instances, the colors of the original stimulus are retained. This is known as a positive afterimage. In other cases, the colors may be reversed. This is known as a negative afterimage.(1)
What is Color Constancy?
Color constancy is the tendency of objects to appear the same color even under changing illumination. A yellow banana appears yellow whether you see it in the tungsten light of the kitchen or in sunlight outdoors. Under these different conditions, the banana reflects very different light spectra to your eyes, but you see instead its unchanging material properties. That is, you see that the banana tends to reflect more light in the ‘yellowish’ region of the spectrum compared with other objects, regardless of the spectrum of light shining on all of them, and you conclude that the banana is inherently ‘yellow’. Color constancy is not a property of objects; it is a perceptual phenomenon, the result of mechanisms in the eye and brain.(2)
As light changes, a red apple in a fruit bowl retains its redness. This happens because our experience of color depends on something more than the wavelength information received by the cones in our retina. That something more is the surrounding context. If you view only part of a red apple, its color will seem to change as the light changes. But if you see the whole apple as one item in a bowl of fresh fruits, its color will remain roughly constant as the lighting and wavelengths shift—a phenomenon known as color constancy. Dorothea Jameson (1985) noted that a chip colored blue under indoor lighting matches the wavelengths reflected by a gold chip in sunlight. Yet bring a bluebird indoors and it won’t look like a goldfinch. Likewise, a green leaf hanging from a brown branch may, when the illumination changes, reflect the same light energy that formerly came from the brown branch. Yet to us the leaf stays greenish and the branch stays brownish.
Though we take color constancy for granted, the phenomenon is truly remarkable. It demonstrates that our experience of color comes not just from the object—the color is not in the isolated leaf—but from everything around it as well. You and I see color thanks to our brains’ computations of the light reflected by any object relative to its surrounding objects. But only if we grew up with normal light, it seems. Monkeys raised under a restricted range of wavelengths later have great difficulty recognizing the same color when illumination varies.(1)
As light changes, a red apple in a fruit bowl retains its redness. This happens because our experience of color depends on something more than the wavelength information received by the cones in our retina. That something more is the surrounding context. If you view only part of a red apple, its color will seem to change as the light changes. But if you see the whole apple as one item in a bowl of fresh fruits, its color will remain roughly constant as the lighting and wavelengths shift—a phenomenon known as color constancy. Dorothea Jameson (1985) noted that a chip colored blue under indoor lighting matches the wavelengths reflected by a gold chip in sunlight. Yet bring a bluebird indoors and it won’t look like a goldfinch. Likewise, a green leaf hanging from a brown branch may, when the illumination changes, reflect the same light energy that formerly came from the brown branch. Yet to us the leaf stays greenish and the branch stays brownish.
Though we take color constancy for granted, the phenomenon is truly remarkable. It demonstrates that our experience of color comes not just from the object—the color is not in the isolated leaf—but from everything around it as well. You and I see color thanks to our brains’ computations of the light reflected by any object relative to its surrounding objects. But only if we grew up with normal light, it seems. Monkeys raised under a restricted range of wavelengths later have great difficulty recognizing the same color when illumination varies.(1)
(1) Psychology, David G Myers
(2) Current Biology Vol 17 No 21
(2) Current Biology Vol 17 No 21
What is Lightness Constancy?
White paper reflects 90 percent of the light falling on it; black paper, only 10 percent. In sunlight, a black paper may reflect 100 times more light than does a white paper viewed indoors, but it still looks black. This illustrates lightness constancy(brightness constancy);we perceive an object as having a constant lightness even while its illumination varies. Perceived lightness depends on relative luminance—the amount of light an object reflects relative to its surroundings. If you view sunlit black paper through a narrow tube so nothing else is visible, it may look gray, because in bright sunshine it reflects a fair amount of light. View it without the tube and it is again black, because it reflects much less light than the objects around it.(1)
(1) Psychology, David G Myers
https://www.diliaranasirova.com/assets/PSYC579/pdfs/01.1-Ware.pdf
Monday, July 13, 2020
Spinning Keystones
Saw this first on Optical Spy but after a bit of searching, I think the creator is Reddit user u/jn3008.