Ebook An introduction to the visual system (2/E): Part 2
➤ Gửi thông báo lỗi ⚠️ Báo cáo tài liệu vi phạmNội dung chi tiết: Ebook An introduction to the visual system (2/E): Part 2
Ebook An introduction to the visual system (2/E): Part 2
7Colour constancyThe colour constancy problemOne of the most important functions of the visual system is to be able to recognise an object under a var Ebook An introduction to the visual system (2/E): Part 2riety of different viewing conditions. For this to be achieved, rhe stimulus features that make up that object must appear constant under these conditions. If stimulus parameters do not form a reliable ‘label’ for an object under dillcrcnt conditions, they are considerably devalued in their use to r Ebook An introduction to the visual system (2/E): Part 2he visual system, l or example, if we perceive a square shape on a video screen and the area it cover’s increases or decreases, we experience a senseEbook An introduction to the visual system (2/E): Part 2
of movement. The square seems to ger closer or further away. The visual system assumes that rhe size of rhe square wi II nor change, so that changes i7Colour constancyThe colour constancy problemOne of the most important functions of the visual system is to be able to recognise an object under a var Ebook An introduction to the visual system (2/E): Part 2rmal conditions, objects seldom change in size. Another example is ligliiness constancy. Over the course of a normal day. light levels change significantly. but tile apparent lightness of an object will change very little. The visual system scales its measure of lightness to rhe rest of rhe environm Ebook An introduction to the visual system (2/E): Part 2ent, so that rhe apparent lightness of an object will appear constant relative to its surroundings. A similar problem exists with the perception of coEbook An introduction to the visual system (2/E): Part 2
lour. Over the space of a day, rhe spectral content of daylight changes significantly (Figure 7.1). Tills means that the spectral content of light ref7Colour constancyThe colour constancy problemOne of the most important functions of the visual system is to be able to recognise an object under a var Ebook An introduction to the visual system (2/E): Part 2ed from them, thus a red object looks red because it reflects more long wave (red) light. However, surfaces and objects retain their colour in spite of wide-ranging changes in the wavelength and energy composition of rhe light reflected from them. This is called colour constancy, and is not only dis Ebook An introduction to the visual system (2/E): Part 2played by humans and primates, bur by a wide range of species from goldfish to honeybees. So it seems there is no prespecified wavelength compositionEbook An introduction to the visual system (2/E): Part 2
that leads to a colour and to that colour alone. If colours did change with every change in illumination,102 COLOUR CONSTANCYFigure 7.1(See also colou7Colour constancyThe colour constancy problemOne of the most important functions of the visual system is to be able to recognise an object under a var Ebook An introduction to the visual system (2/E): Part 2nm (reproduced with kind permission from Bruce McEvoy from the website http:// www.handprint.com).wavelength (in nanometres)then they would lose rheir significance as a biological signalling mcclianism since that object could no longer be reliably identified by its colour.The Land Mondrian experimen Ebook An introduction to the visual system (2/E): Part 2tsSome of the most important and influential studies on colour constancy were made by lidwin Herbert Land (1909-1991). land was a Harvard I JniversiryEbook An introduction to the visual system (2/E): Part 2
drop-out. who went on to become one of the most successful entrepreneurs in America. He developed a method for producing large sheets of artificial p7Colour constancyThe colour constancy problemOne of the most important functions of the visual system is to be able to recognise an object under a var Ebook An introduction to the visual system (2/E): Part 2 soon being used in cameras and sunglasses, and in wartime for range-finders and night adaptation goggles. This development was followed up in 1948 with an instant camera, which could produce a picture in 60 seconds, and Land and his company became very rich. However, for the last 35 years of his li Ebook An introduction to the visual system (2/E): Part 2fe. Land's chief obsession was with colour and colour constancy. As part of his experiments, he had observers view a multicoloured display made of parEbook An introduction to the visual system (2/E): Part 2
ches of paper of different colours pasted together (Land. 1964). Tins display was called a Colour Nfondrian. from the resemblance it bore to the paint7Colour constancyThe colour constancy problemOne of the most important functions of the visual system is to be able to recognise an object under a var Ebook An introduction to the visual system (2/E): Part 2scene with no recognisable objects to control for factors such as loaming and memory. No patch was surrounded by another of a single colour and the patches surrounding another patch differed inREFLECTScolour. This was to control for factors such as induced colours and colour contrast. The patches we Ebook An introduction to the visual system (2/E): Part 2re made of matt papers which reflected a constant amount of light in all directions. As a result, the display could be viewed from any angle without aEbook An introduction to the visual system (2/E): Part 2
ffecting the outcome of the experiment.The display was illuminated by three projectors, each equipped with a rheostat that allowed rhe intensity of rh7Colour constancyThe colour constancy problemOne of the most important functions of the visual system is to be able to recognise an object under a var Ebook An introduction to the visual system (2/E): Part 2reen light and rhe third projector only passed blue light. The intensity of light produced by each projector was measured using a telephotometer, so the relative amounts of the three wavelengths in the illumination could be calculated.In one experiment, rhe intensity of light reflected from a green Ebook An introduction to the visual system (2/E): Part 2parch was set so that it reflected 60 units of red light. 30 units of green light and 10 units of blue light. Test subjects reported the green patch aEbook An introduction to the visual system (2/E): Part 2
s being green in colour even though it reflected twice as much red as green light, and more red light than green and blue light pur together. So. Illi7Colour constancyThe colour constancy problemOne of the most important functions of the visual system is to be able to recognise an object under a var Ebook An introduction to the visual system (2/E): Part 2periment was repeated bur under slightly different conditions. The subject still observed the same patch, illuminated by the same light, but this lime the patch was viewed in isolation. The surrounding colour parches were nor visible. This is called rhe void viewing cơĩuỉữiơn. In Illis case flic per Ebook An introduction to the visual system (2/E): Part 2ceived colour of the patch corresponded to flic wavelength composition of the light reflected from it. If the surround was then slowly brought into viEbook An introduction to the visual system (2/E): Part 2
ew, rhe colour of rhe parch was immediately reported to be green. This suggests that rhe perceived colour of the patch was determined not only by the 7Colour constancyThe colour constancy problemOne of the most important functions of the visual system is to be able to recognise an object under a var Ebook An introduction to the visual system (2/E): Part 2 the position of the green patch was changed within the Mondrian, so Hull the surrounding patches were different, the perceived colour remained rhe same. This suggested that rhe relationship between rhe perceived colour and rhe wavelength composition of flic patch and its surrounding patch or patche Ebook An introduction to the visual system (2/E): Part 2s was not a simple one.Reflectance and lightness: the search for constancy in a changing worldTo construct a representation of colour that is constantEbook An introduction to the visual system (2/E): Part 2
with changes in the spectral illumination of a surface, the visual system must find some aspect of the stimulus which does not change. One physical c7Colour constancyThe colour constancy problemOne of the most important functions of the visual system is to be able to recognise an object under a var Ebook An introduction to the visual system (2/E): Part 2ance for given and blue light. If the intensity of the light104 COLOUR CONSTANCYFigure 7.2surface for light of a given wavelength is its efficiency for r eflecting light of dial wavelength, expressed as the percentage of the incident light of that wavelength which it reflects. The reflectance never Ebook An introduction to the visual system (2/E): Part 2changes, although the amounts incident on. and relcctcd from, the surface change continually. The surface shown here reflects 90%. 20% and s%. respectEbook An introduction to the visual system (2/E): Part 2
ively, of red. green and blue light, irrespective of the intensity of the illuminating light (modified from Zeki. 1993).incidentRedGreenBluelightRefle7Colour constancyThe colour constancy problemOne of the most important functions of the visual system is to be able to recognise an object under a var Ebook An introduction to the visual system (2/E): Part 2he visual system must ignore the information related to light intensities and concentrate purely on relative reflectance.One way of doing this is to compare rhe reflectance of different surfaces for light of the same wavelength. So. for example, consider two surfaces, a rod and a green one. The red Ebook An introduction to the visual system (2/E): Part 2surface will have a liigh reflectance for long-wave light and so reflect a high proportion of red light. The groen surface will have a low reflectanceEbook An introduction to the visual system (2/E): Part 2
for red light, and therefore only a small proportion of red light will be reflected from it. So, if rhe parches are illuminated by a red light, rhe r7Colour constancyThe colour constancy problemOne of the most important functions of the visual system is to be able to recognise an object under a var Ebook An introduction to the visual system (2/E): Part 2993). By determining the efficiency of different surfaces in a scene for reflecting light of a given wavelength, rhe brain builds a lightness record of rhe scene for that particular wavelength.When an entire scene is viewed, each surface will have a different lightness at every wavelength depending Ebook An introduction to the visual system (2/E): Part 2upon its efficiency for reflecting light of that wavelength. The record of that scene in terms of areas that are lighter or darker, is called its lighEbook An introduction to the visual system (2/E): Part 2
tness record (Zeki, 1993). In ordinary daylight, as in most light sources, there is a mixture of wavelengths, and each set of wavelengths will produceGọi ngay
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