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CS32310 Assignment

THE THEORY OF COLOUR, AS USED IN COMPUTER GRAPHICS ail7. CS32310 Assignment. 17 Nov 2009. COLOUR IN COMPUTER GRAPHICS. What is colour? Visible spectrum and CIE Visual response of the human eye. Colour gamut. What is a pixel? Additive and subtractive colour models. RGB vs CMYK.

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CS32310 Assignment

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  1. THE THEORY OF COLOUR, AS USED IN COMPUTER GRAPHICS ail7 CS32310 Assignment 17 Nov 2009

  2. COLOUR IN COMPUTER GRAPHICS • What is colour? • Visible spectrum and CIE • Visual response of the human eye. • Colour gamut. • What is a pixel? • Additive and subtractive colour models. • RGB vs CMYK. • Greyscale. • LUT – Colour look-up table • Colour Quantization.

  3. WHAT IS COLOUR? • Colour is our perception, our response to the combination of light, object and observer. In order to fully use colour, we have to understand all of these, and we have to make sure that visual technologies that we develop are matched to the human visual capabilities.

  4. VISIBLE SPECTRUM It is the portion of the electromagnetic spectrum that is visible to (can be detected by) the human eye.

  5. VISIBLE SPECTRUM • This diagram represents the mapping of human colour perception in terms of two CIE parameters x and y. The spectral colours are distributed around the edge of the "colour space" as shown, and that outline includes all of the perceived hues and provides a framework for investigating colour.

  6. VISIBLE SPECTRUM • The diagram is associated with the 1931 CIE standard. • Human eye has receptors for short, middle and long wavelenghts. • Chromaticity diagram is a tool to specify how the human eye will experience light with given spectrum. It cannot specify colors of object or printing inks, the chromaticity of an object depends on the light source as well.

  7. Addition of three primary colours, in equal amounts, produces white and in some cases, the best trichromatic match is too desaturated, or mixed with white, to mach the test colour exactly. To obtain a match, an additional desaturating colour hat to be added to the test colour. This artifact leads to negative amounts in some colour matching equations. In order to avoid negative values The CIE adopted a system of colour measurement based on theoretical rather than actual colours. CIE SYSTEM AND AVOIDANCE OF NEGATIVE COLOUR INTENSITIES

  8. Perception of colour is different for every human eye (the way an individual's eyes respond to light waves and the sensation of colour in the human mind). Light is also very important in the perception of colour. The physics of the process:the light entering the eye, and the factors determining the spectral composition of the light. The physiology of the process:the retina, and visual pathway's response to the light. The psychology of the process:the colour actually received and the psychological response to the appearance of the colour perceived by the eye. VISUAL RESPONSE TO COLOUR

  9. COLOUR GAMUT The gamut is the set of possible colours within a colour system. No device in a publishing system is capable of reproducing the full range of colours viewable to the human eye. Each device operates within a specific colour space that can produce a certain range, or gamut of colours.

  10. Pixel • Number, Picture Element, a pixel is a single point in a graphic image. • On colour monitors, each pixel is actually composed of three dots: • a red, a blue, and a green one. Ideally, the three dots should all converge at the same point, but all monitors have some convergence error that can make color pixels appear fuzzy.

  11. COLOUR MODELS A colour model is an abstract mathematical model describing the way colours can be represented as tuples of numbers.

  12. ADDITIVE AND SUBTRACTIVE COLOR MODELS • Additive and subtractive colour models are based on absorbed and reflected light. • Additive colour theory deals with the properties of light, and describes the situation where, as you add colours to the projected light, the resulting colour will become closer to white. • Subtractive colour theory deals with reflected light. When colour is applied to a white board, the more colours you add, the more the resulting colour approaches black.

  13. RGB VS CMYK • Relationship between the RGB and CMY systems: • (r,g,b)RGB = (1,1,1) - (c,m,y)CMY • The colours in the RGB model are much brighter than the colours in the CMYK model. It is possible to attain a much larger percentage of the visible spectrum with the RGB model. That is because the RGB model uses transmitted light while the CMYK model uses reected light. The muted appearance of the CMYK model demonstrates the limitation of printing inks and the nature of reected light. If we want to see a properly printed picture, then printing must be in CMYK mode.

  14. PROS: 1. RGB colours are displayed according to digital numbers and are not dependent on the performance of any device, such as a monitor. 2. RGB files have three channels ( therefore require less memory, than for example CMYK). 3. Wide gamut. 4. Images are originally made in RGB. CONS: 1. Files are printed in CMYK (CMYK - narrow gamut, loss of data). THE PROS AND CONS OF THE RGB

  15. GREYSCALE • In photography and computing, a greyscale digital image is an image in which the value of each pixel is a single sample, that is, it carries only intensity information. Images of this sort, also known as black-and-white, are composed exclusively of shades of gray, varying from black at the weakest intensity to white at the strongest.

  16. It's a matrix of colour data that is searched in order to change a source set of colours to a destination set. LUT's are found in graphics devices, such as display adapters, in order to translate the colours in an image to the colours in the hardware. They are also found in graphics formats. A LUT is characterized by: * The number of entries in the palette: the maximum number of colours which can appear on screen simultaneously (a subset of the wider full palette, which is to be understood as the total number of colours that a given system is able to generate or manage, e.g. the full RGB colour palette). * The width of each entry in the palette: the number of colours which the wider full palette can represent. LUT – COLOUR LOOK-UP TABLE

  17. COLOUR QUANTIZATION Inexpensive display hardware stores 8 bits per pixel, so it can display at most 256 distinct colours at a time. To display a full-colour image, the computer must choose an appropriate set of representative colours and map the image into these colours. This process is called "colour quantization" Clearly, colour quantization is a lossy process.

  18. REVIEW OF MATERIAL • Colour and visible spectrum for human eye. • Colour gamut and range of viewable colours. • Pixels and colour models. • Greyscale. • LUT and colours Quantization.

  19. Bibliography • Diagnosis Of Defective Colour Vision. Jennifer Birch, 2001. • Colour Space (10.11.2009) : - http://en.wikipedia.org/wiki/Color_space • Colour Theory (10.11.2009) : - http://www.handprint.com/HP/WCL/color18a.html • LUT (10.11.2009) : - http://en.wikipedia.org/wiki/Colour_look-up_table • Indexed colour (10.11.2009) : - http://en.wikipedia.org/wiki/Indexed_color

  20. Bibliography • Colour Quantization (10.11.2009) - http://www.andreas-schrader.de/research-colorquant.html • Computer Graphic using OpenGL,3rd edition, F.S. Hill, Jr. And Stephen M. Kelly, Jr. , 2007.

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