the eye Light is electromagnetic radiation. The different wavelengths of the (to humans) visible part of the spectra make up the colors.

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1 Computer Assisted Image Analysis TF 3p and MN1 5p Color Image Processing Lecture 14 GW 6 How does the human eye perceive color? How can color be described using mathematics? Different color models. Use of color information in image analysis and visualization. Light is electromagnetic radiation. The different wavelengths of the (to humans) visible part of the spectra make up the colors carolina@cb.uu.se carolina@cb.uu.se 2 Color = the eye s and the brain s impression of electromagnetic radiation in the visual spectra. How is color perceived? illumination s( achromatic light - white or uncolored light, contains all visual wavelengths a complete mix chromatic light - colored monochromatic light - only one wavelength (laser) light source s( reflecting object r( detector rods & cones red-sensitive r( green-sensitive g( blue-sensitive b( reflection r( no color that we see consists of only one wavelength the dominating wavelength reflected by the object decides the color tone or hue if many wavelengths are reflected in equal amounts, the object appears gray (white black) carolina@cb.uu.se carolina@cb.uu.se 4 the eye only rods cone (tapp) rods (stav) ~130 milion rods & cones per eye optical nerve rods Sense luminance, or brightness, but not color. Are spread out across the whole retina, and dominate when the pupil is large, i.e. night vision. Less color is seen at night. The response is not linear, but logarithmic. The appearance of an object s intensity depends on the surroundings; the sensation is relative and not absolute. mostly cones radiance (W=J/s) = the total amount of energy flowing from a light source luminance (lumen=cd*sr) = amount of light an observer perceives (i.e. excludes non-visible energy) carolina@cb.uu.se carolina@cb.uu.se 6

2 red-sensitive r( green-sensitive g( blue-sensitive b( three kinds of cones b=435.8nm r=700nm g=546.1nm standard lightsource object reflectance s( r( CIE 1931 standard observer x x y x = z CIE XYZ values X=14.27% Y=14.31% Z=71.52% CIE standard (Comission Internationale de L Eclairage, 1931) wavelength In order to standardize the description of color, a large number of people were instructed to say what combination of basic colors a certain color sample consisted of in standard lighting. This resulted in the color matching curves, i.e. transform r(, g(, b( x(, y(, z( carolina@cb.uu.se 7 400nm 700nm 400nm 700nm 400nm 700nm X = s( r( x( dλ Y = s( r( y( dλ Z = s( r( z( dλ Each color is represented by a point (X,Y,Z) in the 3D CIE color space. The point is called the tristimulus value carolina@cb.uu.se 8 The projection of X+Y+Z=1 creates the CIE-XYZ chromaticity diagram x=red, y=green, z=blue) wavelengths along the edge CIE standard white when x=y=z it is not possible to produce all colors by mixing three. Light and reflection Achromatic light - white or not colored Chromatic light - color Monochromatic light - only one wavelength No color that we see (reflected) consists of only one wavelength. -the closest we come is the laser The dominating reflected wavelength gives the hue of the color. Reflection of the same amount of all wavelengths gives a greyscale carolina@cb.uu.se carolina@cb.uu.se 10 Color models Color models or color spaces are usually a subspace in a 3D system where each color is represented by a point. Examples of color models CIE-UCS CIE-LUV CIE-LAB RGB CMY } YIQ HSI } NCS } variations of the CIE-XYZ space, ex. UCS=Uniform Chromaticity Scale, has equal visual variance in all parts of space. hardware oriented useful for color image manipulation and color matching carolina@cb.uu.se 11 red yellow Mixing light and mixing pigment green magenta R C G = M R+B+G=white [] B 1-[] Y cyan blue magenta blue green (CMYK common in printing, where K is black pigment) carolina@cb.uu.se 12 red yellow cyan (additive) R+G=Y C+M+Y=black (subtractive) C+M=B etc...

3 RGB/CMY color space Separation of RGB grayscale color red green blue C [] M = Y transform for printing R [] 1- G B (0,0,0) =black (1,1,1)=white Ex. red spot on screen (1,0,0) produces pigment mix (1,1,1)-(1,0,0)=(0,1,1), meaning equal parts of magenta and yellow carolina@cb.uu.se 14 R G GBR RGR HLS (or HIS) color space HueLightnesSaturation (HueIntensitySaturation) Hue=dominant wavelength, tone Lightness=intensity, brightness Saturation=purity, dilution by white RRG Important aspects: Intensity decoupled from color Related to how humans perceive color B BRG carolina@cb.uu.se 15 ** The GW version of HLS space is a double triangle. We use the double cone in the course carolina@cb.uu.se 16 RGB HLS (double-cone model, not the same version as in GW) max=max(r,g,b) R,G,B [0 1] min=min(r,g,b) L=100*(max+min)/2 if (L<50) S=100*(max-min)/(max+min) else if (L=100) S=0 else S=100*(max-min)/(2-(max-min)) Rc=60*(max-r)/(max-min) Gc=60*(max-g)/(max-min) Bc=60*(max-b)/(max-min) if (R=max) H=Bc-Gc else if (G=max) H=120+Rc-Bc else H=240+Gc-Rc if (H<0) H=H+360 L [0 100] S [0 100] H [0-360] carolina@cb.uu.se carolina@cb.uu.se 18

4 Uses of the HLS color space The intensity description is separated from the color: can do operations on intensity (e.g. histogram equalization) but maintaining color. Similar to our way of describing and classifying color. Independent of light conditions/shadows Make a dark color image lighter 1. RGB HLS 2. histogram equalization on L 3. HLS RGB Lightness) (Hue) carolina@cb.uu.se carolina@cb.uu.se 22 YIQ color space Y=Lightness I=Inphase = amount red-green Q=Quadrature = amount blue-yellow Optimized for transmission (TV broadcast). Compatible with BW monitors (use only Y component) Uses the fact that the human eye is more sensitive to variations in lightness than variations in hue and saturation and more band with (bits) is used for Y carolina@cb.uu.se 23 NCS color description NCS=Natural Color System A psychological more than a physiological description of color. Common for description of paint color among artists, designers, in fashion etc R50B= 20% white w 60% black red with 50% blue carolina@cb.uu.se b c

5 Use of color in visualization The eye can distinguish between only about 30 different shades of gray, but something like colors. It is therefore often useful to display grayscale images using color to visualize the information better. This is called PSEUDOCOLORING. grayscale image transform color image Exemple of pseudocoloring Types of transforms: intensity slicing look-up table one transform per R, G, B three different kinds of spatial or frequency filters make up the R, G and B band **** it is very important to include the scale in the final image carolina@cb.uu.se carolina@cb.uu.se 26 Multi-spectral images divide spectra in to different intervals (spectral bands) capture one grayscale image of the intensity in each band Multi-spectral image, of Washington D.C., with R, G, B, and IR band. Bottom left shows RGB image, bottom left shows RIGB (where IR is treated as R). * invisible IR or UV bands can be displayed as R, G or B carolina@cb.uu.se carolina@cb.uu.se 28 Color image processing Color transformations treating R, G, and B equally will not change the image hue. (increase of contrast, exponential and logarithmic transformation). Example: smoothing an RGB image is the same as smoothing the R, G and B component independently. One can also use HLS space to decouple intensity and color, and treat H, L and S separately. (Important in histogram equalization) carolina@cb.uu.se carolina@cb.uu.se 30

6 Adjusting R, G and B, or more commonly C, M and Y, separately will change the hue (or tone) of the image carolina@cb.uu.se 31