Vision & Perception. Simple model: simple reflectance/illumination model. image: x(n 1,n 2 )=i(n 1,n 2 )r(n 1,n 2 ) 0 < r(n 1,n 2 ) < 1

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1 Visio & Perceptio Simple model: simple reflectace/illumiatio model Eye illumiatio source i( 1, 2 ) image: x( 1, 2 )=i( 1, 2 )r( 1, 2 ) reflectace term r( 1, 2 ) where 0 < i( 1, 2 ) < 0 < r( 1, 2 ) < 1

2 Visio & Perceptio Imagig o the retia (back of eye cosistig of photoreceptors) Focal poit of les Eye 2m Retial image 17mm 100m 20mm

3 Visio & Perceptio Visible rage of electromagetic spectrum is 350 m to 780 m. 380m 780m γ rays x rays ultraviolet visible ifrared microwaves radio λ

4 Visio & Perceptio Simple model for HVS eye brai optic erve HVS Iput (spatial pulse) What we see Approximate HVS with a LTI system HVS Primarily a BPF/LPF Output What we thik we see NOTE: The HVS is really a o-liear system.

5 Visio & Perceptio

6 RGB CIE spectral primary sources; CRT moitors CMY Priters; ik-based devices Traditioally, RGB primary colors, CMY complemets of RGB C = W - R M = W - G = R + B Y = W B = R + G R N G N B N NTSC receiver primaries; stadard for televisio receivers; three phosphor primaries that glow i the red, gree, ad blue regios of the visible spectrum YIQ NTSC trasmissio stadard; compatible with B/W TV broadcast; more efficiet trasmissio tha RGB HSV or HSB User-orieted, based o ituitive or perceptual measure Note: NTSC stads for Natioal Televisio Systems Committee

7 RGB (CIE primaries) color matchig fuctios T B (λ ) T R (λ ) T G (λ ) The tristimulus values (weights) of a arbitrary color C(λ ): t k = λ λ max mi C( λ) T k ( λ) dλ k = R, G, B

8 CIE Chromaticity Diagram CIE defied 3 stadard (hypothetical) primary sources called X, Y ad Z to replace R,G ad B. These ew primaries ca match all visible color with positive weights (positive matchig fuctios) Y color matchig fuctio matches the lumious efficiecy fuctio of the eye z y x λ (m)

9 Let The C = xx + yy + ' C = axx + ayy + will produce the same color but with a differet itesity; i.e., same Hue ad Saturatio, but differet Brightess Normalize by settig C x zz X azz a = x + y + y Y = + + z z Z where x = x y ; y = x + y + z x + y + z ; z = x + z y + z

10 Note: z x + y + z =1 x y =1 (Uit Plae) out of the 3 ormalized weights, oly 2 have to be specified oly 2 primaries eeded to defie color CIE diagram = projectio of Uit Plae ito (X,Y) plae x y The three values, ad defie hue ad saturatio but give o ifo about the brightess sice they are relative compoets A extra value is required to determie the itesity (Brightess) ad the value of Y is chose, I practice, ay absolute itesity value (x, y or z) may be specified to determie the brightess. z

11 CIE Chromaticity Diagram y Curve (Horse-shoe) boudary correspods to 100% pure colors 546.1m G All possible colors (of ormalized itesity) are displayed o CIE diagram The (MacAdam) ellipses are the just oticeable color differece ellipses. White: x = y = B 435.8m White Yellow 700m x R z = 1 x y = 0.333

12 YIQ: NTSC trasmissio stadard Y = Lumiace (same as CIE Y primary); color matchig fuctio idetical to lumious efficiecy fuctio V(λ ) I ad Q: chromiace compoets (give hue ad saturatio) Recodig of R N G N B N for trasmissio efficiecy Trasmissio efficiecy: Badwidth of I or Q < half badwidth of Y NTSC ecodig of YIQ ito a broadcast sigal assigs: 4 MHz to Y 1.5 MHz to I 0.6 MHz to Q I ad Q compoets cotai less iformatio less samples (more tha 50%less) used to represet I ad Q Dowward compatibility with B/W TV receivers (Y compoet)

13 Covertig R N G N B N to YIQ: Y = I Q Recall: L λ R G B max ( λ) = C( λ) V ( λ) λ mi dλ C(λ ) cosists of oly three compoets of weights R N at λ R, G N at λ G ad B N at λ B C(λ ) = R N δ(λ -λ R ) + G N δ(λ -λ G ) + B N δ(λ -λ G ) becomes a summatio weighted by the correspodig V(λ R ), V(λ G ) ad V(λ B ) Y = L(λ) = V(λ R ) C(λ R ) + V(λ G ) C(λ G ) + V(λ B ) C(λ B ) Colored light distributio = 0.30 R N G N B N N N N Y = 0.30R G B C(λ) B N G N R N λ B λ G λ R λ

14 Some useful trasformatios betwee color coordiate systems RGB to XYZ X Y Z = R G B R N G N B N to XYZ R G B N N N = X Y Z

15 Temporal Properties of Visio Importat for processig motio images (video) ad i the desig of image displays for statioary images Mai properties: Bloch s law If we expose a observer to flashig light where flashes have differet duratios but same eergy these duratios became idistiguishable below a critical duratio threshold d 1 d 2 Flash 1 duratio Flash 2 duratio d 1 idistiguishable of d 2 if d 1 d c ad d 2 d c This threshold was foud to be about 30 ms whe eye adapted at moderate illumiatio level The more the eye is adapted to dark, the loger the critical duratio

16 Temporal Properties of Visio Critical Fusio Frequecy (CFF) If flashig rate of light > CFF idividual flashes are idistiguishable; i.e., flashes are idistiguishable from a steady light at the same average itesity CFF does ot geerally exceed 50 to 60 Hz Basis for TV raster scaig cameras ad displays Iterlaced image fields sampled ad displayed at rates of 50 or 60 Hz Moder displays are refreshed at 60 frames/sec to avoid flicker perceptio

17 Temporal Properties of Visio Spatial versus Temporal effects: Eye more sesitive to flickerig of high spatial frequecies (i.e. flickerig edges) tha low spatial frequecies Useful i codig of motio video where movig areas are subsampled except at the edges (low spatial areas represeted by less samples)