CS 445 / 645: CS 445 / 645: Introductory Computer Introductory Computer

GraphicsGraphics

Color

Midterm ExamMidterm Exam

The Midterm Exam will be Tuesday, October 23rd

Review Session will be Thursday, October 18th.

Assignment 3Assignment 3

• Assignment 3 goes out today

• Due Sunday, October 21st at Midnight

ColorColor

• Next topic: Color

To understand how to make realistic images, we need a basic understanding of the physics and physiology of vision. Here we step away from the code and math for a bit to talk about basic principles.

Basics Of ColorBasics Of Color

• Elements of color:

Basics of ColorBasics of Color

• Physics: – Illumination

• Electromagnetic spectra

– Reflection• Material properties• Surface geometry and microgeometry (i.e., polished

versus matte versus brushed)

• Perception– Physiology and neurophysiology– Perceptual psychology

Physiology of VisionPhysiology of Vision

• The eye:

• The retina– Rods– Cones

• Color!

Physiology of VisionPhysiology of Vision

• The center of the retina is a densely packed region called the fovea. – Cones much denser here than the

periphery

Physiology of Vision: ConesPhysiology of Vision: Cones

• Three types of cones:– L or R, most sensitive to red light (610 nm) – M or G, most sensitive to green light (560 nm)– S or B, most sensitive to blue light (430 nm)

– Color blindness results from missing cone type(s)

Physiology of Vision: The Physiology of Vision: The RetinaRetina

• Strangely, rods and cones are at the back of the retina, behind a mostly-transparent neural structure that collects their response.

• http://www.trueorigin.org/retina.asp

Perception: MetamersPerception: Metamers

• A given perceptual sensation of color derives from the stimulus of all three cone types

• Identical perceptions of color can thus be caused by very different spectra

Perception: Other GotchasPerception: Other Gotchas

• Color perception is also difficult because:– It varies from person to person– It is affected by adaptation (stare at a light

bulb… don’t)– It is affected by surrounding color:

Perception: Relative IntensityPerception: Relative Intensity

We are not good at judging absolute intensity• Let’s illuminate pixels with white light on scale

of 0 - 1.0• Intensity difference of neighboring colored

rectangles with intensities:– 0.10 -> 0.11 (10% change)– 0.50 -> 0.55 (10% change)

will look the same• We perceive relative intensities, not absolute

Representing IntensitiesRepresenting Intensities

• Remaining in the world of black and white…• Use photometer to obtain min and max

brightness of monitor• This is the dynamic range

• Intensity ranges from min, I0, to max, 1.0

• How do we represent 256 shades of gray?

Representing IntensitiesRepresenting Intensities

• Equal distribution between min and max fails– relative change near max is much smaller

than near I0

– Ex: ¼, ½, ¾, 1• Preserve % change

– Ex: 1/8, ¼, ½, 1

– In = I0 * rnI0, n > 0

I0=I0

I1 = rI0

I2 = rI1 = r2I0

…

I255=rI254=r255I0

Dynamic RangesDynamic Ranges Dynamic Range Max # of Display (max / min illum) Perceived

Intensities (r=1.01)

• CRT: 50-200 400-530• Photo (print) 100 465• Photo (slide) 1000 700• B/W printout 100 465• Color printout 50 400• Newspaper10 234

Gamma CorrectionGamma Correction

• But most display devices are inherently nonlinear: Intensity = k(voltage)

– i.e., brightness(voltage) != 2*brightness(voltage/2) is between 2.2 and 2.5 on most monitors

• Common solution: gamma correction– Post-transformation on intensities to map them to

linear range on display device:– Can have separate for R, G, B

1

xy

Gamma CorrectionGamma Correction

• Some monitors perform the gamma correction in hardware (SGI’s)

• Others do not (most PCs)

• Tough to generate images that look good on both platforms (i.e. images from web pages)

HalftoningHalftoning

• A technique used in newspaper printing• Only two intensities are possible, blob of ink and

no blob of ink

• But, the size of the blob can be varied• Also, the dither patterns of small dots can be used

HalftoningHalftoning

HalftoningHalftoning

Back to colorBack to color

• Color is defined many ways

• Computer scientists frequently use:– Hue - The color we see (red, green, purple)– Saturation - How far is the color from gray (pink

is less saturated than red, sky blue is less saturated than royal blue)

– Brightness (Luminance) - How bright is the color

How well do we see color?How well do we see color?

• What color do we see the best?– Yellow-green at 550 nm

• What color do we see the worst?– Blue at 440 nm

• Flashback: Colortables (colormaps) for color storage

• Can perceive color differences of 10 nm at extremes (violet and red) and 2 nm between blue and yellow

How well do we see color?How well do we see color?

• 128 fully saturated hues can be distinguished

• Cannot perceive hue differences with less saturated light.

• Sensitivity to changes in saturation for a fixed hue and brightness ranges from 16 to 23 depending on hue.

Combining ColorsCombining Colors

Additive (RGB) Subtractive (CMYK)

Color SpacesColor Spaces

• Three types of cones suggests color is a 3D quantity. How to define 3D color space?

• Idea: shine given wavelength () on a screen, and mix three other wavelengths (R,G,B) on same screen. Have user adjust intensity of RGB until colors are identical:

CIE Color SpaceCIE Color Space• The CIE (Commission Internationale

d’Eclairage) came up with three hypothetical lights X, Y, and Z with these spectra:

• Idea: any wavelength can be matched perceptually by positive combinations of X,Y,Z

Note that:X ~ RY ~ GZ ~ B

CIE Color SpaceCIE Color Space

• The gamut of all colors perceivable is thus a three-dimensional shape in X,Y,Z

• Color = X’X + Y’Y + Z’Z

CIE Chromaticity Diagram CIE Chromaticity Diagram (1931)(1931)

For simplicity, we often project to the 2D plane X’+Y’+Z’=1

X’ = X’ / (X’+Y’+Z’)Y’ = Y’ / (X’+Y’+Z’)Z’ = 1 – X’ – Y’

Device Color GamutsDevice Color Gamuts

• Since X, Y, and Z are hypothetical light sources, no real device can produce the entire gamut of perceivable color

• Example: CRT monitor

RGB Color SpaceRGB Color Space

• Define colors with (r, g, b) amounts of red, green, and blue

Device Color GamutsDevice Color Gamuts

• The RGB color cube sits within CIE color space something like this:

Device Color GamutsDevice Color Gamuts

• We can use the CIE chromaticity diagram to compare the gamuts of various devices:

• Note, for example, that a color printercannot reproduceall shades availableon a color monitor

Converting Color SpacesConverting Color Spaces

• Simple matrix operation:

• The transformation C2 = M-12 M1 C1

yields RGB on monitor 2 that is equivalent to a given RGB on monitor 1

B

G

R

ZZZ

YYY

XXX

B

G

R

BGR

BGR

BGR

'

'

'

Converting Color SpacesConverting Color Spaces

• Converting between color models can also be expressed as such a matrix transform:

• Note the relative unimportance of blue in computing the Y

• YIQ is the color model used for color TV in America. Y is luminance, I & Q are color– Note: Y is the same as CIE’s Y – Result: backwards compatibility with B/W TV!

B

G

R

Q

I

Y

31.052.021.0

32.028.060.0

11.059.030.0

HSV Color SpaceHSV Color Space

• A more intuitive color space– H = Hue– S = Saturation– V = Value (or brightness)

ValueSaturation

Hue