kuliah 03 – color perception - ilmu komputer ipbyeni/files/ppcd/kuliah 03... · brightness...

9/11/2008

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Departemen Ilmu Komputer -IPBPengantar Pengolahan Citra Digital

Kuliah 03 – Color Perception

Yeni Herdiyeni

http://ilkom.fmipa.ipb.ac.id/~yeni/ppcd


11 September 2008 21999-2007 by Richard Alan

Peters II

The Eye

Diagram from http://webvision.med.utah.edu/



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The Retina




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The Retina


Light



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Pixelization of Color Images:

All Bands Equal

L – downsample factorR – information content



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16× Pixelization of Color Images:

R, G, & B Bands

L – downsampled bandR – information content

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In the Brain: from RGB to LHS

lum

inance

hue

satu

ratio

n

photo receptorsbrain

The eye has 3 types of photoreceptors:

sensitive to red, green, or blue light.

The brain transforms RGB into separate

brightness and color channels (e.g., LHS).


L H S – Luminance Hue Saturation


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Luminance (brightness) atau kesilauan; dimensi pengalaman visual yang terkaitdengan jumlah cahaya yang dilepaskan atau dipantulkan (intensitas cahaya)oleh sebuah benda

Hue adalah dimensi pengalaman visual yang dinyatakan oleh nama-nama warnadan terkait dengan panjang gelombang cahaya

Saturation adalah kejelasan atau kemurnian sebuah warna; dimensi pengalamanvisual terkait dengan kompleksitas gelombang cahaya , yaitu seberapa lebaratau sempit jangkauan gelombang cahaya. Ketika cahaya mengandung sebuahgelombang tunggal, maka dapat dikatakan bahwa warna murni dan warna yangdihasilkan dapat dikatakan tersaturasi penuh (saturasi sama dengan 1). Dititikekstrem lainnya adalah cahaya putih yang sepenuhnya tidak tersaturasi) atausaturasinya sama dengan nol.


HSV (Hue Saturation Value)


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16× Pixelization of Color Images:Luminance Only




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16× Pixelization of Color Images:

Chrominance (H+S) Only




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16×

Pix

eliz

atio

n


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16×

Pix

eliz

atio

n


These 4 images all have the same amount of digital information…



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16×

Pix

eliz

atio

n


… but different visual information.



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Color Sensing / Color Perception

These are approximations of the responses to the visible spectrum of the “red”, “green”, and “blue” receptors of a typical human eye.



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Note that the “red” receptor exhibits the same response at 4 different wavelengths …

1 2 3 4



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1 2 3 4

b1

g1

r1


… but the responses of the “green” and “blue” receptors differ …



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1 2 3 4

b2

g2

r2


… at each of the 4 locations so that …

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1 2 3 4

g3

b3

r3


… each of the 4 wavelengths is represented by a unique response from the set of 3 receptors.



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1 2 3 4

g4

b4

r4




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These are approximations of the responses to the visible spectrum of the “red”, “green”, and “blue” receptors of a typical human eye.

The simultaneous red + blue response causes us to perceive a continuous range of hues on a circle. No hue is greater than or less than any other hue.


11 September 2008 221999-2007 by Richard Alan Peters II

Color Images• Are constructed from three

intensity maps.

• Each intensity map is projected

through a color filter (e.g., red,

green, or blue, or cyan, magenta,

or yellow) to create a single

color image.

• The intensity maps are overlaid

to create a color image.

• Each pixel in a color image is a

three element vector.



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Color Images

on a CRT or

LCD Display

Intensity images are projected through dot-array color filters which are slightly offset from one another.

NTSC Color Space = Y (Luminance) I (Hue) Q (Saturation)



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Color Images In Print

Images are separated into four color bands, each of which is printed as a grid regularly spaced dots. A dot’s diameter varies in proportion to the intensity of the color.

CYMK Color Space

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Color Images in Print

The four colors are magenta, cyan, yellow, and black

CYMK Color Space



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Standard Halftone Screen Angles

Cyan: 105°

Yellow: 90°

Magenta: 75°

Black: 45°

The dot grids are created with a screen that overlays the intensity images.

The screens are oriented at different angles.

The resulting patterns are called “rosettes”.



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Color Separation / Halftoning

The original is separated into an intensity image for each of the four color bands.



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Each screened image is printed in its own color on the same page.

Each intensity image is multiplied by a corresponding “screen”,

Cyan Magenta

Yellow Black



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Stare at the dot in the center of the image

Color Perception: The Afterimage Effect



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The color “negatives” saturate the local receptors so that when the color is removed the agonist (opposite) color receptors remain saturated.

Color Perception: The Afterimage Effect



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Brightness Perception

Linear intensity changes are not seen as such.

image intensity profile

255

0

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The previous slide demonstrates the Weber-Fechner relation. The linear slope of the intensity change is perceived as logarithmic.

The green curve is the actual

intensity; the blue curve is the

perceived intensity.

1 2

1 2

g gg

g g

1

2

: Intensity 1

: Intensity 2

: Apparent change in brightness

g

g

g

Brightness Perception

kuliah 03 – color perception - ilmu komputer ipbyeni/files/ppcd/kuliah 03... · brightness...

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