Color Image Processing

Vishal Kumar Jaiswal

ColorIf light is achromatic (void of color), its only attribute is its intensity.

The term gray level refers to a scalar measure of intensity that ranges from black, to grays, and finally to white.

A very powerful descriptor that simplifies object identification and extraction from a scene.

Cones are the sensors in the eye responsible for color vision.

A body that reflects light that is balanced in all visible wavelengths appears white to the observer.

Color SpectrumNo single color may be called red, green or blue.

Quality descriptor of light sourceRadiance The total amount of energy that flows from the light source. measured in watt(W).

Luminance Gives a measure of amount of energy an observer perceives from a light source. measured in lumens(lm).

Brightness Subjective descriptor impossible to measure. It embodies the achromatic notion of intensity and is one of the key factors in describing color sensation.

Characteristics of ColorHue Hue represents dominant color wavelength in the mixture of light waves. When we call an object red, orange, or yellow; we are specifying its hue. It’s independent of intensity.

Saturation It refers to the relative purity or amount of white light mixed with a hue.The degree of saturation is inversely proportional to the amount of white light added. The pure spectrum colors are fully saturated and colors such as pink(red and white) are less saturated.

Brightness As defined earlier.

Color ModelA color model is a specification of a coordinate system and a subspace within that system where each color is represented by a single point.

Hardware oriented model

❏RGB(red, green, blue) for color monitors and color video camera.

❏CMY(cyan, magenta, yellow) for color printing.

Color image manipulation oriented model

❏HSI(Hue, saturation, intensity)

❏HSI(Hue, saturation, value)

Color Model: RGBNot very useful for image processing,

since RGB components can not be treated treated independently.

In histogram equalization of R,G,B respectively, three image intensities altered differently will not appear natural.

Color Model: CMYSecondary model of color.

RGB to CMY conversion:

Color Model: HSIWell suited to human perception as one does not refer to the color of

an automobile by giving the percentage of the primaries composing its colors.

Ideal for developing image processing algorithms.

It decouples intensity from color information.

H=0o -> Red,H=60o -> Green, H=120o -> Blue.

S is proportional to distance from point to center.Surface is purely saturated color.

I measured with respect to a line perpendicular to triangle and passing through its center.

Color Model: HSI

Tristimulus valuesThe amount of red, green, and blue needed to form any particular color is called the tristimulus values and are denoted X,Y and Z respectively.A color is then specified by its tristimulus coefficients.

x+y+z=1

Full color image processingApproach:

1.Process each color component individually and then form a composite processed color image from the individually processed components.

2.Work with color pixels directly.

Color images have at least three components, So a pixel is a vector.The pixel vector in RGB space:

Components are spatial variables in x and y.

Spatial Mask for neighborhood spatial processing

Color TransformationProcessing the components of a color image within the context of a

single color model. The transformation is modeled using the expression:

Where, f(x,y) is a color input image, g(x,y) is the transformed color output image, and T is an operator on f over a spatial

neighborhood of (x,y).

The principal difference is that pixel values are triplets from the color space chosen to represent the image.

Intensity TransformationTo modify intensity of an image using where 0<k<1.

In HSI color space, s3=kr3 where s1=r1 and s2=r2. Only HSI component r3 is modified.

In RGB color space, three components must be transformed. si=kri i=1,2,3.

In CMY color space, a similar set of linear transformation. si=kri+(1-k) i=1,2,3.

Color complementsThe hues directly opposite to one another on color circle are called

complements.

Analogous to gray level negative.

Useful for enhancing detail that is embedded in dark regions of a color image- particularly when regions are dominant in size.

Color SlicingHighlighting a specific range of colors in an image is useful for seperating objects from their surroundings.

Map the colors outside some range of interest to a nonprominent neutral color. If the colors of interest are enclosed by a cube (or hypercube for n>3) of width W and centered at a prototypical (e.g. average) color with components (a1,a2,a3,...an), the necessary transformation is

Tones and color correctionProvides facilities similiar to dark room on monitor.

Transformation performed on monitor and effectiveness is desired in print, So it’s necessary to maintain high degree of consistency between monitor and output devices.

The model of choice is the CIE L*a*b model, also called CIELAB.

L*a*b(lightness, red minus green, green minus blue) color space is colorimetric (i.e. colors perceived as matching are encoded identically), perceptually uniform(i.e. Differences among various hues are perceived uniformly) and device independent.

While not directly displayable, it can represent colors of any display, print or input device.

L*a*b color model

Thank You