Color Image Analysis

Chaur-Chin ChenInstitute of Information Systems and Applications

Department of Computer ScienceNational Tsing Hua UniversityE-mail: cchen@cs.nthu.edu.tw

Tel: +886 3 573 1078

Color Image Processing

• Three Primary Signals: Red, Green, Blue

• Representation of (R,G,B) Color Images

• A Palette of 256 colors from HP Hex Triplet Color Chart

• RGB and HSI Conversion

• RGB and YIQ Conversion

Color Images and Their Histograms

Color Images and Their Histograms

Koala and Its RGB Components

(R,G,B) Histograms of Koala

(R,G,B) Histograms of Starfruits

(R,G,B) Histograms of Greentrees

From JPEG to RGB • m=512; n=512; npixel=m*n;• I=imread('koala512.jpg'); % m * n * 3 = (R,G,B)• R=I(:,:,1); G=I(:,:,2); B=I(:,:,3);• hR=zeros(256); hG=zeros(256); hB=zeros(256);• for i=1:256• for j=1:256• r=1+R(i,j); g=1+G(i,j); b=1+B(i,j);• hR(r)=hR(r)+1; • hG(g)=hG(g)+1;• hB(b)=hB(b)+1;• end• end• for k=1:256• hR(k)=100.0*(hR(k)/npixel);• hG(k)=100.0*(hG(k)/npixel);• hB(k)=100.0*(hB(k)/npixel);• end• subplot(2,2,1)• imshow(R)• subplot(2,2,2)• imshow(G)• subplot(2,2,3)• imshow(B)• L=0:255;• subplot(2,2,4)• plot(L,hR,'r-',L,hG,'g-',L,hB,'b-')• title('RGB Histograms of 512 \times 512 Koala512')• xlabel('Intensity Levels')• ylabel('Percentage %')

RGB Hex Triplet Color Chart

• Red = FF0000• Green = 00FF00• Blue = 0000FF• Cyan = 00FFFF• Magenta= FF00FF• Yellow = FFFF00

Hue, Saturation, Intensity (HSI)

• Hue is a color attribute that describes a pure color, e.g., pure orange, pure green, pure cyan, whereas Saturation is a measure of the degree to which a pure color is diluted by a white light. Brightness (Intensity) is a subjective description that is practically impossible to measure.

• HSI is an ideal tool for developing image processing algorithms based on color description that are naturally intuitive to humans.

RGB ←→ HSI

• I = (R+G+B)/3• S=1-3*min{R,G,B}/(R+G+B) =1-min{R,G,B}/I• H=θ if B ≤ G, = 2π- θ if B >G, whereΘ=cos-1 {0.5[(R-G)+(R-B)]/[(R-G)2+(R-B)(G-B)]1/2}• B=I*(1-S)• R=I*{1+S*cos(H)/cos[(π/3)-H]}• G=1-(R+B)

RGB ←→ YIQ

• Convert (R,G,B) signals into uncorrelated (Y,I,Q) components for further processing and analysis, for example, compression

• Y=0.299R + 0.587G + 0.114B

• I =0.596R - 0.275G - 0.321B

• Q=0.212R – 0.523G + 0.312B

RGB YIQ Conversion