Color Standards

1. Color Standards • A pixel color is represented as a point in 3-D space. Axis may be labeled as independent colors such as R, G, B or may use other independent indicators such as Hue, lightness, saturation. • RBG, HSV, HSL are the most popular. 240-373 Image Processing

2. RGB • RGB (Red green blue) • For CRT display (TV, computer monitor) • Additive combination of r g and b 240-373 Image Processing

3. HSV • HSV (Hue Saturation Value) Hue is effectively a measure of the wavelength of the main color. It has a value between 0-255 (0-360o, Red = 0o, Green=120o and Blue=240o.) Hue can be calculated from RGB values as follows: Rh = R - min (R, G, B) Gh = G - min (R, G, B) Bh = B - min (R, G, B) At least on of these values is 0. Hue value is in between. 240-373 Image Processing

4. HSV For example if Rh = 0 Hue angle = (240xBh + 120xGh ) / (Bh + Gh) If two are zero, then hue is the angle corresponding to the third non-zero color. If three are zero, then there is no color hue. The monitor will display a gray level (between black and white). 240-373 Image Processing

5. Saturation • Saturation is the amount of pure hue in the final color If Saturation = 0, final color is without hue ( white light only) If Saturation = 255, no white light in final color Saturation = (max(R,G,B) - min (R,G,B)) / max(R,G,B) 240-373 Image Processing

6. Value • Value (brightness) is a measure of the intensity of the brightest component and given by Value = max(R,G,B) 240-373 Image Processing

7. The HSV Model 240-373 Image Processing

8. HSV Example Given R=100, G=200, B= 40, convert this RGB color model to HSV. Rh = 100 - min(100,200,40) = 60 Gh = 200 - min (100,200,40) = 160 Bh = 40 - min (100,200,40) = 0 H = (60*0 + 160*(120*256/360))/(60+160) 240-373 Image Processing

9. HSV Example (Continued) Saturation = (max(R,G,B) - min (R,G,B)) / max(R,G,B) = (max(100,200,40)-min(100,200,40))/ max(100,200,40) = (200-40)/200 = 0.8 = 80% Saturation = 80*256/100 = 204 Value = max(100,200,40) = 200 = 200*100/256 = 78% 240-373 Image Processing

10. 240-373 Image Processing

11. HLS HLS (Hue Lightness Saturation) • Similar to HSV except that the hue angle start at Blue = 0o, and the model is double cone with a lightness axis going from L=0 (black) to L=1 (white) • For HLS, hue is calculated the same way as for HSV model except Blue = 0 and lightness and saturation are given by: • Lightness = (max(R,G,B) - min(R,G,B))/2 (max+min)/(max-min) if L <= 0.5 • Saturation = (max-min)/(2-max-min) otherwise 240-373 Image Processing

12. HLS 240-373 Image Processing

13. CMY & CMYK • CMY (Cyan Magenta Yellow) เป็นระบบสีที่พัฒนาขึ้นมาใช้สำหรับการพิมพ์ภาพสีโดยมีสีหลักคือสี Cyan Magenta และ Yellow ซึ่งเรียกว่า Subtractive primaries Color (สีแดง เขียวและน้ำเงิน เรียกว่า Additive primaries Color) ระบบสีแบบ CMY สามารถหาได้โดยการนำเอาสีในระบบ RGB ลบกับสีขาวดังนี้คือ 240-373 Image Processing

14. CMY & CMYK • ระบบสี CMY จะนำไปใช้สำหรับการพิมพ์ภาพสีแต่ยังไม่ดีเท่าที่ควรเนื่องจากไม่ยังไม่สามารถสร้างสีดำได้อย่างถูกต้อง ดังนั้นจึงมีการใช้ระบบ CMYK แทนโดย K แทนสีดำ 240-373 Image Processing

15. The Human Vision • Better than any camera yet developed • Eye has 2 classes of discrete light receptors; Cones and rods • 6-7 million cones, sensitive to bright light • >= 75 Million rods, sensitive to light intensity but not color • eye color perceptions 240-373 Image Processing

16. Color Additive • Problem with CRT 240-373 Image Processing