Week 6 Lecture slides

1. Cosc 1P02 Week 6 Lecture slides "To succeed, jump as quickly at opportunities as you do at conclusions." --Benjamin Franklin

2. Light • Light is electromagnetic radiation and has properties of both waves and particles • Human perception of light • the visible light spectrum has wavelength of 370-730 nanommeters (nm, 10-9, 0.00000037-0.00000073 meters) • low acuity • cannot distinguish fine detail • TV screen, monitor, photographs actually contain many dots but we see a clear picture

3. Color • Different wavelengths of visible spectrum are sensed as different colors • ROYGBIV • Eyes have 3 sensors that trigger at different wavelengths • 425 nm (blue) • 550 nm (green) • 560 nm (red) • Brain interprets the values registered by the three censors as colour. • each sensor registers something at all wavelengths • highest value for wavelength closest to trigger wavelength • brightness (luminance) handled separately from colour

4. Picture Representation • Picture represented by a series of dots (pixels – picture elements) • if small enough eye won’t detect • pictures are two-dimensional • pixels arranged in rows and columns • e.g. monitor resolution (1024x768) • Can trick brain into seeing colour by providing 3 sources of light • e.g. orange light • actual wavelength will cause the brain to get a value for blue, green and red from the sensors • if transmit 3 colours (blue, green, red) in close proximity that trigger the 3 sensors at orange level, will “see” orange • Each pixel is really 3 dots: red, green and blue (RGB) • the amount (intensity) of each colour dot varied to produce the colours • e.g. blue at high intensity and others at low intensity will be seen as blue

5. Encoding (Digitizing) Pictures • Picture is a collection of pixels • width & height • Each pixel has three Color values • red (R), green (G) and blue (B) • represent the intensity of the light of that color • 1 byte gives 256 different intensities for each color • 3 bytes or 224 = 16,777,216 different colors • Digital camera • array of sensors (3 per pixel) pick up R, G and B (like eye) • 3 values (bytes) recorded per pixel • e.g. 10 megapixel camera • 3648 x 2726 = 9,980,928 pixels • at 3 bytes per pixel = 29,942,784 bytes or approximately 28Mb per picture

6. Simplest Representation

7. Colour Representation

8. Exploring Pictures • PictureInspector tool • Java program built using BasicIO & Media libraries • displays picture and allows zooming on a portion • E.g. • beach.jpg • Coordinate system • picture has width & height (in pixels) • top left corner is (0,0) • x-coordinate is column • y-coordinate is row • RGB values

9. Picture and PictureDisplayer • Picture • class in Media library • access to attributes and pixels of picture • pictures can be loaded from a file (.jpg) • PictureDisplayer • class in BasicIO library • provides a window on which a picture can be displayed • picture objects placed on PictureDisplayer • E.g. load and display a picture

10. Choosing a Color • Set color of each pixel • Sequence through all pixels • Picture is a collection • for-each • ASCIIPrompter to get RGB values • Pixel class • methods to get & set color of pixels • Example • Color constructor allows setting RGB • Black & white • Gray • if R=G=B we get gray • different shades with different intensities

11. Grayscale • Convert a picture into grayscale • essentially what we call a black and white picture • shades of gray indicating the luminance of the picture • Gray if R=G=B • set RGB to same value • average of RGB values • Color methods • Example • Note method may change the pixels of the color parameter • they stay changed • just like a method moving a turtle changes its position

12. The end