CS 450: COMPUTER GRAPHICS REVIEW: RASTER IMAGES SPRING 2016 DR. MICHAEL J. REALE

Transcription

1 CS 450: COMPUTER GRAPHICS REVIEW: RASTER IMAGES SPRING 2016 DR. MICHAEL J. REALE

2 RASTER IMAGES VS. VECTOR IMAGES Raster = models data as rows and columns of equally-sized cells Most common way to handle both displays and images Alternative: Vector image = stores mathematical descriptions of lines, curves, and shapes effectively instructions Advantages: Resolution-independent Good for precision Disadvantages: Must be rasterized before display (on raster display devices)

3 CRT: CATHODE-RAY TUBE Vector Cathode-Ray Tube Used electron gun to draw on phosphor-coated screen Largely replaced by LCD monitors Two variants: Vector = draw curves explicitly Precise, no aliasing Raster-scan Inflexible (curves, no filling, few colors) Raster-scan = scan across every vertical line Flexible, complexity of image doesn t matter Aliasing (jagged edges)

4 PLASMA DISPLAYS Plasma Displays Electricity passes through gas glowing plasma Very good display quality, but very expensive

5 LED DISPLAYS LED displays Each pixel is three LED lights (red, green, and blue) Mostly used for large billboard displays NOT usually what is meant when people say LED monitor 25,000-square-foot billboard in Times Square Resolution: 10,048 x 2,368

6 LCD DISPLAYS LCD = Liquid Crystal Displays Most common display device type Liquid crystal controls how much light passes through it Current passed through crystal changes structure Backlight can be: Mirror (ambient light) Fluorescent light LED lights Usually, LED monitor = LCD display backlit by LEDs

7 REFRESH RATE Refresh rate = frequency that picture is redrawn Usually expressed in Hertz (e.g., 60 Hz)

8 3D DISPLAYS Show two different images to each eye brain meshes together to get depth perception If using display device (monitor): Active 3D Special shutter glasses swap eyes and image on monitor Full resolution, BUT slightly darker image Passive 3D Polarized lens glasses special display showing both images at once Brighter image, BUT see only half resolution If using VR headset: No loss in resolution or brightness Heavier headset

9 RGB(A) COLOR MODEL Red, green, and blue = additive primary colors = mixtures of light Examples: Add red and green light get yellow light Add red and blue light get magenta light Add green and blue light get cyan light Add all three lights get white light Alpha = uses for transparency and blending

10 DISPLAY COLOR Common color storage variations: 8-bit indexed color 1 byte per pixel index into color palette with 256 colors 15/16-bit high color 2 bytes per pixel either or variations 24-bit true color 3 bytes per pixel 256 variations for each CHANNEL (red, green, blue) over 16 million colors Most common option now Deep color 30+ bits per pixel Not well supported yet Color depth = how many bits we use to represent color for each pixel

11 COLOR ARTIFACTS Clipping Clipping Brightness value clamped to maximum Image looks whitewashed Banding Caused by quantization Taking continuous range of brightness values cutting up in discrete levels Weird discontinuity between brightness levels Banding 4d1a6d370f448_hires.jpg 01.png

12 GAMMA CORRECTION Nonlinear relationship between voltage and output intensity in display devices Intensity Gamma correction Find power function that fits voltage-intensity relationship Apply inverse function Voltage

13 GAMMA CORRECTION EXAMPLE Example: Intensity (S = r 2.5 ) S = r 2.5 Gamma correction: S = r (1/2.5) = r 0.4 voltage r 2.5 r 0.4 intensity 0 Intensity (S = r 0.4 ) Voltage (r) 0 Voltage (r)

14 ALPHA COMPOSITING AND BLENDING Use alpha channel (or alpha bit, if only 1 bit) to store transparency information Alpha compositing Putting a foreground image (with transparency information) over the background If foreground pixel partially covers background need to know pixel coverage a C C f ( 1 ) C b Alpha blending Blending colors from semi-transparent foreground image/object and background image Final color Alpha * C (1 Alpha )* C source source source dest