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Graphical images

Graphical images. Bit-mapped (or raster-based) image: Matrix describing the individual dots that are the smallest elements (pixels) of resolution on a computer screen or printer Bit-map tools ‘paint’ pixels of images Vector-based image: Formulas representing geometric shapes

Jeffrey
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Graphical images

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  1. Graphical images • Bit-mapped (or raster-based) image: • Matrix describing the individual dots that are the smallest elements (pixels) of resolution on a computer screen or printer • Bit-map tools ‘paint’ pixels of images • Vector-based image: • Formulas representing geometric shapes • Vector-based tools ‘draw’ shapes of images

  2. Bit map color depths • N bit depth can represent a range of 2N colors • What is the range of 8-bit color depth (GIF)? • How many bits per pixel for monochrome? • How many bits per pixel for grey scale?

  3. More color depth • Why is 24-bit color called “millions of colors”? • What are the tradeoffs of more or less depth? • Photorealism vs. file size • JPEG preserves 16-bit color depth • Photoshop saves in 32-bit color depth: • 24-bit color: 8 bits each for red, green & blue • 8-bit alpha: transparency mask

  4. RGB model is additive Red=Green=Blue=0 is black (no color) Add Red only How to make pure white? Add some blue

  5. HSB model • Based on human perception • Hue: color reflected from or transmitted through an object • Saturation: strength of color (% grey) • Brightness: relative lightness from black (0%) to white (100%)

  6. CMYK model • Based on light-absorbing quality of ink printed on paper • As light is absorbed, part of spectrum is absorbed and part reflected back to eyes • So CMYK is a subtractive model (RGB is additive) • 4 channels: Cyan (C), magenta (M), yellow (Y), black (K) • In theory, pure colors should produce black, but printing inks contain impurities • So combining 3 pure colors produces muddy brown • Therefore pure K is needed to produce pure black • Hence CMYK is four-color process printing

  7. Color gamut • Eyes see more than screens or printers can show • Gamut is the range of a color model • RGB and CMYK show different subsets of colors

  8. Alpha channel • Alpha channel stores mask information • A mask lets part of an image be transparent so that other layers show through • 8-bit mask value determines degree of transparency • So Photoshop files have 3 channels for color and 1 alpha (32 bits per pixel) + =

  9. CLUT or Palette • 8-bit color is low-cost, limited gamut • GIF and web-safe colors are 8-bit • Indexing millions of colors to 256 is arbitrary • Here are some standard and custom palettes Macintosh CLUT (Color Look-Up Table) Web-safe palette (216 colors)

  10. Palette issues • What happens when converting from 24- to 8-bit color depth? • Dithering: finding closest possible color in palette • Why might a custom palette be useful? • Matching colors of a photograph • What happens when switching palettes? • Screen flashing • Avoid it by fading images to white or black

  11. Bitmap compression • .bmp, .pict or .psd files can be large • CODEC (compressor/decompressor) algorithms reduce file size • GIF codec removes repetition • E.g., if there’s a string of 100 pixels with the code 196, GIF encodes one 8 bit number to represent the color then two numbers that repeats that value 196 times

  12. Bitmap compression (2) • JPEG uses a more complex codec: • Since most neighboring pixels have similar colors, group them into blocks of 4 to 16 pixels. • Then determine average values for a block. E.g., for pixels 459BF0, 459BF2, 459BED and 459BF0, the average is 459BF0. • Then encode the average, plus differences for individual pixels. • Next, quantize the blocks, by calculating the average of a group of adjacent blocks. • GIF is a lossless algorithm, whereas JPEG is lossy (averaging may lose information).

  13. GIF or JPEG? • See graphics comparisons here • What rules of thumb do you infer about when to use GIF vs JPG? • GIFs work best large areas of the same color and a moderate level of detail • JPEG works best for continuous tone images, such as full-color photographs

  14. Vector-drawn images • Created from lines, rectangles, ovals, polygons using mathematical formulas • RECT 0,0,300,200,RED,BLUE says: • ”Draw a rectangle starting at 0,0 (upper left corner of screen) going 300 pixels horizontally right and 200 pixels downward, with a RED boundary and filled with BLUE.” • How can vectors be smaller than bitmaps? • Macromedia Freehand, Corel Draw & Adobe Illustrator are vector-drawing applications • Flash puts vector-drawing on the Web

  15. Drawing vs. painting • Drawing tools feature tools for drawing rectangles, polygons, ellipses, lines, freehand, plus reflect and scale • Convert bitmaps to vector formulas by autotracing shapes • E.g., in Flash 5, Modify > Trace BitMap • Paint/image processing feature tools for paint bucket, sharpen/blur, magic wand (to copy pixels) • Convert vector-based drawings to bitmaps by rasterizing

  16. 3D graphics • Extend vector-drawn graphics in 3 dimensions x, y and z • Objects as a whole have properties such as shape, color, texture, shading & location. Extrusion and lathing. • Modeling also deals with lighting:set camera view to project shadows • A 3-D applications lets you model an object’s shape, then render it completely • To create Toy Story, Pixar used a farm of 117 SparcStations, each with 192-384MB RAM plus swap space, running Pixar’s proprietary Renderman software.

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