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# Graphical images - PowerPoint PPT Presentation

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

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## PowerPoint Slideshow about 'Graphical images' - Jeffrey

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Presentation Transcript

• 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

• 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?

• 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

Red=Green=Blue=0 is black

(no color)

How to make pure white?

• 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%)

• 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

• 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

• 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)

+

=

• 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)

• 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

• .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

• 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).

• 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

• 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

• 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

• 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.