Graphical images презентация

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

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?

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

only

Add some blue

How to make pure white?

an object
Saturation: strength of color (% grey)
Brightness: relative lightness from black (0%) to white (100%)

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

the range of a color model
RGB and CMYK show different subsets of colors

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)

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)

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

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

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).

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

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

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

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.