Lecture 3. Fundamentals of Computer Graphics

Computer Graphics, a very broad term Fields Related to Computer Graphics Bitmap/Vector graphics, 2D/3D graphics, Computer-aided design, Digital video, Interactive Multimedia, Net Art, Subcategory of Computer Science Digital Imaging A field that includes digital photography, scanning, and composition and manipulation of bit-mapped graphics.

Analog to Digital Conversion Digital signals are generated by sampling analog signals Digital information is always a subset of analog information Digitization allow processing of images using computer, by providing better noise correction.

Bitmap (Raster) Graphics made up of a grid of dots known as pixels most common electronic medium for continuous- tone images, such as photographs or digital paintings resolution-dependent ( causes pixellation ) Photoshop

Vector Graphics made up of mathematically defined lines and curves called vectors move, resize, or change the color of a graphic object without losing the quality of the graphic resolution-independent Computer generated curves & objects Flash, Illustrator, Maya…

Bitmap vs Vector image

Anatomy of Bitmap Image Pixel Basic building block of a bitmap image Each pixel in the grid is the same size and has a tonal value 256 different levels or separate tones to create a smooth transition from dark to light

Anatomy of Bitmap Image Digital Data Based on binary numbers 1bit has a value either 0 or 1 8 bits=1 byte ( 2*2*2*2*2*2*2*2 = 256 combination, 0 – 255 ) 24 bits = 3 byte ( 256(8bit)*256*256 = 16,777,216 combination 0 – 16,777,215 ) 1024 bytes=1 kilobyte 1024 kilobytes=1 megabyte 1024 megabytes=1 gigabyte

Anatomy of Bitmap Image Bit Depth Number of bit needed to define a ton for a pixel

Resolution Pixel Dimension -The number of pixels along the height and width of a bitmap image - A pixel has no fixed size (it can be scaled or shrink ) - The pixel size is determined by the image resolution( for print) or by monitor resolution (for display)

Resolution Image Resolution -how large or small the pixels appear in the final printed image -the number of pixels per inch (ppi) printed on a page

Resolution 104 pixel wide at 60 ppi104 pixel wide at 30 ppi The greater the resolution the smaller the pixels and the greater the apparent sharpness of the image

Resolution

Resampling - changing the pixel dimensions (and therefore display size) of an image by interpolation technique such as pixel replication/ elimination. - downsample (decrease the number of pixels), information is deleted from the image. - resample up (increase the number of pixels), new pixels are added. - bilinear, bicubic and nearest neighbor

Resolution Downsampled image Original image Resampled-up image

Resolution Pixellation A status when the individual pixels of an image are visible for human eyes due to the low resolution.

Resolution Monitor Resolution - Image data are translated directly into monitor pixels. - Monitor resolution depends on the size of the monitor(15”, 21”) plus its pixel setting(1024*768, 640*480)

Resolution Printer Resolution - the number of ink dots per inch (dpi) Screen frequency - lines of cells per inch in a halftone screen. (lpi) - use an image resolution that is from 1.5 to at most 2 of screen frequency

Tips for Deciding Image resolution Output to Print -Divide the print resolution by a factor of 3. -When screen frequency is offered : Multiply the screen frequency by a factor of 2. -Minimum 150 ppi, ppi for high quality Screen display ( ex. web, digital video ) - 72 ppi  When you first scan images, use a resolution high enough to avoid resampling up

Digital Image File Size Determined by -pixel dimension (inter-related with image resolution and output dimensions) -bit depth -number of channels -embedded layers - Compression

Digital Image File Size Example. File size of an 8bit RGB (10 by 10) image Pixel dimension = 10*10 Bit depth = 8bit # of Channel = 3 (for Red, Blue, Green) Estimated File size = (Total pixel per channel) * (Bit Depth) * (# of channel ) = (10*10)*8bit*3 = 2400 bit = 300 byte ** The file size calculated in this manner does not imply the actual file size since an image file contains other information about the file as well as pixel information.

Reference Digital Imaging by Mark Galer and Les Horvat, Focal Press © 2001 Ars Electronica: Facing the Future, A Survey of Two Decades by Timothy Druckrey (ed), The MIT Press © 1999 Adobe Photoshop CS online help