Lesson 2:Introduction to Digital Imaging

Digital Photography

MITSAA

IAP 2003

Rob Zehner

Film vs. digitalFilm

• Mature technology• Very high resolution• Many form factors• Low equipment cost• Large infrastructure• Can be archival• Each exposure costs $• Harmful to the environment

Digital• Immature technology• Low resolution• Non-interchangeable• Expensive equipment• Little infrastructure• Infinitely archival (?)• Photos for free• Environmentally friendly

Pixels, Resolution, etc.

• Pixel = picture element– Only appropriate for use with displays, input

sources– 1 pixel is the smallest unit in a digital file– a pixel can take on many intensities / colors

• Printers work in dots or lines– dots are typically either on or off– 1 dot does not equal 1 pixel

Pixels, Resolution, etc.

• Color depth– Refers to the number of colors that each pixel can show

– Given in bits -- for N bits, actual # is 2N

– For color, total depth is 3X the depth for each primary color (red, green and blue)

• Effective resolution is determined by color depth, dots per inch– # gray values = (printer res./screen frequency)2 + 1

Pixels, Resolution, etc.• “Resolution” is a misnomer

– Usually refers to total number of pixels in an image (i.e. 3 megapixel camera)

– May also refer to dpi or lpi

– Meaningless without specifying a size for the output, and a corresponding color depth

• 150-300 dpi at 24-bit color is approximately photo-quality

• Computer monitors are usually 72-125 dpi at 24-bit color

Choosing a File Format

• Ultimate reference:– “Encyclopedia of Graphics File Formats,”

O’Reilly

• Many possible ways to save your images

• There are advantages and disadvantages to each

• Many formats are closely tied to particular functions

Compression

• Compression is crucial for storing image files– A 3Mpix file with 24-bit color takes up 9MB

uncompressed !

– This would make digital cameras impractical

• Two kinds of compression: lossless and lossy– Lossless: all original data is preserved

– Lossy: Relies on information theory, human perception, to throw out info with low info content

Graphics File Formats• GIF: Graphics Interchange Format

– Developed by CompuServe

– Only supports 256-color images (palette)

– Lossless compression algorithm

– Good for WWW• supports animation, transparency

• TIFF: Tagged Image File Format– The Swiss Army Knife of graphics formats

– Can be losslessly compressed, or uncompressed

– Immense variability in type can cause problems

Graphics File Formats

• JPEG: Joint Photographic Experts Group– Very common format for photographs– Supports full color depth images– Lossy compression

• Can give artifacts

• User can typically adjust the compression ratio

• Artifacts are worst in images with solid colors, sharp lines

Graphics File Formats

• PNG: Portable Network Graphics– An up-and-coming format– Attempting to replace GIF, TIFF– Free from IP issues

• PSD: Photoshop Project File– AFAIK, only supported by PhotoShop– Allows multiple layers, history support, etc.– Good for storing works-in-progress