Preliminaries

• Lab Discussion: Contrast and Color Control

Discretization

A digital image is a grid of numbers.

255 = white 0 = black

This is more of an opportunity than a limitation.

Basic Concepts of Image Discretization

Image size and resolution

Print Size

The physical dimensions of a printed image or original (e.g. 4 x 6 inches)

Pixel Dimensions

The height and width of the digital image measured in pixels (e.g. 1000 x 1500 pixels)

Resolution

The number of pixels per linear inch used in scanning or printing. Typically expressed as PPI (pixels per inch). The term DPI is also commonly used but often has different meanings in different contexts; this is a common source of confusion.

The basic relationship of print size, pixel dimensions and resolution:

Print size 3x5 in. Pixel dimensions 30x50 Resolution 10 ppi.

Scale Print Size 50% without resampling:

(same effect as doubling resolution without resampling)

(same effect as scaling pixel dimensions 50%)

Change resolution from 10 ppi to 5 ppi with resampling

Spots and Dots: Monitors, Printers, and Resolution

Many printers say things like "1440 dpi" on the case, implying that it can print each pixel of a 1440 dpi image.

Don't believe it.

For the printer, the "D" in dpi means a dot of ink or toner, not a pixel. There may be dozens or hundreds of such dots composing a single pixel.

Compare screen and printed output

Screen Resolution: It's not really 72 dpi

File formats and color depth