Glossary for Geospatial Science

  Technical vocabulary defined by MicroImages

book Glossary

framegrabber and framegrabbing:  Background: Composite video and US standard broadcasts repeat each field every 1/60 of a second.  Two interlaced fields, each containing alternate lines of the image make up one video frame that lasts 1/30 of a second.

A video framegrabber is a microcomputer interface board that accepts a video input signal and passes it to a color monitor.  A program signals the video frame-grabber to both freeze and digitize one video frame.

Digitizing a video frame may transform each picture element in the frame to a single byte in the board’s memory.  More commonly, it simultaneously captures, digitizes, and stores the video’s separate red, green, and blue color values.  Some framegrabbers can be set to grab only a single field to avoid the relative movement between a frame’s two fields.  If the video comes from a camera that has high-speed electronic shuttering (like 1/1000 of a second), movement in the 1/30 of a second between the primary field and the secondary field causes saw-toothed edges on alternate lines in straight features like road edges, and vertical poles.

As soon as the video is saved in the board memory (1/30 or 1/60 of a second), picture motion on the monitor freezes (even if the live video input continues) while the data in the board memory is converted into a display image.  Then TNTmips reads the memory of the board and transfers the image into project file raster objects.

Framegrabber boards should not be confused with video digitizing boards, which gradually sample and construct a digital representation of a still scene video image.  Sampling video boards represent an older technology, but are still used for non-standard, higher resolution video sources.  (See also: video digitizing board.)