Original Article Date: Feb. 5, 2001
Need to calulate the compression ratio, dots per inch or displacement size of your raster images? Give this article a read.
Dimensions of the imageTo determine the dimensions in working units of the area covered by the raster document:X number of pixels * Pixel size = X dimensionY number of pixels * Pixel size = Y dimensionFor example, if you have 10 pixels and each pixel covers 2 feet on the ground then the dimension for that axis would be 10 x 2 = 20 feet.Image ResolutionIs used to define the number of dots that fit into one inch of the image. To determine the size in inches of each dot:1 inch / Dots per Inch = Image ResolutionFor example, you have an image scanned in at 600 DPI, what's the image resolution?Image Resolution = 1 / 600 DPI = 0.00166666Terrain ResolutionIs the 'ground distance' covered by each image dot and is calculated by:Scanned Scale / Dots per Inch = Terrain ResolutionFor example, you've got an image which was scanned at 600 DPI at a scale of 1:500, how much ground does one dot cover?Terrain Resolution = 500 / 600 DPI = 0.833333 inchesOutput ResolutionIs the maximum output resolution of the image when printing at a given size.Output Resolution = Image width in pixels / Paper width in inchesFor example, an image has 4800 pixels across it's width and needs to be plotted on 42" paper. What's the maximum output resolution? Output resolution = 4800 / 42 = 114dpiTo calculate image file size in bytes: Using sample data: 10 inch x 10 inch photo, Scanned at 600 DPI.8 bit gray scale Width of image in inches X length of image in inches X DPI X DPI = Size in bytesSample data: 10 x 10 x 600 x 600 = 36,000,000 bytes or 36mb24 bit RGB width of image in inches X length of image in inches X DPI X DPI X 3= Size in bytesSample data: 10 x 10 x 600 x 600 x 3 = 108,000,000 bytes or 108mb1 bit Binary (width of image in inches X length of image in inches X DPI X DPI) / 8 = Size in bytesSample data: (10 x 10 x 600 x 600) / 8 = 4,500,000 bytes or 4.5mb
AskInga Article #43