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Pixels and Points: How Much Is That Picture In The Window?

by Lee Alexander

WYSIWYG (What You See Is What You Get) does not always hold between your monitor and your printer. Sister Susie sent you a photo via e-mail. It looked fine when viewed on your monitor's screen, but when you printed it – it came out the size of a postage stamp. What is this all about? The problem lies with a parameter called RESOLUTION.

In our fairly new world of electronics, the scene is digital – a land of zeros and ones, dots and pixels. Pixel is a newly coined word from Picture Element and is the unit of measurement for display devices. Dots or lines are the units for printers. These incremental units are measured against a unit of length – inches in the U.S. Thus we have ppi, pixels per inch, and dpi, dots per inch as the resolution of, respectively, your monitor and printer. Scanners' resolution is also in dpi.

Do you think of your monitor as a TV for the PC? Don't!

The NTSC standard for U.S. TV's established 525 horizontal lines at 60 Hz vertical frequency for the picture on the screen. That's it – 525 lines whether it is a seven inch “personal TV” or a 52 inch diagonal BIG SCREEN TV. The resolution of your PC monitor is adjustable in discrete steps. The size and range are dependent upon the monitor, the video card, and the driver software. IBM started this mish mosh with its VGA (Video Graphics Adapter) at 640 by 480 (width x height) pixels. Subsequent technological advances gave us SVGA (Super VGA) at 800 x 600 pixels, XGA (Extended VGA) at 1024 x 768 pixels, and so forth. These odd sounding combinations are a result of the digital schema – a mathematical base 2 of 1's and 0's – and the CRT's (Cathode Ray Tube) format of 4:3 aspect ratio. Consider the laptop's screen; it measures 14” diagonally (as advertised) giving a 8.4” high x 11.2” wide display. At the native resolution of 600 x 800 pixels, this computes to 71.4 pixels per inch, commonly referred to as 72 ppi.

“Standards” can be perplexing: we have 3 x 5 and 4 x 6 index cards; 8 ½ x 11, 8 ½ x 14 (legal) stationary; 4 x 6, 5 x 7, 8 x 10 photo prints. Now throw in the computer with its ability to “customize” to non-standard dimensions and you have a versatile but not always compatible situation.

How good a linguist must you be to cope with your PC? The video is in ppi, the text characters are in points (approximately 72 points to the inch), and the printer and scanner are in dpi. If you would like to see more than you ever wanted to know about printers' points, visit
www.shaunf.dircon.co.uk/shaun/metrology/points.html .

Getting back to Susie's photo – it was most likely a photo file intended for a PC monitor at 72 ppi and, when viewed at a fairly standard 800 x 600 resolution, it looked like a 4 x 6 print. Typical printer resolution for color photos is 300 dpi. The driver software can convert the ppi to dpi on a near one-to-one basis. Four inches of 72 ppi = 288 pixels, thus the printer put out a 288/300 dpi = 0.96 inch high picture instead of 4” high. On the other hand, if Susie had scanned a photograph at 1200 dpi and sent that file, the 4 x 6 would consist of 7200 pixels wide x 4800 pixels high. Trying to print that file with no adjustments would produce a printed image 24 inches wide x 16 inches high. Here is a note of caution – my printer is an HP 855c Deskjet. For Normal and Best black and white printing the resolution is 600dpi and for Draft, it is 300 dpi; for color, all modes are 300 dpi. This is just another instance where you should familiarize yourself with the equipment.

Photo editing software and an understanding of image file formats can solve this dilemma. The number of formats is manifold. Some of the more popular formats are:

•  Bitmap (.bmp) as in Microsoft's Windows application, Paint. This is a raster format, meaning the picture is constructed as of mosaic tiles. If you zoom in close enough, you will see the “jaggies,” the stair-like steps from pixel to pixel.

•  Tagged Image File Format –TIFF (.tif) is the most popular format for full information of images, including 24-bit color (more on that later).

•  Graphics Interchange Format (.gif) bought and licensed by Compuserve. This is one of the most compressed formats and is used extensively for transmitting images via the Internet. A major limitation is its support for only 8-bit (256) color.

•  Joint Photographic Experts Group (.jpg) is a very efficient compression scheme for images. The amount of compression is variable but lossy, pixels are removed to reduce the file size. For most digital cameras, once you reduce the resolution from the device's native value, the typical format is .jpg.

Color depth is the number of bits associated with each pixel. It can vary from 1-bit (black and white), 8-bit grayscale (256 shades of gray), 4-bit (16 colors), to 24-bit “True Color” or “RGB [Red, Green, Blue] capable of 16.7 million colors. Another specification is 32-bit CMYK [Cyan, Magenta, Yellow and Black]. Your eyes and the monitor live in the transmitted light of a RGB world; your printer, in the reflected light of CMYK inks. For this reason, do not expect an exact match of color between the two.

PMS (with respect to the ladies, not an ailment) is the abbreviation for the Pantone Matching System of inks, an industry standard. Eight basic colored inks: yellow, three reds, purple, two blues, and green; together with black and transparent white comprise a system yielding 505 standard colors. Deviations are, of course, possible. Richard Petty, of NASCAR auto racing fame, explains the color Petty Blue came about when they were starting. Low on cash, they simply mixed two cans of different shades of blue paint they found in their barn. It has been a trademark for them ever since.