Resolution and Color Depth Issues

(C) Mustek, 1996

All rights reserved. This document may not be reprinted,reproduced or distributed in anyway without the express written permissionof Mustek.

Is More Really Better?

Every day we are bombarded with ads telling us more is better. MoreRAM is better. More CPU speed is better. Faster modems are better.In most cases it’s true, more is better. But is it true concerning scanners.Is a 1200 DPI scanner better than a 600 DPI scanner? In an absolutesense, yes. Particularly if you are scanning small items that you wantto enlarge before printing. But the question you should be asking yourselfis, "How often will I actually need the higher resolution capability?"

Prior to scanning any image, the first thing you should do is determinewhat resolution to scan at. Since modern advertising has conditioned usto think that more is always better, new scanner owners frequently scanat higher resolutions than they need to. In actuality, there is rarelya need to scan higher than 240 DPI. The resolution should always be determinedby the capability of the output device. I will try to put the resolutionissue in perspective by discussing scanning for glossy magazines.

Magazines are printed at 133 Lines Per Inch. Magazine printing technologyis not quite equivalent to Laser and Inkjet printer technology, so thelayout artists scan at 1.5 times the printing resolution. This means thatpictures in magazines are scanned at 200 DPI. That’s right, those beautifulpictures in the pile next to your easy chair were scanned at a mere 200DPI! Clearly more is not always better.

When scanning for output on an inkjet printer, you should scan at 1/3of the resolution you are going to print at. Modern inkjet printers havea maximum resolution of 720 DPI. Dividing 720 by 3 is how I got the 240DPI number mentioned above. But the 720 DPI inkjet printers can’t printat 720 on plain paper. When using plain paper on such a printer, you shouldset the printer driver to 360 DPI. Dividing 360 by 3 means you should scanat 120 DPI for printing on plain paper with your 720 DPI inkjet printer.

Does that mean if you scan at 720 DPI and print at 720 DPI on specialcoated paper that the picture is not going to look good? No. The picturewill look fine, but it will create a larger file on your hard drive. Tounderstand all of this, I recommend that you perform the following testwhich I recently did.

I scanned a 4" x 6" photograph at three different resolutions.240 DPI, 360 DPI and 720 DPI. I then printed all three images at 720 DPIusing the special coated paper recommended by my printer manufacturer.I wrote the scanned resolution number on the back of each print. When Ishowed all three to colleagues here at Mustek, none of them were able totell which was which. There was absolutely no difference in the qualityof the printed images!

However there was considerable difference in the size of the three filesthat I had saved on my hard disk! Here are the file sizes using the *.TIFfile format:

240 DPI = 4.032Mb

360 DPI = 9.074Mb

720 DPI = 36.295Mb

If I had done what most people do, scanned the image at the maximumresolution of my printer (720 DPI), I would be wasting about 32 megabytesof precious hard disk space storing data that I don’t really need.

My 720 DPI inkjet printer holds three different colors of ink.It can only print 240 dots of each color. The dots are printed close togetherand they blend with each other to produce the variety of colors in my scannedpicture. This is why I can get the results I need by scanning at 1/3 ofmy printer’s resolution.

So why do we make 1200 DPI scanners? Because some people need to scanpostage stamps and turn them into billboards. Well, not really. But someusers do have the need to enlarge their scanned images by several 100%.It takes a higher resolution scanner to do this. Unfortunately, it is beyondthe scope of this article to teach you how to scale images. In the nextfew months, we will have a tutorial section on this Web Site that willcover this type of task.

Color Depth Issues

If you have not read the article on UnderstandingScanner Mechanics, you should read it carefully prior to reading thissection.

Computers use a Binary number system to store information. We are usedto using a Decimal number system which has 10 possible digits (0-9). TheBinary number system only has 2 digits (0&1). These correspond to switchesbeing either on or off. The chips in your computer are nothing more thana complex system of switches that can be turned on and off by program codes.

When you only have 2 possible digits to work with, it takes more ofthem to represent 256 possible integers (0-255 being the 256 integers)than it does using the Decimal number system. Here is what 255 looks likein Binary:

11111111

Here is what 0 looks like in binary when stored as 8-bits:

00000000

All numbers between 0 and 255 will be various combinations of 1s and0s. (If you desire to understand the Binary number system further, talkto a mathematician or a computer programmer. This is about as far as Icare to understand it! You can play with the Windows calculator which iscapable of converting between Decimal and Binary. Open the calculator andchange it to Scientific Mode from the View Menu. Enter a number between0 and 255 then click the Binary button and see what happens. Enter 256and see what happens when converting to binary – you get 9-bits!)

It takes 8 digits (8-bits) to store the Decimal number 255 in computercode.

24-bit scanners can detect up to 256 different levels of intensity forthe three colors used by a monitor (Red, green and blue). The monitor usesdots of each of these three colors placed very closely together to trickyour eyes into seeing a broad spectrum of colors (16.77 million to be precise).

To store a single dot of color, the computer needs a value between 0-255for each of the three colors the monitor blends together to produce theperceived color. That means it takes 24-bits to store one dot of colorin a file on your hard disk. Below are the 24-bits for a dot of white:

11111111,11111111,11111111

A dot of black would be stored as 24 0s. there are 16.77 million uniquecombinations of 1s and 0s possible when you have 24 digits to work with.This is why 24-bit color scanners can scan up to 16.77 million colors.

When you scan in Gray Scale Mode, the computer only needs 8-bits ofinformation. These bits contain 1s and 0s representing gray intensitiesbetween 0 and 255 which makes a total of 256 gray shades. Eight 0s wouldbe black and eight 1s would be white.

When you scan in Lineart or Black&White mode, only 1-bit is neededto store the color of each dot of image data. A 0 represents black anda 1 represents white.

Monitor ColorDepth

All computer monitors use dots of red, green and blue to represent thecolors of an image. Current Video Card technology only allows 256 intensitylevels for each of these dots. Therefore, current computer monitors canonly display 24-bit color or 16.77 million colors. I doubt that this willever change because the human eye cannot distinguish more colors than this.There also would be a quadruple increase in file sizes which would clogthe "arteries" of your computer if video card technology advancesto 48-bit technology (the next logical step). Keep this 24-bit limitationof your video card in mind as it is quite important when we get to thediscusion of 30-bit and 36-bit scanners.

Video cards allow you to run your monitor in several different modes.You can set your monitor to display 16 colors, 256 colors, 16-bit color(65,000 colors) and 24-bit color (16.77 million colors). Some video cardshave a 32-bit mode, but this is merely a speed trick. Computers move informationin 16-bit or 32-bit chunks. When a video card allows 32-bit mode, it isstoring 24-bits of color information in 32 digits merely to make data transfersfaster. The extra 8-bits contain null information.

The amount of colors your computer monitor can display is dependententirely on the video card you have installed. If your computer is morethan 4 years old, you probably don’t have a 24-bit capability. Most machinespurchased within the last few years are capable of displaying 24-bit color.

The amount of RAM on your video card limits the color depth for eachresolution of your monitor. Most current video cards can display resolutionsup to 1,024 x 768. That is 1,024 pixels horizontally by 768 pixels vertically.(A pixel is one set of three dots, one each of red, green and blue. A pixelcan display one dot produced by the scanner.) If your video card has 1MBof RAM, you can display 24-bit color at 640 x 480 resolution. If you seta 1MB video card to 800 x 600 pixels, you will only be able to display16-bit color. If your video card has 2MB of RAM, you can display 24-bitcolor at 800 x 600 resolution. It takes 4MB of RAM on your video card todisplay 24-bit color at 1,024 x 768 resolution.

So how do I know what resolution and color depth my monitor is usingyou ask? Well, in Windows 95 it is easy to adjust. When you click the Displayicon in your control panel, a screen will come up which allows you to setthe resolution and color palette for your monitor. Click on the SettingsTab to get to the adjustments for your video card.

In Windows 3.1x, open the Main program group and double click WindowsSetup. The next screen will show your Screen Resolution and Color Depthsettings.

Gamma Settings

The images below were scanned at 72 DPI on a Mustek Paragon 800 II SPat various gamma settings. The first row of images are a portion of anAgfa IT8.7/2 Calibration Target.

Understanding the mechanics of your scanner, video card, monitor andvarious file formats will enable you to get good results from your Mustekscanner. With a little practice you will be producing the best possibleimages while keeping file sizes to a minimum. That is the test of a goodscanner operator. If you are putting the resulting images on a Web Page,those who access your page will appreciate your keeping image file sizesas small as possible.