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Flatbed Scanning Hints
|PIXELS and RESOLUTION|
Suggested reading before these pages:
All of the topics in the 'GENERAL' classroom.
All of the topics in the 'COLOUR' classroom.
All of the topics in the 'IMAGE OUTPUT' classroom.
The picture above is typical of a scanning interface program. While the main window will vary from program to program, and options only available via drop-down menus and buttons, the options should be familiar to anyone that has attempted flatbed scanning.
The Preview area is necessary for checking placement and cropping of the new image. Comparing scans of equal size and resolution (ppi or Pixels per Inch) we find that an RGB or True Colour scan would be about 3 times the size in bytes as a Black & White Greyscale scan and a CMYK scan would be about 4 times the size of a Greyscale scan.
ie. Each colour level is correctly referred to as a channel (B&W Greyscale = 1, RGB = 3, CMYK = 4) and the colour values for each channel in every pixel has to be recorded (256 possible values, 0 to 255 per pixel).
Because of the large file sizes often encountered, it is wise to crop your scan to the smallest area. Not only will it scan faster but the file should be more manageable in a Bitmap Retouching Program (scanners only scan the cropped area, not the whole page).
Also ALWAYS crop your preview inside the extremities of the copy. Modern scanners usually (or can be set to) auto-calculate the full tonal range of the cropped area, and if any of the scanner-cover background is included in the crop area, the result will be very inaccurate scans.
The problems encountered when scanning Black & White LINE pictures and the different resolutions required are covered on the 'GENERAL - Resolutions' tutorial page, but compared as above with equal resolution, a Line scan or 1bit Bitmap will be smaller (in bytes) than a B&W Greyscale because each pixel is only represented as either a 0 or 1 (however, keep in mind that we never scan Line Scans at the same resolutions as Greyscales).
Now, perhaps the most important decision to be made before a scanned image is produced, is the question of what resolution to use. The following broad rule-of-thumb is applicable for any output device (i.e. printer) other than the drum output device connected to a high end trade scanner. This rule of thumb is an accepted Industry standard.
Firstly a number can be chosen between 1.6 and 2.4 and the number finally decided on can be arrived at from tests on your own equipment, but 2.0 can be used with confidence in most situations - the industry default. This number is then multiplied by the FINAL PRINT RESOLUTION represented as Halftone Dots per inch or the SCREEN RULING. This screen ruling is the number of Halftone DOTS printed over a linear inch.
1) A 300 pixel/inch (laser pixels) Laser Printer has a default or preferred maximum Halftone resolution of 47 dots/inch. Therefore, Scan Resolution = 47 x 2 = 94.
2) A commercially printed B&W or CMYK Colour picture is often printed at a screen ruling of 150 dots/inch. Therefore, Scan Resolution = 150 x 2 = 300.
Any output from bitmap/pixel information at resolutions above that required for a given screen ruling will not show any improvement on the final printed picture-detail quality. As explained in the 'Image Output' tutorial, the 2X rule of thumb equates to approximately four pixels of image data averaged for each single halftone dot, and is proven adequate.
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