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Image Output
 
Following the introduction to halftones, we can now proceed to learn the complex association between the desired screen ruling and required output resolution. If you have not read the Halftone Screens pages, then please do so first.

The rasterised page

CELLS & Dots
Digital halftone dots are constructed within a Cell or defined grid area, and each Halftone Cell is the same size as the others. Output devices that use a laser for the creation of our output images also work with a defined grid where the entire page is divided into areas like pixels in a bitmap (sample on the left), except these tiny virtual areas in an output device are called Dots.

Therefore we can have Halftone Dots and Output Device Laser Dots. Remember these very different but correct references.

Laser scanning film

Various techniques are used to direct the laser light onto the film emulsion, but shown on the left is one where the laser beam is directed by a rotating mirror.

The film is advanced by the equivalent of the device resolution each time the mirror completes a scan across the width of the film. As the mirror scans across, the laser beam is either switched on or off in sync according to laser resolution, so producing (or stepping over) the tiny image dots of the mapped array (raster).

Your simple text and line images (solid colour) will be constructed in a fashion similar to that shown by the top image.

Animated Halftone Dot sizesGrey Levels
The ideal Halftone cell has a grid matrix of 16 X 16 = 256, allowing up to 256 different halftone dot sizes. The small animation above shows this, only on a 5 X 5 cell array.

A defined 'Grey Level' is one of the different values of halftone dots, 100% to 0% and any value in between, that can be output on a Laser Output Device.

To ensure that the maximum number of grey levels can be obtained at output for a single channel of a Black to White or CMYK image, we must be able to use the 16 X 16 Cell, at the required Halftone Screen Ruling, at an Output Device Resolution that will allow this.

Consider:
• A 150 screen ruling has 150 halftone dots per inch.
• To allow any one of the 256 grey values to be produced for any halftone dot, the Output Device Resolution will have to be 150 X 16 or 2,400 Laser Dots per inch (150 X 16 = 2,400).
• Postscript can only output a maximum of 256 different Grey Levels or Halftone Dot values.
• The finer the Screen Ruling, The higher the resolution required on the Output Device.
• The lower the number of Grey Levels that can be output, the less image detail that can be reproduced.

If an image (or page) is output for a screen ruling of 85# on a laser printer with a fixed resolution of 600 laser dots, then only 50 Grey Levels can be achieved, a pretty poor result. (600 / 85 = a little over 7. So a 7 X 7 Cell gives us 49 + 1 Grey Levels. Always add 1 to the result)

Dividing 600 by 16 gives us an ideal number of Grey Levels at a screen ruling of 37.5, but again a poor result because this time the screen ruling would be too coarse for normal viewing!.

# Dot size calculation
When creating a bitmap image, the accepted rule is to make the pixel resolution twice that of the required output halftone screen resolution (also see the Bitmap Resolutions page). From this we can deduce that there would be 4 bitmap pixels under any virtual or calculated halftone cell area. The average colour value of those 4 pixels (256 maximum per channel or colour) is used to calculate the desired halftone dot size or grey level for that cell.

Image Output CONTINUES on PAGE 2

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