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# Digital Halftone Method for Matrix Displays

IP.com Disclosure Number: IPCOM000088466D
Original Publication Date: 1977-Jun-01
Included in the Prior Art Database: 2005-Mar-04
Document File: 3 page(s) / 44K

IBM

Thompson: AUTHOR

## Abstract

A method of generating digital halftones for matrix displays, e.g., cathode-ray tube and gas discharge display devices. The steps for carrying out the method of generating digital halftones are as follows: 1. Calculate the required enlargement/reduction ratio. 2. Define the output display or plot (PEL) resolution. 3. Calculate the gray level scanner sample spacing. 4. Define the input sharpness and tone control parameters. 5. Scan the photo and generate the digital halftone image.

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Digital Halftone Method for Matrix Displays

A method of generating digital halftones for matrix displays, e.g., cathode-ray tube and gas discharge display devices. The steps for carrying out the method of generating digital halftones are as follows: 1. Calculate the required enlargement/reduction ratio. 2. Define the output display or plot (PEL) resolution.
3. Calculate the gray level scanner sample spacing. 4. Define the input sharpness and tone control parameters. 5. Scan the photo and generate the digital halftone image.

Step 1 - The enlargement/reduction ratio may be calculated at the host by the equation E/R = output size over input size, e.g., 1 over 1 = 1X (1 to 1). 2 over1 = 2X (enlargement) or 1 over 2 = .5X (reduction).

Step 2 - The output matrix display picture element (PEL) spacing may be defined by the value P, as shown in Fig. 1, which, by way of example, may have a value equal to .012".

Step 3 - The gray level sample spacing of the input photo to be scanned may be defined by the value D, as shown in Fig. 2. This value may be calculated by the equation D = 2P over E/R. Thus, for example, if P = .012" and E/R = 3(enlargement), then D can be calculated to have a value equal to .008".

Step 4 - A variable factor K is defined to control the extent of sharp signal modification. Additionally, the tonal relationship between the input digital gray level and the output halftone is variable. Accordingly, a series of tonal conversion sets may be defined in a conversion table, and the set selected is the one best suited for the best reproduction of an individual photo.

Step 5 - Fig. 3 illustrates the apparatus for scanning the input photo and generating the digital halftone image. The scanner portion of the apparatus includes a drum on which a document is mounted. The drum is rotated to give motion in the "X" dimension while the carriage housing the scan-head assembly is driven by a leadscrew to impart motion in the "Y" dimension. An optical shaft encoder is mounted on the drum shaft and contains two tracks, one providing equally spaced pulses per revolution and the other providing an index pulse for registration purposes. The index and timing pulses are applied to a control unit for controlling the scan and generating apparatus in combination with a host processor.

The scan-head assembly, which houses the scan optics, is mounted on a carriage which is driven by the leadscrew operated by a stepping motor driven by the control unit. The scan optics consists of a lens-beam splitter combination which magnifies the image from the drum surface and splits the image through different sized aperture masks to two photosensors. The image surface is illuminated by two scan lamps focused through lenses onto the photograph. Thus, for each scan spot two concentric gray level measurements (sharp and unsharp) are made for an equivalent halftone cell area. The analog signals produced by the two photosensors undergo logarithmic amplification and a...