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Browse Prior Art Database

Overall Gamma Correction for Halftone Displays

IP.com Disclosure Number: IPCOM000062357D
Original Publication Date: 1986-Nov-01
Included in the Prior Art Database: 2005-Mar-09
Document File: 2 page(s) / 14K

Publishing Venue

IBM

Related People

Chase, BD: AUTHOR

Abstract

In a graphics display for halftone images, the grey levels are arranged so that the eye perceives them as being equally distributed in steps from black to white. For a display gamma of n the digital data to be displayed is preprocessed using a power law with the reciprocal of the display gamma as the exponent to provide visually corrected display intensity with an overall gamma of one to match that of the eye. In the case of a display with no grey level capability the pels used are simply on or off and the overall effect tends to be somewhat granular, although this depends on the pel density. In the case where the display has a small number of grey levels available, say, between 4 and 16, then halftoning can still be used such that by varying the dot density all intermediate grey levels can be represented.

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Overall Gamma Correction for Halftone Displays

In a graphics display for halftone images, the grey levels are arranged so that the eye perceives them as being equally distributed in steps from black to white. For a display gamma of n the digital data to be displayed is preprocessed using a power law with the reciprocal of the display gamma as the exponent to provide visually corrected display intensity with an overall gamma of one to match that of the eye. In the case of a display with no grey level capability the pels used are simply on or off and the overall effect tends to be somewhat granular, although this depends on the pel density. In the case where the display has a small number of grey levels available, say, between 4 and 16, then halftoning can still be used such that by varying the dot density all intermediate grey levels can be represented. This is accomplished by displaying a mixture of dots from two adjacent grey levels such that the integrated light output is that of the desired intermediate. The image can still have a granular appearance if the visual difference between the two adjacent levels is large. In the method disclosed, the levels displayed are selected to balance out the visual difference between levels over the tonal range such that the granularity is spread evenly over the tonal range and hence minimized by preprocessing the image so that the original contrast ratios are maintained. This can also be referred to as ensuring that the overall image capture, process and display system has a gamma of 1. In a CRT display: Light out from display = (light from original image)(gamma) The gamma correction can be applied to image data as follows: new data = constant x (original data)(gamma) The constant can be chosen so that the maximum value of the new data is the same as that of the original. If a display has a gamma characteristic of unity, then the grey level range displayed by applying a linear range of input steps appears to the eye to be weighted heavily towards the white. In other words the eye can more easily distinguish differences between adjacent lower levels of greys than between high levels. When an image is halftoned to a set of levels such as 8, then the lower intensity levels in the image will be represented by a pattern of dots of the lowest displayed grey level on a black background. If the gamma of the display is unity, then the eye perceives this halftoning as a clearly distinguishable set of dots and this can be very distracting. In the lighter shades formed by white on near white the dots are not so noticeable or distracting. Here, the gamma of the display has a value, typically 2, where the grey levels (assumed 8) are equally distinguishable from one ano...