Method for producing contrast-controlled grayscale characters

Roger D. Hersch, Claude Bétrisey, Justin Bur

U.S. Patent No. 5,796,409

The invention teaches grayscale specific character outline modification techniques for synthesizing contrast controlled grayscale pixmap characters of improved appearance at any resolution and size. In addition, methods for computing the visually optimal spacing of any two characters taking into account outline shape modifications induced by the character contrast control method. The synthesis of contrast controlled grayscale characters may include the following outline modification steps: (1) modifying the height of the character by placing its reference lines on pixel boundaries of the target pixel array, (2) verifying and correcting the width of character strokes such as bars and curved stems, (3) modifying the placement of vertical and horizontal bars and of bar-like terminal elements so as to ensure that each bar or terminal element has at least one high-contrast edge, (4) modifying the placement of vertical and horizontal curved stems so as to ensure similar optical correction behavior and similar intensity profile behavior of curved character parts throughout the characters of the considered character set. Visually optimal spacing of characters is obtained by converting the geometric space between the right, respectively left border of a character and its right, respectively left reference lines into its visually perceived equivalent. Converting between geometric and visually perceived spaces implies smoothing out the right, respectively left borders of the character and replacing them by virtual borders which represent the perceived space boundaries. The optimal visual distance between the origin of the first character and the origin of the next one is obtained by requiring the characters to have a perceived visual space equal or close to an ideal perceived visual space called ideal optical space extracted from adequately spaced character pairs such as "nn" for lower-case and "HH" for capital letters.

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