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Pel Position Skewing for Anti-aliased Font Resolution Reduction

IP.com Disclosure Number: IPCOM000108651D
Original Publication Date: 1992-Jun-01
Included in the Prior Art Database: 2005-Mar-22
Document File: 3 page(s) / 156K

Publishing Venue

IBM

Related People

Edgar, AD: AUTHOR [+2]

Abstract

This article relates to the automatic translation of digital font characters to a lower resolution sampling grid. Rather than use a constant sampling interval, the sampling grid is skewed according to a desirability function to articulate the grid around the features of each character.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 43% of the total text.

Pel Position Skewing for Anti-aliased Font Resolution Reduction

       This article relates to the automatic translation of
digital font characters to a lower resolution sampling grid.  Rather
than use a constant sampling interval, the sampling grid is skewed
according to a desirability function to articulate the grid around
the features of each character.

      The present method of resolution conversion lays a rigid grid
of the new lower resolution over the character, and assigns to each
pixel in that grid a value based on the character coverage under that
pixel.  Such sampling at regular intervals does not account for
character structure.  Blurring or raggedness of character edges
occurs when that edge is randomly unaligned with the low resolution
pixels.

      The present invention preprocesses the font to create a map of
desirable sample points.  The lower resolution grid is made flexible,
and is warped slightly as it is laid over the character to align as
much as practical to the desirable sample points.

      For thin lines, these sample points should be placed in the
center of the line, and for thick lines, the sample points should be
placed a half-pixel width on each side of the boundary.  Thus, for
one case, pixels are positioned on a line center, irrespective of
edges, and in the other, they are positioned relative to edges,
irrespective of the center.  A specific algorithm to do this will be
given below.

      An expedient limits the grid warpage so horizontal and vertical
lines remain straight and orthogonal, thus the problem decomposes
into two passes: a one-dimensional problem for vertical sample
points, and a one-dimensional problem for horizontal sample points.
A third pass merges the one-dimensional vertical case of all letters
to stabilize baseline, lowercase height, and uppercase height across
the font.

      The method of building the vertical transition information for
desirability searches each column for horizontal pel transitions from
off to on or from on to off.  Only horizontal lines are counted, as
defined by a transition occurring in the present column as well as
the column to the left and to the right at the same height.  When the
transition is off-on, a desirability function (defined in the next
paragraph) is added to a work array offset to align with the height
of the transition, and when the transition is on-off, the function is
subtracted.  Following each processing of a column in this manner,
the absolute value of the work array is added to the vertical
desirability array and the work array cleared to zero until all
columns are processed.  Note that by following this algorithm, an
on-off transition in one column does not cancel an off-on transition
in another at the same height, but rather further adds to the
importance of that height as a desirable alignment height.

      The preferred desirability function is a modified sine wave
where a half cycle is approxima...