Pixel Borrowing, ClearType and Antialiasing


ClearType

During his keynote address at the most recent COMDEX conference,
Microsoft President Bill Gates announced "a  breakthrough technology",
called ClearType, to display type on LCD screens. The story was covered
thoroughly in the mainstream media as well as by the technical press and
the omnipresent Microsoft press relations machinery.

This article is partially about ClearType. But mostly it is about the problem
ClearType is intended to address: the low visual quality of most of the
information presented on computer screens. Here is a summary of what
I think (the same topics are also discussed in a recent article in the New
York Times.)

As this outline should make clear, I am no friend of ClearType. I agree with
the assertion  made by Bill Gates in the ClearType announcement that the
type we read on  our screens today is in need of immediate and substantial
improvement - although I would immediately add that vector graphics are
also equally wretched and in need  of similar improvement. I differ in my
prescription to deal with the problem: my bet is a combination of antialiasing
and higher display resolutions. I consider ClearType as a distraction. At best
it is  a benign placebo for a serious problem that requires surgery. At worst it
my be an excuse to have us talk about Microsoft in a context unrelated to
antitrust law.
 

ClearType or Pixel Borrowing?

The issue of ClearType's novelty - more precisely, the lack thereof - is
addressed by two online articles one in The New York Times and another
by Steve Gibson. Both point out  that similar, if  not identical, techniques
have been known for many years. The basic idea was used in the Apple II
computer,  where it was known as "pixel borrowing" and was the  subject
of a patent granted to Steve Wozniak, the Apple Computer co-founder.

More important than the question of novelty is whether pixel borrowing can
indeed improve substantially the visual quality of what we see in our screens.
The answer seems to be negative. In addition to some static examples of
pixel borrowing at work, Steve Gibson's web site also offers a small program
that allows you to experiment with the technique by using it on text set with
various fonts and your choice os sizes while varying certain rendering
parameters. Because there seems to be no available samples or demos from
Microsoft, the discussion below is based on Gibson's demo and my own
experiments.
 

The limitations of Pixel Borrowing

Steve Gibson describes the limitations of pixel borrowing as follows:

  • It only works on color LCD displays and does not work on high

  • resolution CRT displays.
     
  • It works well only for black shapes over a white background  or

  • white on black - high color contrast in all three primary colors is
    required for pixel borrowing to be effective.
     
  • It improves shapes in only one direction - the horizontal direction

  • for  most LCD displays.
    Another way to state these limitations is as follows: pixel borrowing will not
    help for colored text or black text over a colored background, will not help
    for most graphics, and will help very little for Kanji and other complex
    scripts that, unlike roman text, are helped little by resolution increases in
    only one direction.

    In addition to these limitations, I am a lot less enthusiastic than Microsoft
    about the basic promise of pixel borrowing even in the case where it does
    help, that of black latin text over a white background. My experiments and
    Steve Gibson's demo program show that the purest forms of pixel borrowing
    exhibit substantial color artifacts, similar to the jaggies of typical display text
    but at the color subpixel level. Careful (and computationally expensive)
    processing can reduce or eliminate these color artifacts but at the expense of
    spreading the intensity of each color subpixel across its neighbors, reducing
    the sharpness of edges which Microsoft claims as the main advantage of
    ClearType over antialiasing.

    Everything I have seen and heard tells me that the 200-300% increase in
    horizontal resolution claimed by Microsoft's literature for ClearType is based
    on little more that hopeful arithmetic. The argument seems to run along the
    following lines. There are three color subpixels per pixel and if they were fully
    utilized the resolution would increase by  300%; but one primary is likely to be
    less helpful  than the others (blue: we are less sensitive to it than to red or
    green) and some of that resolution needs to be "spent" to reduce the color
    artifacts, so maybe 200% is a more reasonable expectation.  Using  my own
    arithmetic to quantify the result of pixel borrowing is unlikely to shed much light
    on the matter, so I will refrain from it. Whatever the numerical value, I would not
    use "breakthrough" to describe the improvement produced by pixel borrowing.

    What kind of LCD screen do I have?
    How the examples were produced
    The actual pixel borrowing examples:
    Nearly vertical lines: the best situation for pixel borrowing
    Vertical lines: color artifacts galore
    A simple vector graphics example
    A very small text sample
    A more complex vector illustration



    References

    Naiman, Avi.C and Makous, Walter, "Undetected gray strips displace perceived edges
    non linearly". J. Opt. Soc. Am. A/ Vol. 10, No. 5/ may 1993.
     



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