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Re: [ft-devel] Revisiting LSB [2]

From: George Williams
Subject: Re: [ft-devel] Revisiting LSB [2]
Date: 28 Oct 2005 14:08:17 -0700

On Fri, 2005-10-28 at 03:54, Ulrich wrote:
> This is correct but I still think that this fact doesn't imply that it isn't 
> possible to set up a test suite. As far as I understand FreeType will 
> generate a certain rasterisation result depending on the parameter settings 
> and the environment it detects. There might be and probably will be other 
> rasterisation results which are good or even better but they aren't 
> generated by FreeType because the algorithms chosen simply don't generate 
> them. 
The algorithms used by FreeType change over time.

> > Perhaps with truetype, with the bytecode interpreter turned on there is
> > a deterministic process which should always produce the same answer. 
> > 
> > But with the autohinter (or postscript hints) I do not believe this is
> > the case. Requiring such leaves no room for improvement of hinting.
> It seems that we're still talking about slightly different things. So far I 
> don't see why some bits of the rasterisation result are randomly set. Which 
> part of FreeType does introduce random? 
Sorry. I mean that using the bytecode interpreter it might be possible
to define a "correct" result for a glyph. Using the autohinter it is not
possible because today's autohinter is different from last year's. If
the autohinter chooses to move a spline a little bit more to the left
because we think it looks better today that will invalidate all test
suites based on the previous behavior.
> Of course, after improving the rasterisation algorithm checking the result 
> by a human being is required and the data of the test suite which are used 
> while comparing the results when the test suite is executed have to be 
> replaced by the new samples.
That's why I believe the testsuite is useless. I think this will happen
far too frequently, that the overhead involved in checking minor changes
will not be worth the results gained. 

It is not like a compiler testsuite where there is only one right answer
(except possibly in the case above). So any change to the autohinter
will probably introduce irrelevant changes in the results which will
require much checking and correction.
>  But even in that case a test suite might well 
> be helpful as not necessarily all generated glyphs get a new shape.
It's more than just shape. It's the difference between a pixel having
0xfe and 0xfd.
>  Work can 
> then be focussed upon the ones that have changed. 
> I would appreciate it if one of the experts shed some light on that. 
Anyway that remains my opinion. Your view keeps being repeated, so
obviously I am not convincing. Perhaps if you write the testsuite and
show that it is useful you can confound me.

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