Re: [Freetype-devel] Re: GSOC - Distance Fields

[Werner LEMBERG]

>> How many times do you have to solve this for each glyph?
> 
> A considerable amount of time.  If you want the exact number it is
> `width * rows * number_of_conic_curves', without any optimization.

Ouch.

>> How many different solutions (aka curve points) do you have to find
>> for each curved segment?  I figure there are a lot of grid-curve
>> pairs to enumerate with most of the curve segment sampled fairly
>> frequently...
> 
> This can be as many as 65536, this is tied to the width and rows of
> the bitmap being generated.  The more the resolution the more curve
> points have to be checked.
>
> And yes there are various grid points which correspond to the same
> curve point.  If the grid point lies on the same perpendicular from
> the curve then their curve point might be the same.
> 
>> Do you see where I am going?
> 
> I'm not sure.  Are you telling me to use the results of the previous
> iteration?

I think he means that the code must be optimized as much as possible
to get fast rendering.  In particular, it is important to find and
eliminate hotspots.

> I have fixed the overflow, it was just my stupid calculation
> mistake.  [...]

Excellent.  Will check the code soon.

> I have updated the repositories and perhaps I can now start
> integrating it into freetype?

Certainly.  Please create a branch for your stuff and try to add your
code in small (if possible), concise commits.  This doesn't hold for
new files, which should be simply added as-is.

Some comments to your code.

* What does line 38 do?

    
https://github.com/preversewharf45/freetype2-sdf/blob/dcedba69423fc169a9ca95b6391902e1cf27e0b6/src/sdfgen.c#L38

  It seems that `pup` and `roots` aren't used at all.

* Function `get_min_conour` should probably be called
  `get_min_contour`.  Otherwise please explain in a comment what
  'conour' means.

* Our coding style is to use

    if ( foo )
      bar;

  for single-line statements instead of

    if ( foo )
    {
       bar;
    }

* Have you thought about iterative solutions to get the cubic roots
  necessary for the quadratic case?  Maybe this would be faster.

* Maybe there is a mathematical approximation to solving the
  fifth-grade polynomial.  I don't mean a better root-finding
  algorithm but a simpler representation of the curves so that we can
  avoid a fifth-grade polynomial altogether.  The same holds for the
  third-grade equation, of course.

* Typo: s/simliar/similar/

* If you end a comment with a full stop, please start it with an
  uppercase letter so that it becomes a normal English sentence.


    Werner


PS: Did you have a look at Behdad's `GLyphy` implementation?