# [OpenJDK 2D-Dev] Fix for uniformly scaled dashed lines.

Jim Graham james.graham at oracle.com
Sat Jun 19 01:19:59 UTC 2010

```Hi Denis,

Here are my thoughts on it:

- Lines are affinely transformed into lines.  The slope may be different
before and after the transform, but both have a single slope.

- The ratio of a line length to its transformed line length is a scale
factor that depends solely on the angle of the line.  Thus, for
determining dashing you can simply compute this scale factor once for a
given line and then that single scale factor can be applied to every
dash segment.

It appears that your setup code takes these factors into account, though
I haven't done a grueling line by line analysis as to whether you got
the math right.

One more optimization is that once you know the angle of the line then
you have a factor for how the length of a segment of that line relates
to its dx and dy.  Note that for horizontal and vertical lines one of
those factors may be Infinity, but every line will have a non-zero and
non-infinity factor for one of those two dimensions.

This means that you can calculate the dashing by simply looping along
the major axis of the line and comparing either the dx, or the dy to
scaled "lengths" that represent the lengths of the transformed dashes
projected onto the major axis.

Finally, the other dx,dy can be computed from the dx,dy of the major
axis with another scale.  I am pretty sure that this dx=>dy or dy=>dx
scale factor might be zero, but it would never be infinite if you are
calculating along the major axis of the transformed line, but I didn't
write out a proof for it.

Taking both of these concepts into account - can that make the inner
loop even simpler?

...jim

Denis Lila wrote:
> Hello.
>
> I think I have a fix for this bug: http://icedtea.classpath.org/bugzilla/show_bug.cgi?id=504
>
> The problem is caused by the "symmetric" variable in pisces/Dasher.java.
> symmetric is set to (m00 == m11 && m10 == -m01), and never changed.
>
> It is only used in one place (in lineTo) to simplify the computation of
> the length of the line before an affine transformation A was applied to it.
>
> This is why it causes a problem:
> If A = [[a00, a01], [a10, a11]] and (x,y) is a point obtained by applying
> A to some other point (x',y'), then what we want is the length of the vector
> (x',y'), which is ||Ainv*(x,y)||. Ainv = (1/det(A)) * [[a11, -a01],[-a10, a00]],
> so, after some calculations, ||Ainv*(x,y)|| ends up being equal to
> sqrt(x^2*(a11^2 + a10^2) + y^2*(a00^2 + a01^2) - x*y*(a11*a01 + a00*a10)) * 1/|det(A)|.
> If symmetric==true, this simplifies to:
> sqrt((a11^2 + a01^2) * (x^2 + y^2)) * 1/|det(A)|, and
> |det(A)| = a11^2 + a01^2, so, the final answer is:
> sqrt((x^2 + y^2)) / sqrt(det(A)). Therefore the problem in Dasher.java
> is that it divides by det(A), not sqrt(det(A)).
>
> My fix for this was to remove the "symmetric" special case. Another possible fix
> would have been to introduce an instance "sqrtldet" and set it to sqrt(det(A)),
> and divide by that instead of det(A). This didn't seem worth it, because the only
> benefit we gain by having the "symmetric" variable is to save 3 multiplications
> and 1 division per iteration of the while(true) loop, at the expense of making the
> code more complex, harder to read, introducing more opportunity for bugs, and adding
> hundreds of operations of overhead (since PiscesMath.sqrt would have to be called to
> initialize sqrtldet).
>
> To make up for this slight performance loss I have moved the code that computes
> the transformed dash vectors outside of the while loop, since they are constant
> and they only need to be computed once for any one line.
> Moreover, computing the constant dash vectors inside the loop causes
> them to not really be constant (since they're computed by dividing numbers that
> aren't constant). This can cause irregularities in dashes (see comment 14 in
> http://icedtea.classpath.org/bugzilla/show_bug.cgi?id=197).
>
> I would very much appreciate any comments/suggestions.
>
> Thank you,
> Denis Lila.
>

```