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.
