Re: metapost dashpattern (on 1) not shown in Mountain Lion Preview

[Xiao-Yong Jin <jinxiaoyong@gmail.com>]

On Sep 25, 2012, at 5:45 PM, Mojca Miklavec <mojca.miklavec.lists@gmail.com> wrote:

> On Tue, Sep 25, 2012 at 7:28 AM, Xiao-Yong Jin wrote:
>> 
>> Dear all,
>> 
>> I just upgrade to Mountain Lion and found out that "dashed dashpattern (on
>> 1)" no longer shows any line in Preview.  The attached file is a snapshot of
>> the Preview window showing the pdf file generated by the following code.
>> 
>> \starttext
>> \startMPpage
>> 
>> draw unitsquare scaled 4cm dashed dashpattern (on 1cm off 1cm) ;
>> draw unitsquare scaled 4cm shifted (5cm,0) dashed dashpattern (on 1cm) ;
>> draw unitsquare scaled 4cm shifted (10cm,0) dashed dashpattern (off 1cm) ;
>> 
>> setbounds currentpicture to boundingbox currentpicture enlarged 0.1cm ;
>> \stopMPpage
>> \stoptext
>> 
>> You can see, from the attached file, that the unitsquare which was
>> supposed to appear in the middle does not show up in the Preview window.
>> However the same file is displayed correctly in Acrobat Reader.
> 
> Awesome ;)
> 
> Just curious: what do you get if you try to draw the same plots with
> PostScript terminal in gnuplot?

The PostScript terminal in gnuplot works fine.  However, if I wrap the metapost code in my previous message with begin/endfig and run it with "mpost" to generate postscript, Preview does not show lines with "dashpattern (on 1)" either.

>> I am not sure whether it is a context problem, or it is just a bug in
>> Mountain Lion.  I found out this issue when I saw that all the pdf files I
>> previously generated with gnuplot context term are showing up incorrectly
>> under the Preview after upgrading to Mountain Lion.  I had to change the
>> dash pattern in file "metapost/context/third/gnuplot/mp-gnuplot.mp", so the
>> pdf generated by gnuplot context term can be viewed correctly with Preview
>> under Mountain Lion.
> 
> What modifications did you do exactly?

I simply changed "on" to "off" for line type -2 and 0.

> 
> It seems that "(on 1cm)" generates
>    [28.34645 0 ] 0 setdash
> and "(off 1cm)" generates
>    [] 0 setdash
> in PostScript file.
> 
> What I had in mind when I was writing the code was for "(on 1cm)" to generate
>    [] 0 setdash
> meaning a solid line. The other pattern "(off 1cm)" for linetype -3 is
> described in gnuplot's README as follows:
>    An lt of -3 is solid and drawn with xor (for temporary interactive
> annotations).
> and I actually had no idea what to draw since PostScript cannot draw
> XORed lines - I was most probably hoping to get an invisible line ;)
> 
> Thinking again about it, different transparency models in PDF are
> actually able to do some weird transformations (color subtraction
> etc.), but I'm not sure if I could get what I want.
> 
> Now, according to specification, the pattern
>    [28.34645 0 ] 0 setdash
> should actually draw almost "a solid line", or, to be mathematically
> precise, a line composed of 1cm long segments with zero-width gaps.
> The difference might be evident when drawing semi-transparent lines
> and linecaps might overlap. If you ask me, it's definitely a bug in
> Mountain Lion's renderer if it doesn't draw the line. On the other
> hand, it probably makes sense to patch MetaPost to generate "[] 0
> setdash" instead of "[<x> 0 ] 0 setdash" in case when it gets "(on
> <x>)" as an argument. But then again, the original code should
> probably call dashpattern() without any argument to get solid lines.

Simply call dashpattern() works as expected.

> 
> The argument "(off 1cm)" is weird anyway (my "bug") and honestly I'm
> not sure what would be the most appropriate behaviour in the eyes of
> metapost. Maybe not drawing anything at all? Throwing an error?
> Inserting "(on 0pt)" would be wrong as that actually draws dots. When
> using "(off 1cm on 0cm)" it works as it should - it draws dots (when
> using round linecaps).
> 
> I believe the best course of actions would be:
> - Filing a bug to Apple with a minimal EPS example. (I can try to
> prepare one.) But that won't help anyway. I've filed several bug
> reports and the only time when they replied was with asking if it was
> fixed when it wasn't. And 4 years have passed since, nothing happened.
> - I will replace "dashpattern(on 1cm)" with "dashpattern()" for
> linetype 0 and -2 in mp-gnuplot.mp. Can you please confirm if that
> works?
> - After ConTeXt Meeting/EuroTeX is over, we could discuss with Taco
> what MetaPost should do when it gets "(on 1cm)" or "(off 1cm)" as an
> argument. If nothing else, the manual should clearly describe the
> behaviour. Are you willing to open a ticket at
> http://tracker.luatex.org?

I'm not sure what to put in the ticket.  I guess it is more appropriate if you can open a ticket, since you know the details of the matter.

Best,
Xiaoyong

> Mojca
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