Re: [NTG-context] drawarrow and transparency

3 May 2012

      On Wed, 2 May 2012 21:42:24 +0200
Marco  wrote:
...
On 2012-05-02 Alan BRASLAU  wrote:
...
For  example, in  the minimal  example below,  how can  one obtain
a  transparent  arrow  without  getting an  X-ray  vision  of  the
arrowhead?
Maybe I'm  reinventing the  wheel here,  but you  can write  a small
macro that only draws the lines you want. Here is an idea:
\starttext
\startMPpage
linecap  := squared;
  penscale := .5bp;
  pickup pencircle scaled penscale;
def drawmyarrow expr p = _apth:=p; _myfinarr enddef;
  def _myfinarr text t =
      draw _apth cutafter point (-ahlength+.1penscale) on
_apth t; fill arrowhead _apth  t
  enddef;
drawarrow   origin    -- (1cm,0)     withtransparency(1,.5) ;
  drawarrow   origin    -- (0,1cm)     withtransparency(1,.5) ;
  drawmyarrow (1.5cm,0) -- (2.5cm,0)   withtransparency(1,.5) ;
  drawmyarrow (1.5cm,0) -- (1.5cm,1cm) withtransparency(1,.5) ;
\stopMPpage
\stoptext
The (-ahlength+.1penscale)  are of  course empiric. You  should find
the correct  formula to always have  a nice match of  line and arrow
whatever ahlength and ahangle values are in use.
Marco
Yes, this is somewhat reinventing the wheel.

I believe that this is somewhat a bug with transparency under MetaFun.
For I tried the following:

\startMPcode
	picture pic ; pic := image(drawarrow origin--(1cm,0)) ;

	draw pic withtransparency(1,.5) ;
	draw pic rotated 90 withtransparency(1,.5) ;
\stopMPcode

Here, the arrowhead is totally opaque and only the "stem"
has transparency. Something is fishy.

Alan