ly:stencil-extent gives not the exact value?

[harm6]

Hi,

the following function should rebuild a beam. 

\version "2.14.2"

#(set-global-staff-size 20)

#(define (test-stencil grob)
 (let* ((beam (ly:beam::print grob))
        (beam-extent-x (ly:stencil-extent beam X))
        (beam-length-x (interval-length beam-extent-x))         
        (single-beam-thickness (ly:grob-property grob 'beam-thickness))
        (beam-positions (ly:grob-property grob 'positions))
        (mrkp (markup #:beam    (* beam-length-x 0.97)  ;; why??
                        0               ;; level Beams only!
                        single-beam-thickness))
        (new-Beam (ly:stencil-in-color
                    (ly:stencil-translate-axis 
                      (ly:stencil-translate-axis 
                        (grob-interpret-markup grob mrkp)
                        -0.025 X)               ;; why??
                      (car beam-positions)
                      Y)
                      1 0 0)))

   new-Beam))

\relative c' {
  \override Beam #'layer = #-4
  \once \override Beam #'stencil = #test-stencil
  e'8 [e] 
  \override Beam #'color = #green
  f8 [f]
}

But I noticed two problems:
First I have to adjust the beam-length. (This needs to be done in addiction
to the global-staff-size.) Seems that ly:stencil-extent gives not the exact
value!?
What am I missing?

And second there's need to use ly:stencil-translate-axis to move the new
beam a little in X-axis-direction. But why?

Any hint is appreciated!

Cheers,
  Harm
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