#(define path->ps 
  (lambda (cmds) 
    (apply string-append 
           (map
             (lambda (f)
               (string-append 
                 (apply string-append 
                        (append 
                          (map 
                            (lambda (e) 
                              (string-append 
                                (number->string (exact->inexact e)) 
                                " ")) 
                            (cdr f))
                          (cons (symbol->string (car f)) '())))
                 " "))
             cmds))))
             
#(define-markup-command 
  (gradient-path layout props startr startg startb endr endg endb path res) 
  (number? number? number? number? number? number? list? integer?)
   (let*
    ((ps (path->ps path))
     (markup-test 
       (ly:text-interface::interpret-markup 
         layout props (markup #:line (#:path 0.1 path))))
     (xextnt (ly:stencil-extent markup-test X))
     (yextnt (ly:stencil-extent markup-test Y))
     (xlen (- (cdr xextnt) (car xextnt)))
     (ylen (- (cdr yextnt) (car yextnt)))
     (xstep (/ xlen res))
     (rshadestep (/ (- endr startr) res))
     (gshadestep (/ (- endg startg) res))
     (bshadestep (/ (- endb startb) res))
     (xlist (iota res (car xextnt) xstep))
     (xextls (map (lambda (e) (cons e xlen)) xlist))
     (rshadelist (iota res startr rshadestep))
     (gshadelist (iota res startg gshadestep))
     (bshadelist (iota res startb bshadestep))
     (mkps 
       (map 
         (lambda (a) 
           (markup 
             #:line 
               (#:postscript 
                 (string-append 
                   (number->string (exact->inexact (car a))) 
                   " -1000 " 
                   (number->string (exact->inexact (- (cdr a) (car a))))
                   " 2000 rectclip " 
                   ps 
                   " fill")))) 
         xextls))
     (colored 
       (map 
         (lambda (a r g b) (markup #:line (#:with-color (list r g b) a)))
         (reverse mkps) 
         rshadelist 
         gshadelist 
         bshadelist))
     (finalmkp (reduce make-combine-markup empty-markup colored)))
    (interpret-markup layout props finalmkp)))

samplePath =
#'((moveto 0 0)
   (lineto -10 10)
   (lineto 10 10)
   (lineto 10 -10)
   (curveto -50 -50 -50 50 -10 0)
   (closepath))
   
\markup 
  \translate #'(40 . -20) 
  %% \with-dimensions is needed to return a nice image in LSR
  \with-dimensions #'(-40 . 10) #'(-20 . 15)
  { \gradient-path #1 #0 #0 #0 #0 #1 #samplePath #100 }