Question

I’m learning Postscript I’m trying to create a method for that would draw a vertical gradient.

Here is my code:

%!PS-Adobe-3.0

%%%%%%%%%%%%%%%%%%%%%%%
% draw a RECTANGLE
/Rect {
/h exch def % height
/w exch def % width
w 0 rlineto
0 h rlineto
-1.0 w mul  0 rlineto
0 -1.0 h mul rlineto
} def
%%%%%%%%%%%%%%%%%%%%%%%
% draw a Gradient
/VGrad {
/h exch def % height
/w exch def % width
/c2 exch def %gray-end
/c1 exch def %gray-start
/index 0.0  def %loop-index

0 1 h { %loop over height
    gsave
    c2 c1 sub  index h div mul c1 add setgray

    w h index sub Rect
    stroke

    /index index 1.0 add def % index++
    grestore
    } for

} def
%%%%%%%%%%%%%%%%%%%%%%%
%test script
200 600 moveto
.1 .9 100 10 VGrad

showpage

But GS raises an error:

GPL Ghostscript 8.70 (2009-07-31)
Copyright (C) 2009 Artifex Software, Inc.  All rights reserved.
This software comes with NO WARRANTY: see the file PUBLIC for details.
Error: /undefinedresult in --div--
Operand stack:
   0   1   2   3   4   5   0.8   5.0   0.0
Execution stack:
   %interp_exit   .runexec2   --nostringval--   --nostringval--   --nostringval--   2   %stopped_push   --nostringval--   --nostringval--   --nostringval--   false   1   %stopped_push   1862   1   3   %oparray_pop   1861   1   3   %oparray_pop   1845   1   3   %oparray_pop   1739   1   3   %oparray_pop   --nostringval--   %errorexec_pop   .runexec2   --nostringval--   --nostringval--   --nostringval--   2   %stopped_push   --nostringval--   6   1   10   --nostringval--   %for_pos_int_continue   --nostringval--
Dictionary stack:
   --dict:1150/1684(ro)(G)--   --dict:0/20(G)--   --dict:75/200(L)--
Current allocation mode is local
Current file position is 588
GPL Ghostscript GPL Ghostscript 8.708.70: : Unrecoverable error, exit code 1
Unrecoverable error, exit code 1

where am I wrong ?

Answer 1

Ok, I found the problem: It seems that index is a reserved word. Here is a functional version:

/box
{
4 dict begin
/height exch def
/width exch def
/y exch def
/x exch def
x y moveto
width 0 rlineto
0 height rlineto
width -1 mul 0 rlineto
0 height -1 mul rlineto
end
} bind def


/gradient
{
4 dict begin
/height exch def
/width exch def
/y exch def
/x exch def
/i 0 def
height 2 div /i exch def

0 1 height 2 div {
    1 i height 2.0 div div sub setgray
    newpath
    x  
    y height 2 div i sub  add
    width
    i 2 mul
    box
    closepath
    fill
    i 1 sub /i exch def
    }for
newpath
0 setgray
0.4 setlinewidth
x y width height box
closepath
stroke
end
} bind def