GhostScript draw text with alpha - pdf

Question

GhostScript version: 9.53.3
I need to stamp a pdf with (Entwurf) for all pages.
<<
/EndPage
{
gsave
/Helvetica-Bold 120 selectfont .5 .setfillconstantalpha .45 setgray
130 430 moveto 30 rotate (Entwurf) show -30 rotate
130 130 moveto 30 rotate (Entwurf) show -30 rotate
grestore
true
} bind
>> setpagedevice
Without .5 .setfillconstantalpha this script runs successfully. I have used 0.5 .setopacityalpha before but it crashes the same way and is deprecated since 9.53.
cat start.pdf | gs -q -dBATCH -dNOPAUSE -sDEVICE=pdfwrite -sOutputFile=/workspace/test.pdf Watermark_def.ps -
Error: /undefined in /--.endpage--
Operand stack:
(/tmp/gs_KvXeWS) --nostringval-- --dict:7/16(L)-- false --dict:4/6(L)-- 2 0 2 0.5
Execution stack:
%interp_exit .runexec2 --nostringval-- .endpage --nostringval-- 2 %stopped_push --nostringval-- .endpage .endpage false 1 %stopped_push 1990 1 3 %oparray_pop 1989 1 3 %oparray_pop 1977 1 3 %oparray_pop .endpage 1978 3 3 %oparray_pop .endpage .endpage 2 1 12 .endpage %for_pos_int_continue .endpage 1938 4 7 %oparray_pop .endpage 1820 6 7 %oparray_pop .endpage 9 .endpage
Dictionary stack:
--dict:731/1123(ro)(G)-- --dict:1/20(G)-- --dict:80/200(L)-- --dict:80/200(L)-- --dict:133/256(ro)(G)-- --dict:320/325(ro)(G)-- --dict:31/32(L)--
Current allocation mode is local
GPL Ghostscript 9.53.3: Unrecoverable error, exit code 1

Answer 1

That isn't a crash. It's a graceful exit on an error condition.
.setfillconstantalpha is a non-standard PostScript extension, PostScript does not support transparency (except in some limited fashions) so you can't use alpha blending in a normal PostScript program.
Ghostscript, because of the needs of PDF, does support transparency in the graphics library, and some limited support was added to the PostScript language to enable its use. This is decidedly non-standard and will not work on any other PostScript interpreter.
However, the use of these extensions is dependent on various operations happening in the correct sequence; get it wrong and it may crash. So by default, you can't use them in order to prevent malicious programs crashing the interpreter (or worse).
If you want to use these extensions (and take the risk of things blowing up) then you need to specifically allow them. This is documented, see the online documentation here and look for -dALLOWPSTRANSPARENCY. Also here.