0000881: Incorrect MPOST output in degenerated closed path case - MantisBT

ID	Project	Category	View Status	Date Submitted	Last Update

0000881	MetaPost	bug	public	2014-01-24 13:05	2014-01-24 13:05

Reporter	piotrst	Assigned To
Priority	normal	Severity	tweak	Reproducibility	always
Status	new	Resolution	open
Platform	PC	OS	Linux and Windows
Product Version	1.780

Summary	0000881: Incorrect MPOST output in degenerated closed path case
Description	Closed degenerated MPOST path: beginfig(1) draw (0,0)--(0,0) & cycle withpen pencircle scaled 1; endfig; bye. yields a strange and `too simple' result: %!PS %%BoundingBox: -1 -1 1 1 %% ... 0 0 0 setrgbcolor 0 1 dtransform truncate idtransform setlinewidth pop [] 0 setdash 1 setlinejoin 10 setmiterlimit newpath 0 0 moveto closepath stroke showpage which is incorrectly displayed in both Ghostscript and Adobe Reader.
Steps To Reproduce	Second, let's consider another degenerated case, not covered by Hans's amendment: beginfig(1) draw (0,0)--(0,0) withpen pencircle scaled 1; endfig; bye. The output PostScript code reads (most of comments skipped): %!PS %%BoundingBox: -1 -1 1 1 %% ... 0 0 0 setrgbcolor 0 1 dtransform truncate idtransform setlinewidth pop [] 0 setdash 1 setlinecap 1 setlinejoin 10 setmiterlimit newpath 0 0 moveto 0 0 lineto stroke showpage Ghostscript happily displays the correct, i.e., expected result (Adobe Reader also, after the distillation, of course). A slight modification of the degenerated case: beginfig(1) draw (0,0)--(0,0) & cycle withpen pencircle scaled 1; endfig; bye. yields a slightly modified result: %!PS %%BoundingBox: -1 -1 1 1 %% ... 0 0 0 setrgbcolor 0 1 dtransform truncate idtransform setlinewidth pop [] 0 setdash 1 setlinejoin 10 setmiterlimit newpath 0 0 moveto closepath stroke showpage Now, Gostscript displays nothing, and the respective PDF file does not contain a visible object according to Adobe Reader. The culprit is, somewhat surprisingly, the phrase "1 setlinecap", missing from the PostScript code in the latter case. (Observe that "1 setlinejoin" does not help.)
Additional Information	So, we would advise that the phrase "1 setlinecap" should be issued in degenerated cases (at least), to avoid admittedly rare but surprising results. Perhaps advisable would be replacing the phrase "0 0 moveto" by "0 0 moveto 0 0 rlineto", but we're not sure.
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Date Modified	Username	Field	Change
2014-01-24 13:05	piotrst	New Issue