Tk: bad output when converting image to PNG

[Christopher A. Chavez]

Ubuntu 16.04 (amd64)
Tk 8.6.5 (tk8.6 8.6.5-1)

The PNG support in Tk causes bad PNG files to be written.

Note that the TkImg extension (libtk-img) is NOT what exhibits this issue.

This issue was noticed as part of a test suite for Tcl::pTk, a Perl Tcl/Tk wrapper: https://rt.cpan.org/Ticket/Display.html?id=128803

I ran a Lubuntu 16.04.6 amd64 live CD, installed tk8.6 8.6.5-1, and could reproduce the reporter's issue. Using a Tcl/Tk example, I confirmed the issue was in Tcl/Tk rather than the Perl wrapper. However, I could not reproduce this issue on 18.04 i386 (tk8.6 8.6.8-4) nor 19.04 amd64 (tk8.6 8.6.9-2), so I suspect this issue has been resolved by the newer upstream releases of Tk 8.6 in more recent Ubuntu versions.

It turns out that the release notes for Tcl/Tk 8.6.7 mention a "PNG writing" issue having been addressed: see https://sourceforge.net/projects/tcl/files/Tcl/8.6.7/tcltk-release-notes-8.6.7.txt/view . I believe this is the corresponding upstream Tk ticket: https://core.tcl-lang.org/tk/tktview/10f2e7872b . That issue turned out to be caused Tcl, rather than Tk; ticket: https://core.tcl-lang.org/tcl/tktview/b26e38a3e4 , check-in: https://core.tcl-lang.org/tcl/info/e5b3cf7279 . Even if this is indeed the issue I've observed, I am not sure if it is practical to patch this issue, or to backport a later Tcl/Tk release to 16.04.

Below is a test script to reproduce the issue (run using tclsh8.6 or wish8.6). Xcamel.gif can be obtained from https://github.com/eserte/perl-tk/raw/master/Tk/Xcamel.gif or likely can be substituted.

package require Tk
# not doing `package require Img`--use PNG support included in Tk

image create photo icon -file "Xcamel.gif" -format gif
button .b -image icon
pack .b

icon write "Xcamel.png" -format png

# the following line will fail with "unfinalized data stream in PNG data"
image create photo icon2 -file "Xcamel.png" -format png
button .b2 -image icon2
pack .b2

[Christopher A. Chavez (chrstphrchvz)]

I believe this bug can be closed since Ubuntu 16.04 is EOL.