SNG has problems with control chars in zTXt
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#4 SNG has problems with control chars in zTXt
I'm attaching a file that has control characters in its zTXt text chunk. After converting the .png file to .sng, converting it back to another .png, and once again converting to .sng, I can see that these control characters get stripped out by SNG when I perform a diff of the two .sng files:
< #SNG: from am_engine.png
SNG: from am_engine3.png
44c44
< text: "# BEGIN DMI\nversion = 4.0\n\^Iwidth = 32\n\^Iheight = 64\nstate = \"core_on\"\n\^Idirs = 1\n\^Iframes = 7\n\^Idelay = 3,3,3,3,3,3,3\n\^Irewind = 1\nstate = \"injector_matter_antimatter\"\n\^Idirs = 1\n\^Iframes = 1\nstate = \"injector_antimatter\"\n\^Idirs = 1\n\^Iframes = 1\nstate = \"injector_matter\"\n\^Idirs = 1\n\^Iframes = 1\nstate = \"injector\"\n\^Idirs = 1\n\^Iframes = 1\nstate = \"antimatter_cell\"\n\^Idirs = 1\n\^Iframes = 1\nstate = \"matter_cell\"\n\^Idirs = 1\n\^Iframes = 1\nstate = \"core_connected\"\n\^Idirs = 1\n\^Iframes = 1\nstate = \"core_off\"\n\^Idirs = 1\n\^Iframes = 1\n\^Irewind = 1\n# END DMI\n";
text: "# BEGIN DMI\nversion = 4.0\n^Iwidth = 32\n^Iheight = 64\nstate = \"core_on\"\n^Idirs = 1\n^Iframes = 7\n^Idelay = 3,3,3,3,3,3,3\n^Irewind = 1\nstate = \"injector_matter_antimatter\"\n^Idirs = 1\n^Iframes = 1\nstate = \"injector_antimatter\"\n^Idirs = 1\n^Iframes = 1\nstate = \"injector_matter\"\n^Idirs = 1\n^Iframes = 1\nstate = \"injector\"\n^Idirs = 1\n^Iframes = 1\nstate = \"antimatter_cell\"\n^Idirs = 1\n^Iframes = 1\nstate = \"matter_cell\"\n^Idirs = 1\n^Iframes = 1\nstate = \"core_connected\"\n^Idirs = 1\n^Iframes = 1\nstate = \"core_off\"\n^Idirs = 1\n^Iframes = 1\n^Irewind = 1\n# END DMI\n";
Hm, that png does not seem to contain any zTXt chunk (looking at the file with a hex editor seems to confirm that). And with sng 1.0.5 it seems to survive the roundtrip just fine.
Also note that the PNG ISO spec says the following on the content of tEXt and zTXt chunks (see http://www.libpng.org/pub/png/spec/iso/index-object.html#11tEXt): "Characters other than those defined in Latin-1 plus the linefeed character have no defined meaning in tEXt chunks." -- in particular, control characters have undefined meaning.
That said, I still think they should survive a sng roundtrip; I merely wanted to point out that by including them in the first place, you may already be making a mistake...