proc write_png_comment {file keyword text} {
set fh [open $file r+]
fconfigure $fh -encoding binary -translation binary -eofchar {}
if {[read $fh 8] != "\x89PNG\r\n\x1a\n"} { close $fh; return }
while {[set r [read $fh 8]] != ""} {
binary scan $r Ia4 len type
if {$type == "IDAT"} {
seek $fh -8 current
set pos [tell $fh]
set data [read $fh]
seek $fh $pos start
set size [binary format I [string length "${keyword}\x00${text}"]]
puts -nonewline $fh "${size}tEXt${keyword}\x00${text}\x00\x00\x00\x00$data"
close $fh
return
}
seek $fh [expr {$len + 4}] current
}
close $fh
return -code error "no data section found"
}--AFDoes not calculate the crc for the comment chunk. Todo: add crc obviously (maybe using tcllib crc package).
