Reading PNG Comments

This procedure reads the comment blocks from a PNG file. This functionality is also present in the tcllib png module.

proc read_png_comments file {
    set fh [open $file r]
    fconfigure $fh -encoding binary -translation binary -eofchar {}
    if {[read $fh 8] ne "\x89PNG\r\n\x1a\n"} {
        close $fh
    set text {}

    while {[set r [read $fh 8]] ne {}} {
        binary scan $r Ia4 len type
        set r [read $fh $len]
        if {[eof $fh]} {close $fh; return}
        if {$type eq {tEXt}} {
            lappend text [split $r \x00]
        } elseif {$type eq {iTXt}} {
            set keyword [lindex [split $r \x00] 0]
            set r [string range $r [expr {[string length $keyword] + 1}] end]
            binary scan $r cc comp method
            if {$comp == 0} {
                lappend text [linsert [
                    split [string range $r 2 end] \x00] 0 $keyword]
        seek $fh 4 current
    close $fh
    return $text

AF 2003-12-18:

Currently only supports uncompressed comments. Does not attempt to verify checksum. Todo: add inflate support.

Returns a list where each element is a comment.

Each comment is itself a list.

a plain text comment consists of 2 elements: the human readable keyword, and the text data.

a Unicode (international) comment is 4 elements: the human readable keyword, the language identifier, the translated keyword, and the unicode text data.