From a news:comp.lang.tcl posting by Berry Kercheval < mailto:berry@kerch.com >:

And this works for simple GIF files:

 proc gifsize {name} {
    set f [open $name r]
    fconfigure $f -translation binary
    # read GIF signature -- check that this is 
    # either GIF87a or GIF89a 
    set sig [read $f 6]
    switch $sig {
        "GIF87a" -
        "GIF89a" {
            # do nothing
        }
        default {
            close $f
            error "$f is not a GIF file"
        }
    }
    # read "logical screen size", this is USUALLY the image size too.
    # interpreting the rest of the GIF specification is left as an exercise
    binary scan [read $f 2] s wid
    binary scan [read $f 2] s hgt
    close $f

    return [list $wid $hgt]
 }
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It leaked channels. Added some missing closes 11-jan-2002, Dave Griffin

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http://www.onicos.com/staff/iz/formats/gif.html
is a reference.
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[MG] Apr 30 2004 - Since GIF support is built into Tcl, could you not just do 
 
 catch {package require Img};# allow for jpeg/png/etc support, if possible
 proc imageSize {file {format ""}} {
 
  if { [catch {image create photo -file $file -format $format} img] } {
       if { $format == "" } {
            set form ""
          } else {
            set form "$format "
          }
       error "'$file' is not a recognisable ${form}image"
     }
  set info [list [image width $img] [image height $img]]
  image delete $img
  return $info;
 };# imageSize
 
 % imageSize ./path/to/image.gif
 % imageSize ./page/to/other/image.jpeg
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Peter Newman 1 May 2004: [MG]'s solution above is probably the best - because the GIF spec. allows various ways of specifying an image's height and width. As it says in the ''gifsize'' proc above; ''"logical screen size" ... is USUALLY the image size too''. That's absolutely true (and I've personally never come across a GIF that didn't do this). But the GIF spec leaves the door open for people to do things differently - so you'd imagine that a purpose built routine that (HOPEFULLY) copes with all the possibilities would be the most reliable solution. ''gifsize'' will probably work 99 times out of 100. But if you were coding (say) a ''gimp'' or ''photoshop'' clone, I'd personally go for something more thorough.

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See also [Reading JPEG image dimensions] and [Reading PNG image dimensions].
[Category Graphics]