proc webpsize {fname} {
    set fin [open $fname rb]
    set chunk0 [read $fin 12]
    set chunk1 [read $fin 28]
    close $fin

    binary scan $chunk0 "a4ia4" riff size id
    if {$riff ne "RIFF" || $id ne "WEBP"} {
        error "not a webp file"
    }
    binary scan $chunk1 "a4" vp8
    if {$vp8 eq "VP8L"} { return [webp.VP8L $chunk1]}
    if {$vp8 eq "VP8 "} { return [webp.VP8 $chunk1] }
    # VP8X extended format blocks should be skipped but are handled.
    # Just seek $size bytes forward but I haven't seen an example to test.
    error "unknown VP8 block"
}
proc webp.VP8 {chunk1} {
    # http://tools.ietf.org/html/rfc6386#page-30
    binary scan $chunk1 "a4cu7cu3tt" vp8 frameTag startCode width height
    lassign $startCode b0 b1 b2
    if {$b0 != 0x9d || $b1 != 0x01 || $b2 != 0x2a} {
        error "missing start code block"
    }
    set width [expr {$width & 0x3fff}]
    set height [expr {$height & 0x3fff}]
    return [list $width $height]
}

proc webp.VP8L {chunk1} {
    # https://gerrit.chromium.org/gerrit/gitweb?p=webm/libwebp.git;a=blob;f=doc/webp-lossless-bitstream-spec.txt;hb=master
    binary scan $chunk1 a4icucu4 vp8 size signature sizeInfo
    lassign $sizeInfo b0 b1 b2 b3
    set width [expr {1 + ((($b1 & 0x3F) << 8) | $b0)}]
    set height [expr {1 + ((($b3 & 0xF) << 10) | ($b2 << 2) | (($b1 & 0xC0) >> 6))}]
    return [list $width $height]
}