:: png-unfilter-line ( width prev curr filter -- curr' )
    prev :> c
    prev width tail-slice :> b
    curr :> a
    curr width tail-slice :> x
    x length [0,b)
    filter {
        { filter-none [ drop ] }
        { filter-sub [ [| n | n x nth n a nth + 256 wrap n x set-nth ] each ] }
        { filter-up [ [| n | n x nth n b nth + 256 wrap n x set-nth ] each ] }
        { filter-average [ [| n | n x nth n a nth n b nth + 2/ + 256 wrap n x set-nth ] each ] }
        { filter-paeth [ [| n | n x nth n a nth n b nth n c nth paeth + 256 wrap n x set-nth ] each ] }
    } case
    curr width tail ;

:: reverse-png-filter ( n lines -- byte-array )
    lines dup first length 0 <array> prefix
    [ n 1 - 0 <array> prepend ] map
    2 clump [
        first2 [ [ n ] 2dip ] [ n 1 - swap nth ] [ [ 0 n 1 - ] dip set-nth ] tri
        png-unfilter-line
    ] map B{ } concat-as ;
: png-image-bytes ( loading-png -- byte-array )
    [ png-bytes-per-pixel ] [ inflate-data ] [ png-group-width ] tri group rever
se-png-filter ;