\d png
PNGHDR:0x89504e470d0a1a0a
A7:(0 5 3 5 1 5 3 5
    6 6 6 6 6 6 6 6
    4 5 4 5 4 5 4 5
    6 6 6 6 6 6 6 6
    2 5 3 5 2 5 3 5
    6 6 6 6 6 6 6 6
    4 5 4 5 4 5 4 5
    6 6 6 6 6 6 6 6)

/ shell out for bit-oriented algos
decmp:{`x(("./png.py";"--decompress");x)}
cmp:{`x(("./png.py";"--compress");x)}
crc:{`c$256\.`x(("./png.py";"--crc");x)}

/ Basic chunk deciphering
ispng:{PNGHDR~8#x}
chunk:{(ln;nm):2 4#@[x;y+!8];ln:256/+/1(256*0>)\ln;(ln;nm)}
chnks:{+1_*+(~"IEND"~*|*:){(ln;nm):chunk[x;*|y];((ln;nm);ln+12+*|y)}[x]\(();8)}
data:{(ls;ns):chnks x;+(ns;+/(8+-1_+\8,12+ls;!'ls))}
idat:{x@,/(*|)'("IDAT"~/:*:')#data@x}
ihdr:{(w;h;f):256!0 4 8_x@16+!13;(256/+(w;h);f)}

/ filter routines
paeth:{(x;y;z)@*<{x|-x}((x+y)-z)-/:(x;y;z)}

unnone:{z}
unsub:{,/+\(0N,x)#z}
unup:{y+z}
unavg:{x_{[x;a;b;c]a,256!c+-2!b+a@(#a)-x}[x]/[x#0;y;z]}
unpaeth:{x_{[x;a;b;c;d];a,256!d+paeth.(a@(#a)-x;b;c)}[x]/[x#0;y;(-x)_(x#0),y;z]}

unfilt:{`c${(t;r):0 1_z;256!(*(unnone;unsub;unup;unavg;unpaeth)t)[x;y;r]}[x]\[(-1+#*y)#0;256!y]}

/ sizes of interleaved images
iszs0:{(-8!({x,y}/#''='x)*((!7)!7-(|/,/&')'(!7)=\:x)+y)}
iszs:{[w;h](iszs0.'+((A7;+A7);(w;h)))@\:!7}

/ Manual "compression".  (Just use tools above.)
(CMF;FLG):120 1
/ No compression [[https://datatracker.ietf.org/doc/html/rfc1950]]
nozip:{`c$CMF,FLG,,/,'/|1(1_|128,#/|0,#:)\,'/|1(,'/2/'''0N 8#/:/:1~:\+(16#2)\#:')\0N 65535#x}
/ [[https://datatracker.ietf.org/doc/html/rfc1950#section-9]]
ad32:{b:+/a:(1+\256!x);`c$2/'0N 8#,/(16#2)\'65521!/:(b;*|a)}
nocmp:{,/(nozip;ad32)@\:x}

/ Manual crc32 algo      (Just use tools above.)
/ [[http://www.libpng.org/pub/png/spec/iso/index-object.html#D-CRCAppendix]]
P:,/+(8#2)\256!0xedb88320
crctbl:(8{$[*|x;~P=-1_0,x;-1_0,x]}/(32#2)\)'!256
/crc:{`c$256\2/~{~crctbl[2/-8#~x=(32#2)\y]=-8_(8#0),x}/[32#1;256!x]}
mkchnk:{(`c$(4#256)\#y),,/1 crc\x,y}

\d .

/
SPEC: https://www.w3.org/TR/png/