overlay_in_pic.sh

function dist(){
	RET=`echo "$1 $2 $3 $4" | \
	awk '
	function ceil(valor)
    {
      return (valor == int(valor)) ? valor : int(valor)+1
    }
	function a(dx,dy) {
		return sqrt(dx*dx+dy*dy);
	}
	{
		print ceil(a($2-$4,$1-$3));
	}'`
}
function rot() {
	DX=`expr $3 - $1`
	DY=`expr $4 - $2`
	DIST=$5
	RET=`echo "$DX $DY $DIST" | \
	awk '
	function asin(x) {
	  return atan2(x, sqrt(1-x*x))
	}
	{
		print asin($2/$3);
	}'`
}

TL[0]=154
TL[1]=501

BL[0]=142
BL[1]=651

TR[0]=355
TR[1]=515

dist ${TL[@]} ${TR[@]}
WIDTH=$RET
dist ${TL[@]} ${BL[@]}
HEIGHT=$RET
rot ${TL[@]} ${TR[@]} $WIDTH
ROT=$RET
echo "$WIDTH x $HEIGHT, ${TL[0]}:${TL[1]}, @$ROT"

OVERLAY_DX=0
OVERLAY_DY=0

if (( $(echo "$ROT > 0" | bc -l) )); then
  OVERLAY_DX="${HEIGHT}*sin($ROT)"
fi
if (( $(echo "$ROT < 0" | bc -l) )); then
  OVERLAY_DY="${WIDTH}*sin(-$ROT)"
fi

FILTER="
	[0:v] scale=${WIDTH}x${HEIGHT}[src];
	[src] rotate=$ROT:ow=rotw($ROT):oh=roth($ROT):fillcolor=magenta [vid];
	[1:v][vid] overlay=x=${TL[0]}-${OVERLAY_DX}:y=${TL[1]}-${OVERLAY_DY} [scaledVid];
	[scaledVid][1:v] overlay='"
echo "===="
echo $FILTER
echo "===="
ffmpeg -i animated.gif -i transparent_frame.png \
-filter_complex "${FILTER}" \
-pix_fmt yuv420p \
output.mp4