# Rectangle detection

Peter Reid preid at reid-it.co.uk
Mon Sep 7 12:40:47 CEST 2015

```With some input from Bernd Niggemann, I've been able to get my rectangle detection and extraction working OK.  Just in case anyone else needs to do something similar, here's my rectangle hunting code:

-- start looking for top line of params box:
put empty into tXleft
put empty into tXright
put empty into tYtop
put empty into tYbottom
put (tImgWth * 2) div 3 into tX
put tImgWth into tMaxX
put 0 into tY
put 0 into tMinY
put (tImgHght * 3) div 4 into tMaxY
put tImgWth div 4 into tMinX
put false into gotTopLine
set itemDelimiter to comma
repeat while tY <= tMaxY and not gotTopLine
put item 1 of getPixelColour(tX,tY,tImgWth) into tRedVal
if tRedVal = cBlack then
put true into gotTopLine
else
end if
end repeat
-- found top line, search for left & right ends of the line:
if gotTopLine then
-- search left for start of top line:
put tX - 1 into tXchk
repeat while getPixelColour(tXchk,tY,tImgWth) = cBlack and tXchk >= tMinX
subtract 1 from tXchk
end repeat
if tXchk > tMinX then
put tXchk + 1 into tXleft  -- note Xleft
end if
-- search right for end of top line:
put tX + 1 into tXchk
repeat while getPixelColour(tXchk,tY,tImgWth) = cBlack and tXchk <= tMaxX
end repeat
if tXchk < tMaxX then
put tXchk - 1 into tXright  -- note Xright
end if
end if
-- found top line OK, search for right line:
if gotTopLine and tXleft is not empty and tXright is not empty then
-- yes, note top Y:
put tY into tYtop
-- search down for bottom of right line:
put tY + 1 into tYchk
repeat while getPixelColour(tXright,tYchk,tImgWth) = cBlack and tYchk <= tMaxY
end repeat
if tYchk < tMaxY then
put tYchk - 1 into tYbottom
end if
end if

and here's my getPixelColour function:

function getPixelColour pX, pY, pImgWidth
local tPix, tImgPosn, tRdVal, tGrVal, tBlVal, tRbyte, tGbyte, tBbyte
put pY * pImgWidth + pX into tPix
put tPix * 4 into tImgPosn
put byte (tImgPosn + 2) of gImage into tRbyte
if tRbyte is empty then
put 0 into tRdVal
else
put byteToNum(tRbyte) into tRdVal
end if
-- greyscale images so ignore green & blue values:
return (tRdVal)
end getPixelColour

Bernd asked me for timings for the above – I get the following timings for 7 graphs converted 1 after each other: 4 ms, 3 ms, 3 ms, 4 ms, 3 ms and 3 ms for this under LC 6.7.6 on my iMac.

I'm sure others can come up with better/faster/cleverer ways of doing this but what I have works and is fast enough for my purposes.

Thanks to anyone who gave this some thought.

Peter
--
Peter Reid
Loughborough, UK

```