iOS Question Canvas Resize Edge Detect...

Hello,

I'd like suggestions for this little project...

I am trying to detect the edges (x1,x2,y1,y2) of a drawing on a canvas made by the user.
And crop/resize that new zone to 50x50 or 100x100.

I am unsure if that's clear enough...

I was thinking of doing a getpixelcolor from the

x = 0 to x = width to find x1
y = 0 to y = height to find y1
x = width to x = 0 to find x2
y = height to y = 0 to find y2

???

Thanks.

Y.