How can I use PIL to crop a select area based on face detection?

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How can I use PIL to crop a select area based on face detection?

Hi I am wanting to use the python imaging library to crop images to a specific size for a website. I have a problem, these images are meant to show people's faces so I need to automatically crop based on them.

I know face detection is a difficult concept so I'm thinking of using the face.com API http://developers.face.com/tools/#faces/detect which is fine for what I want to do.

I'm just a little stuck on how I would use this data to crop a select area based on the majority of faces.

Can anybody help?

Joe

+1 A:

If you have some rectangle that you want to excise from an image, here's what I might try first:

(optional) If the image is large, do a rough square crop centered on the face with dimensions sqrt(2) larger than the longer edge (if rectangular). Worst-case (45° rotation), it will still grab everything important.
Rotate based on the face orientation (something like rough_crop.rotate(math.degrees(math.atan(ydiff/xdiff)), trig is fun)
Do a final crop. If you did the initial crop, the face should be centered, otherwise you'll have to transform (rotate) all your old coordinates to the new image (more trig!).

Nick T 2010-10-13 19:33:13

the angle of rotation seems to be directly provided by the api as `roll` value, so we can go even without trig knowledge :)

mykhal 2010-10-13 19:39:16

That's good. I don't like trig (i've done it in school and don't really see how it works!).I'll see what that does :)

Joe Simpson 2010-10-13 19:43:19

@{Joe Simpson} oops, bad news.. i take it back.. roll, pitch and yaw seem to be 3D rotation coords :) so roll is probably not exactly 2D rotation angle

mykhal 2010-10-13 19:46:55

The API provides all three axes of rotation...`yaw`, `pitch`, and `roll`, so `roll` is probably a 3-D property, not a *direct* correlation to the face on the 2-D image. In most circumstances though, they should be pretty close, and it's not like the value is exact anyways.

Nick T 2010-10-13 19:51:15

@mykhal Ugh, I type slow... I'm thinking that if faces are generally pointed at the camera (huge assumption), `yaw` and `pitch` should be fairly small.

Nick T 2010-10-13 19:55:07

I will have to see what I can play around with :/

Joe Simpson 2010-10-14 20:05:10

ansaurus

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How can I use PIL to crop a select area based on face detection?

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