James Hensman’s Weblog

May 15, 2009

Tracking a finger

Filed under: Uncategorized — jameshensman @ 11:29 am

Right. Having broken my avi file into pngs, how do I keep track of an object in the vid? Check this out to see what I mean:

I’m not sure what the tapping of the finger is about (at the start of the vid), but I have data from the experiment which I’d like to correlate to the position of the finger on the rig. the idea is to follow that black blob.

Having broken the vid into png images, I loaded them into python using PIL:

from PIL import Image
images = [Image.open('./tmp/'+f) for f in imagenames]

and then made them into numpy arrays, slicing off only the red channel and the middle part of the image(by inspection, the dot was most visible on the red channel):

images = [np.asarray(image)[upper:lower,:,0] for image in images]#0 indexes red

Now, scipy.ndimage has an awesome tool for identifying regions of an image. I used a threshold (set by inspection) first:

images = [imagehttp://www.youtube.com/watch?v=Cu08aLAUVuY%5D the top image is the (sliced) original image, with a green dot where I think the block pen mark is. The middle plot is a pylab.imshow() of the regions, and the final subplot is the same after applying my set of rules. Thanks to Sarah T. for letting me post the videos. Edit: corrected some code

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Making and Breaking Videos into pngs

Filed under: Uncategorized — jameshensman @ 10:59 am

I’ve got some videos that I’d like to do a little bit of processing on. nothing spectacular, I just want to track a black blob.

Luckily numpy/Scipy has all the tools I need to get started… on images. I need to break the (avi) video file into a series of png files (i could use jpeg, I suppose, but it’s not like hdd space is limited here…). The tool I’ve been using for this is ffmpeg.

Now, ffmpeg is a phenomenal beast. It can re-scale videos, re-encode them, crop them, and all kinds of other things that I’m not interested in. The command I’ve used to break a avi file into pngs is:

ffmpeg -i ./tmp/%03d.png

It’s nice that ffmpeg recognises the %03d format string, huh? Now that I’ve got some images, I can load them into numpy and process them (future blog post!)

After doing said processing, ffmpeg should be able to re-code my processed images into a video file for convenience. However, I’m really lazy, and can’t be bothered to read the whole of the ffmpeg doc. So I’m copying Mike here and using:

mencoder ‘mf://tmp/*.png’ -mf type=png:fps=10 -ovc lavc -lavcopts vcodec=wmv2 -oac copy -o <whatever>.mpg

I’m sure ffmpeg can do the job too, but it just isn’t working for me.

In fact, I’m wrapping both of those commands up in python, using the os module.

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