Wednesday, October 6, 2010

Converting VLFeat SIFT output to Lowe-compatible SIFT output for Bundler

I've recently been trying to get Bundler working on my Mac. Bundler is a part of a set of software (also including PMVS, CMVS, and others) that provides an open-source alternative to Microsoft PhotoSynth. Unfortunately, it has a dependency on a closed-source implementation of the SIFT keypoint-detection algorithm written by David Lowe.

There are a few open-source implementations of the SIFT algorithm, and one is part of the VLFeat toolkit. Unfortunately, VLFeat's SIFT tool generates differently formatted output from the Lowe version, even though it's the same sort of information. I spent far too long hunting for a conversion script, and found no standalone (or even Bundler-ready) one, so I did it with awk.

If you are using Bundler, you can just update the script, replacing the line:

echo "mogrify -format pgm $IMAGE_DIR/$d; $SIFT < $pgm_file > $key_file; rm $pgm_file; gzip -f $key_file"


echo "wc -l $key_file.vlfeat | awk '{ print \$1 \" 128\" }' > $key_file"
echo "awk 'BEGIN { split(\"4 24 44 64 84 104 124 132\", offsets); } { i1 = 0; tmp = \$1; \$1 = \$2; \$2 = tmp; for (i=1; i<9; i++) { i2 = offsets[i]; out = \"\"; for (j=i1+1; j<=i2; j++) { if (j != i1+1) { out = out \" \" echo "rm $key_file.vlfeat"

Once that's done, the script can be run as usual.

If you'd rather just convert existing VLFeat-generated key files, you can use:

zcat vlfeat.key.gz | wc -l | awk '{ print $1 " 128" }' > lowe.key

zcat vlfeat.key.gz | awk 'BEGIN { split("4 24 44 64 84 104 124 132", offsets); } { i1 = 0; tmp = $1; $1 = $2; $2 = tmp; for (i=1; i<9; i++) { i2 = offsets[i]; out = ""; for (j=i1+1; j<=i2; j++) { if (j != i1+1) { out = out " " }; out = out $j }; i1 = i2; print out } }' >> lowe.key
gzip lowe.key

Hope this helps someone avoid the effort!


Nate Lawrence said...

You should come visit Ólafur Haraldsson's Photogrammetry Forum and keep us informed of your progress getting these tools running on OSX. I know you'd find an interested audience and possibly some collaborators, if you'd like them.

Siddhartha Chandra said...

Thanks for the post regarding this. However, the current version of bundler does not seem to use '', but uses another script file ''. I have tried the changes suggested by you in both files in various combinations, but in vain. Please can you suggest if I might be missing out something.