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would it be best for me to just make my own BOW routine for a hog feature ?

i'm afraid, you have to. a HOGDescriptor is not a FeatureExtractor.

The authors refer to 32 * 32 "patches",

yes, you have to sample your image with a grid of patches, then compute a hog descriptor per patch. (those are actually column vectors, so you'll have to reshape() them to a row, and stack them hoizontally, so with 10x10 patches, you should have a 100 rows, 1760 columns Mat per image. for 30 images, that's [1760x3000]. then you can throw that into kmeans, and reduce it to 128 rows and 1760 cols again, that's your vocabulary.

would it be best for me to just make my own BOW routine for a hog feature ?

i'm afraid, you have to. a HOGDescriptor is not a FeatureExtractor.

The authors refer to 32 * 32 "patches",

yes, you have to sample your image with a grid of patches, then compute a hog descriptor per patch. (those are actually column vectors, so you'll have to reshape() them to a row, and stack them hoizontally, so with 10x10 patches, you should have a 100 rows, 1760 (sorry, forgot the correct number) columns Mat per image. for 30 images, that's [1760x3000]. then you can throw that into kmeans, and reduce it to 128 rows and 1760 cols again, that's your vocabulary.