gcinbis / NonIIDImageModels

Source Code for non-i.i.d. Image Models

Source Code for Non-i.i.d. Image Models

This package provides the Matlab source code for training and extracting Fisher vectors of Latent Bag-of-Words (LatBoW) and Latent Mixture-of-Gaussians (LatMoG) models, as explained in

• Ramazan Gokberk Cinbis, Jakob Verbeek, Cordelia Schmid, “Approximate Fisher Kernels of non-iid Image Models for Image Categorization”, accepted for publication in IEEE Transactions on Pattern Analysis and Machine Intelligence (PAMI), 2015,
• Ramazan Gokberk Cinbis, Jakob Verbeek, Cordelia Schmid, “Image categorization using Fisher kernels of non-iid image models”, in IEEE Conference on Computer Vision & Pattern Recognition (CVPR), Providence, USA, June 2012.

Call

[genm,eprm] = fv_fisher_latentgmm_variationalestimate(p,fvbase,N,D,K,[])

to train a generative model genm using variational expectation-maximization (EM) procedure. This function also returns the posteriors eprm for the training examples.

Call

    [eprm] = fv_fisher_latentgmm_estep(fvbase,N,D,K,[],genm,p)

to estimate posteriors on test images.

Call

    [desc] = fv_fisher_latentgmm_grads(N,D,K,gradopt,genm,eprm)

to extract LatMoG Fisher vectors. Use grapopt=alpha for the LatBoW model and grapopt=all for the LatMoG model to get all Fisher vector components. In our experiments, we then apply per-dimension whitening and L2 normalization to the resulting descriptors, as explained in the paper.

In order to utilize the aforementioned functions, the following per-image statistics over local descriptors should to be provided in the fvbase struct:

field	Size	Contents
E_x	(N D K)	E_x(j,:,k)=sum_i( p(k|x_i) * x_i ) / sum_i( p(k|x_i) ), over x_i \in image_j
E_x2	(N D K)	E_x2(j,:,k)=sum_i( p(k|x_i) * x_i^2 ) / sum_i( p(k|x_i) )
counts	(N K)	counts(j,k)=sum_i( p(k|x_i) ), over x_i \in image_j

where

• x_i i-th local descriptor in an image.
• N: number of training images
• D: local descriptor dimensionality
• K: vocabulary size
• E_x and E_x2 fields are not needed when training LatBoW models.

A couple of other options should be provided in the p struct. See example.m for default values and their explanations.

A final note: When using spatial grids, we train per-cell models independently.