Flexible constrained sparsity preserving embedding

•Two non-linear semi-supervised embeddings are proposed.•These methods elegantly integrate sparsity preserving and constrained embedding.•The second framework provides a non-linear embedding and its out-of-sample extension.•Classification performance after embedding is assessed on eight image datasets.•KNN and SVM classifiers are used after getting the embedding.•Experimental results on eight public image datasets show the outperformance of the methods.

In this paper, two semi-supervised embedding methods are proposed, namely Constrained Sparsity Preserving Embedding (CSPE) and Flexible Constrained Sparsity Preserving Embedding (FCSPE). CSPE is a semi-supervised embedding method which can be considered as a semi-supervised extension of Sparsity Preserving Projections (SPP) integrated with the idea of in-class constraints. Both the labeled and unlabeled data can be utilized within the CSPE framework. However, CSPE does not have an out-of-sample extension since the projection of the unseen samples cannot be obtained directly. In order to have an inductive semi-supervised learning, i.e. being able to handle unseen samples, we propose FCSPE which can simultaneously provide a non-linear embedding and an approximate linear projection in one regression function. FCSPE simultaneously achieves the following: (i) the local sparse structures is preserved, (ii) the data samples with a same label are mapped onto one point in the projection space, and (iii) a linear projection that is the closest one to the non-linear embedding is estimated. Experimental results on eight public image data sets demonstrate the effectiveness of the proposed methods as well as their superiority to many competitive semi-supervised embedding techniques.