Categorizing plant images at the variety level: Did you say fine-grained?

This paper addresses the problem of categorizing plant images at the variety level, i.e. at a finer taxonomic grain than state-of-the-art studies usually working at the species level. It therefore introduces two new evaluation datasets of agro-biodiversity interest, each being related to concrete scenarios on large-scale plant resources. They have been chosen so as to involve very different acquisition protocols and visual patterns in order to evaluate if state-of-the-art image classification techniques can generalize to such specific contexts and avoid the cost of building specific ad-hoc solutions. The first one is a collection of 2071 pictures of loose rice seeds built from 95 accessions kept in a bank of seeds. The second one is a collection of 2037 pictures of grape leaves taken in the fields and belonging to 34 varieties among the most commonly ones used in viticulture. Both datasets exhibit a very low inter-class variability resulting in two challenging fine-grained classification tasks, even for expert human operators. A baseline experimental study was conducted on the two datasets using the two most effective families of classification techniques in the state-of-the-art, i.e. convolutional neural networks on one side and fisher vectors-based discriminant models on the other side. It shows that the achieved classification performance is very different between the two problems. It is actually pretty bad for the grape leaves collection but much better in the case of the rice seeds collection for which the acquisition protocol was much more constrained and the morphological variability more visible. The conclusion is that automatically identifying plant varieties might already be feasible for some specific scenarios and in controlled environments but that it is still an open problem in the general case.