Multiple kernels for object detection

Our objective is to obtain a state-of-the art object category detector
by employing a state-of-the-art image classifier to search for the
object in all possible image sub-windows. We use multiple kernel
learning of Varma and Ray (ICCV 2007) to learn an optimal combination
of exponential chi-squared kernels, each of which captures a different
feature channel. Our features include the distribution of edges, dense
and sparse visual words, and feature descriptors at different levels
of spatial organization.

Such a powerful classifier cannot be tested on all image sub-windows
in a reasonable amount of time. Thus we propose a novel three-stage
classifier, which combines linear, quasi-linear, and non-linear kernel
SVMs. We show that increasing the non-linearity of the kernels
increases their discriminative power, at the cost of an increased
computational complexity. Our contributions include (i) showing that a
linear classifier can be evaluated with a complexity proportional to
the number of sub-windows (independent of the sub-window area and
descriptor dimension); (ii) a comparison of three efficient methods of
proposing candidate regions (including the jumping window classifier
of~Chum and Zisserman (CVPR 2007) based on proposing windows from
scale invariant features); and (iii) introducing overlap-recall curves
as a mean to compare and optimize the performance of the intermediate
pipeline stages.

The method is evaluated on the PASCAL Visual Object Detection
Challenge, and exceeds the performances of previously published
methods for most of the classes.