libLDB is a C++ library for extracting an ultrafast and distinctive binary feature LDB (Local Difference Binary) from an image patch. LDB directly computes a binary string using simple intensity and gradient difference tests on pairwise grid cells within the patch. Relying on integral images, the average intensity and gradients of e
ach grid cell can be obtained by only 4~8 add/subtract operations, yielding an ultrafast runtime. A multiple gridding strategy is applied to capture the distinct patterns of the patch at different spatial granularities, leading to a high distinctiveness of LDB. LDB is very suitable for vision apps which require real-time performance, especially for apps running on mobile handheld devices, such as real-time mobile object recognition and tracking, markerless mobile augmented reality, mobile panorama stitching. This software is available under the GNU General Public License (GPL) v3.