6. A computer-implemented method for generating and storing invisible junctions, the method comprising: receiving, with a processor, an electronic document; applying, with the processor, a distance transformation to the electronic document to generate a distance transform; detecting, with the processor, a skeleton in the distance transform; determining, with the processor, junction points in the skeleton as the invisible junctions, the junction points having a distance transformation value based on the distance transform; determining, with the processor, a junction size for at least one junction point, the junction size based at least in part on the distance transformation value; extracting, with the processor, an invisible junction feature descriptor from the electronic document for each invisible junction, the invisible junction feature descriptor including the including the junction size; creating, with the processor, a feature index from the invisible junction feature descriptors; and storing the feature index.
6. A computer-implemented method for generating and storing invisible junctions, the method comprising: receiving, with a processor, an electronic document; applying, with the processor, a distance transformation to the electronic document to generate a distance transform; detecting, with the processor, a skeleton in the distance transform; determining, with the processor, junction points in the skeleton as the invisible junctions, the junction points having a distance transformation value based on the distance transform; determining, with the processor, a junction size for at least one junction point, the junction size based at least in part on the distance transformation value; extracting, with the processor, an invisible junction feature descriptor from the electronic document for each invisible junction, the invisible junction feature descriptor including the including the junction size; creating, with the processor, a feature index from the invisible junction feature descriptors; and storing the feature index. 9. The method of claim 6 , wherein creating the feature index from the invisible junction feature descriptors comprises: creating a quantization tree; and adding a feature point to the quantization tree for the invisible junction feature descriptors.