1. A computer-implemented method comprising: obtaining an image of a sequence of symbols, based on a document capture process performed on a rendered document; segmenting a portion of the image into multiple segmented sub-images, each of the segmented sub-images corresponding to a single symbol in the sequence of symbols; for each segmented sub-image of at least a subset of the segmented sub-images, in an order that corresponds to the sequence of symbols: determining whether one or more features of the segmented sub-image are classified as being similar to one or more corresponding features of any stored sub-image in a cache of stored sub-images; based on determining that one or more features of the segmented sub-image are not classified as being similar to one or more corresponding features of any stored sub-image, selecting an unassigned symbol identifier, and storing the segmented sub-image in association with the selected symbol identifier in the cache of stored sub-images; and attributing the selected identifier to any segmented sub-images that follow the segmented sub-image and have one or more features that are classified as being similar to one or more corresponding features of the stored segmented sub-image; and generating an encoding of the sequence of symbols that is based on the identifiers attributed to the segmented sub-images.