The subject matter of this specification can be implemented in, among other things, a method including identifying one or more blocks in an electronic image that depicts text characters. The method includes identifying one or more text blocks among the blocks that depict the text characters. The method includes identifying an average text contrast for each of the text blocks. The method includes identifying a type for each pixel in each of the text blocks based on the average text contrast. The method includes performing local adaptive filtering on a first neighborhood of pixels around each pixel in each of the text blocks to determine a brightness for the pixel based on the identified type. The method includes storing, in at least one memory, the electronic image including the determined brightness for each pixel in each of the text blocks.