There are a lot of scanned books in the DjVu format, but many of them are not professionally scanned. One of the DjVu books I encounter has different page sizes (page size changes every few pages). There are nice pdf scaling programs, but no such program for DjVu exists.
So I made my own. The python 3 program searches for INFO blocks which specify sizes and DPIs of images and change DPIs to match a constant width. The program needs improvement though – it currently reads the entire file into memory for processing. However, this is not a problem for my files so I’m not going to change it.
# usage: python3 djvu.py [FILE] [PAGE_WIDTH(IN)] import sys import struct def main(): args = sys.argv if len(args) != 3: print('filename? page_width?') return filename = args page_width = float(args) file_bytearray = None with open(filename, mode='rb') as f: file_bytearray = bytearray(f.read()) for i in range(len(file_bytearray) - 8): if file_bytearray[i:i+7] == b'INFO\\0\\0\\0': info_start = i+8 (width, height) = struct.unpack('>HH', file_bytearray[info_start:info_start+4]) desired_dpi = int(width / page_width) file_bytearray[info_start+6:info_start+8] = struct.pack('<H', desired_dpi) with open(filename+'mod.djvu', mode='wb') as f: f.write(file_bytearray) if __name__ == '__main__': main()