preprocess.c

uses ImageMagick to convert a directory of images <input_image_dir> (image 
format support from libmagickcore) into a greyscale pixel [n][h_res][v_res] 
byte array, and write the bytes row-major order to <output_file>

output_file format:
0x00 - 0x01: num_images
0x02 - 0x03: out_h_res
0x04 - 0x05: out_v_res
0x06 - ... : pixel_data

./preprocess <input_image_dir> <output_file> <out_h_res> <out_v_res>