Small and Large Stones of Programming

The usage when the program below is stored in file "caltechVideo.py" is as follows. This invocation converts a video file 'V000' in directory 'set00' to an AVI file in directory 'Images'.

import caltechVideo caltechVideo.genVideo('set00', 'V000')

### Generate an AVI file from Caltech Dataset ###
 
import cv2, cv, time, struct
import Image, utils, random, os, gzip
import numpy as np
 
 
def createPedestrianWriter(outFile, width, height):
    writer = cv2.VideoWriter()
    writer.open(outFile, cv.CV_FOURCC('P','I','M','1'), fps = 30,
                frameSize = (width, height), isColor = False)
    return writer
 
 
def genVideo1(inFile, outFile):
 
    def read_header(ifile):
        feed = ifile.read(4)
        norpix = ifile.read(24)
        version = struct.unpack('@i', ifile.read(4))
        length = struct.unpack('@i', ifile.read(4))
        assert(length != 1024)
        descr = ifile.read(512)
        params = [struct.unpack('@i', ifile.read(4))[0] for i in range(0,9)]
        fps = struct.unpack('@d', ifile.read(8))
        # skipping the rest
        ifile.read(432)
        image_ext = {100:'raw', 102:'jpg', 201:'jpg', 1:'png', 2:'png'}
        return {'w':params[0],
                'h':params[1],
                'bdepth':params[2],
                'ext':image_ext[params[5]],
                'format':params[5],
                'size':params[4],
                'true_size':params[8],
                'num_frames':params[6]}
   
    print('From {} to {}'.format(inFile, outFile))
    params = read_header(open(inFile, 'rb'))
    bytes = open(inFile, 'rb').read()
 
    writer = createPedestrianWriter(outFile, 640, 480)
 
    # this is freaking magic, but it works
    extra = 8
    s = 1024
    seek = [0]*(params['num_frames']+1)
    seek[0] = 1024
   
    # print("Params:{0}".format(params)) #!!!!!!
    for i in range(0, params['num_frames']-1): ##!!! why -1?
        tmp = struct.unpack_from('@I', bytes[s:s+4])[0]
        s = seek[i] + tmp + extra
        if i == 0:
            val = struct.unpack_from('@B', bytes[s:s+1])[0]
            if val != 0:
                s -= 4
            else:
                extra += 8
                s += 8
        seek[i+1] = s
 
        nbytes = struct.unpack_from('@i', bytes[s:s+4])[0]
        # print("Frame:{0} seek:{1}".format(i, s))
        I = bytes[s+4:s+nbytes]
       
        tmp_file = '/tmp/img{0}_{1}.jpg'.format(os.getpid(), i)
        open(tmp_file, 'wb+').write(I)
 
        img = cv2.cvtColor(cv2.imread(tmp_file), cv2.COLOR_BGR2GRAY)
        writer.write(img)
    return
 
def genVideo(dir, name):
    inFile = dir + '/' + name + '.seq'
    outFile = 'Images/' + dir + '_' + name + '.avi'
    genVideo1(inFile, outFile)