RAVL failed to load video file

[Y bob]

hi,RAVL is really important for me
but i don't know how to let it read a video file
i run the example "Ravl/Image/VideoIO/exImgSeq.cc"
and i downloaded a avi file and convert it to a yuv (420) format file named stefan_sequence.yuv
but the excutable said： Failed to open input sequence 'stefan_sequence.yuv'

and i tried another format ppm ,but got a same result;

how can i set the variants and option to load a video file ,suppose i use stefan_sequence.yuv

thanks a million in advanced
here is my code, just changed a little from the exImgSeq.cc

#include "Ravl/Option.hh"
#include "Ravl/DP/SequenceIO.hh"
#include "Ravl/Image/Image.hh"
#include "Ravl/Image/ImgIO.hh"

using namespace RavlN;
using namespace RavlImageN;

int main(int argc,char **argv)
{ 
  // Process options.
 
  OptionC option(argc,argv);
  //option.CompulsoryArgs(1); // input sequence name is compulsory
  option.Check();
  StringC ifmt = option.String("if","yuv","Input format. ");
  StringC ofmt = option.String("of","yuv","Output format. ");
  bool verbose = false;
  StringC ifilename("stefan_sequence.yuv");
  StringC ofilename("stefan_sequence2.yuv");

  DPIPortC<ImageC<ByteT> > inputStream;
  DPOPortC<ImageC<ByteT> > outputStream;
 
  if(!OpenISequence(inputStream,ifilename,ifmt,verbose)) {
    cerr << "Failed to open input sequence '" << ifilename << "' \n";
    return 1;
  }
 
  if(!OpenOSequence(outputStream,ofilename,ofmt,verbose)) {
    cerr << "Failed to open output sequence '" << ofilename << "' \n";
    return 1;
  }
 

  /*****  other code of exImgSeq.cc  *****/

  return 0;
}