Any ideas for really messing up XVID?

[eyemessiah]

Hello!

I am doing some video editing and I am looking for ways to introduce errors into my video. I've downloaded plenty of corrupted videos by accident and am interested in producing the blocky picture warping and colour smudging in my own work.

I have tried messing up the video file using a hex editor, but much to my surprise XVID seems pretty error resilient. Interestingly I have found that sometimes messing up pages and pages of hex will produce no noticeable effect, and sometimes just changing one value from D5 to D6 will cause massive distortions. The trouble is that getting a good effect by hex editing seems very laborious, and more difficult than I had thought.

Are there any strings I could search for that flag particularly vital data?

Ideally I want the video to remain playable, and experimentation with mass replacements have proved catastrophic in this sense.

Any one got any ideas for quick ways of producing interesting, non-fatal corruption?

thanks,

[Manao]

Have a look here (http://forum.doom9.org/showthread.php?s=&threadid=68442&highlight=gmc)

[eyemessiah]

Great! Thanks, I'll take a look.

...

Interesting, but no solution yet.

I suppose what I am hoping for is someone who might be able to point me in the right direction for identifying key frames, or b frames or i frames, or something like those in a hex editor so I can write a script to automatically mess up more than a few seconds worth of them. I have tried messing with encoder settings, and reducing the bit rates and things but they arent quite what im looking for. That said, XVID does some pretty evil looking stuff to video at very low bit rates :)

Although my understanding of how XVID works is only getting better quite slowly, I have a hunch that what is happening is the the key frame(?) (B frame? I frame?) is getting corrupted (in those cases where this happens accidentaly) and the decoder is 'drawing' the motion, or the "differences between frames" onto the garbled key frame until it hits a fresh one. Or something crudely like that.
So what you get is something that looks like you froze a frame of the video, and then just drew the motion from the subsequent frames onto the frozen frame, without updating the image.

And what I want to do is find the key\b\i (sorry if this seems like a poor understanding of XVID, im still reading!) frame data in a hex editor and mess with it, and subsequent key\b\i frames to produce interesting effects.

Im also thinking of doing stuff with CRC's. Like maybe generating CRC's for snippets from one video clip, and then trying to "fix" the bits in another video clip to fit the CRC.

I'd also like to do something like extracting the motion information for a sequence of frames from one clip and using it to replace the motion information in another clip.

Any thoughts?

Should I maybe take this thread to the general forum?

I just posted here because I love XVID!

Its /the/ codec!

[eb]

eyemessiah,

Try to use VirtualDubMod program and use for example logo or delogo plugins and then you can make some picture changes on individual or on serial frames. I think that also frames with "burned" solid subtitles can be used for your tests.

eb

PS
quickest way to foolout any codec is is to present to it records let say from tvsat with fading or interrapted signal