divx5 Scene Change Threshold - how works?

[john blox]

I've been unable to find out exactly how this works. I searched these forums and read that a lower % will give more keyframes, but a test has confused me. Can someone who knows clarify the way it works.

test-25% = 288,062k
test-50% = 313,914k
test-75% = 233,098k

weird eh?

My reason for wondering about this is that divx'ing tv shows then stripping the ads out from the avi is somewhat awkward as divx5.05 doesn't seem to insert a keyframe where the ads start despite the video usually changing to a logo at start/end of video. I wondered how best the threshold can be adjusted to ensure I get a keyframe at this point without adversely affecting encoding performance?

[Awatef]

Change the threshold in 10% steps until it puts a keyframe there, no one can tell you which threshold you should use ¬.¬

BTW, you should cut the adds BEFORE DivX encoding.

[dTb]

From my understanding the sct is directly related to the number of blocks trackable between frames. It basically defines the percentage of untrackable blocks required to force a keyframe. Say the sct is 30%, then if 30% of the blocks are untrackable between two frames a keyframe will be inserted.

Apparently with tv captures due to the noise in the video this will affect the effectiveness of the sct.
Like Awatef says unless your capturing directly to divx then cut the ads before conversion.

For more info check out this (http://forums.divx.com/viewtopic.php?topic=50689&forum=6) thread which is where I've obtained most of my info on the subject.

[mothman]

is there a way to manually introduce keyframes or be able to cut video on nonkey frames, virtualdub will not cut on nonkey frames and usually divx will not insert key frames where i need them to cut commercials out.

[BluDChyLD]

I remember a program called "keyframe enforcer", that should do the trick. Never tried it myself, search the forums for it :)

[fedge]

i thought id just post my 1/2 cent worth here...

I have had problems with leaving the settings at 50% like the divx people suggest. Often in high aciton sceens the codec will drop TONS of HUGE I=frames into the video. and sometimes on simple camera movements of jiggles as a call them, like in the movie Master of Disguise, it drops them in also. I set Scene change threshold for movies with lots of action and or a lot of camera movements to 20% -40%... this may degrade some of the quality, but for the most part, it plays without any jitter at all, and looks passable for my tastes. But im still learning.

[kenobi51]

Ok, i have met the same problem in the past and so i write a tool to correct the shitful detection algorythm of DivX codec.
It's based on IA algos, go to my (little) home page and try it :

http://membres.lycos.fr/kenobi51/

bonus is that it permit BFrame adaptative Encoding (very good for action movie if you want BFrame - Lot of action + BFrames = :(

after tweaking my tool it seems that best result are obtain for a max motion of 1.30.

Have a nice day, and good encoding.

NB: Don't forget to use DivX 5.0.2 Pro

[dTb]

It looks like there might not be a scene change threshold in a future release of divx. In the Tahanea release the newer performance modes don't use it. For more info go here
http://forums.divx.com/viewtopic.php?topic=50724&forum=25