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FORUMS Post Processing, Marketing & Presenting Photos Video and Sound Editing 
Thread started 15 Nov 2014 (Saturday) 12:58
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Weird compression artifcacting problem

 
Christopher ­ Steven ­ b
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Nov 15, 2014 12:58 |  #1

This question is pertaining to a video slideshow that uses large res images and employs basic panning / zooming. Because the problem is visible in the video output, I thought that this forum section was appropriate.

The problem, in short, is that my HD exported videos look fabulous EXCEPT every few seconds there are 2-3 frames that look very very blocky.

My exports are H.264 between 5-6Mb/s and I'm using VBR 2 pass. Interestingly I note that the problem occurs even if I use CBR. That makes no sense to me.

Any idea what is going on there ?



christopher steven b. - Ottawa Wedding Photographer

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joeblack2022
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Nov 17, 2014 16:40 |  #2

Sounds like your key frames setting could use some tweaking.

http://help.adobe.com …59e6131255b4dd2​-8000.html (external link)

Post a screenshot of your settings?


Joel

  
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Christopher ­ Steven ­ b
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Nov 17, 2014 17:01 |  #3

Thanks Joel--I've attached a screen.


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joeblack2022
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Nov 17, 2014 17:10 |  #4

Perhaps uncheck the "Key Frame Distance" setting to let the software automatically set it and see if the results are any better. For some reason I have in my head that key frame distance should be about 10 seconds but I can't reference why right now.

Are you experiencing playback issues on your computer and / or playing back the file from Vimeo / YouTube, etc.?

EDIT: For streaming purposes, 10 seconds is a good baseline. I bolded the text below.

http://www.lighterra.c​om/papers/videoencodin​gh264/ (external link)

Maximum Keyframe Interval (--keyint)

The maximum amount of time between keyframes (I-frames) has a major impact on quality, which makes it one of the most important settings to tune, and one of the most difficult decisions. The encoder will try to use keyframes at scene changes, of course (see above), but for a lot of content this value is important because many scenes are longer than 5 or even 10 seconds. Having too many keyframes severely reduces quality, because the efficiency of reusing image areas from previous frames is completely lost at each keyframe – the encoder has to "start over" at every keyframe. Therefore, we want as few keyframes as possible to achieve the highest quality for the given target bitrate.

On the other hand, we still want enough keyframes that seeking and fast-forwarding behavior is good, because players can only jump directly to keyframes "under the hood" during playback, and will usually only display the keyframes during fast-forwarding and rewinding at higher speeds (at low speeds such as 2x or 3x they can often play every frame). Jumping to an arbitrary point in the timeline therefore becomes more sluggish the fewer keyframes there are, because more intervening delta-frames need to be decoded just to reconstruct the final target frame, even though those intervening frames between the previous keyframe and the target frame won't actually be displayed. If the video is deployed using adaptive streaming, where the player might dynamically switch between different versions during playback based on the available network bandwidth, then such switching can also only occur at the keyframes (of the stream being switched to), so again we don't want the keyframes too far apart.

x264's default is 250 frames (8.3 seconds at 30fps), but for no good reason the x264Encoder QuickTime plug-in's default is only 60 frames (2 seconds). The x264 team have recently been using 500 frames (16.7 seconds) for their settings at the annual MPEG-4 AVC/H.264 Video Codecs Comparison competition, but that's definitely pushing things a bit too far. Digital TV normally uses 1 or 2 seconds, but that's deliberately quite short to make channel switching fast and to have quick error recovery in case of interference. DVD uses a very short ~0.5 second keyframe interval, and Blu-ray 1 second, because they use very high bitrates (so quality isn't an issue) and they want to guarantee good fast-forwarding behavior despite being read from a relatively slow optical disc. The most common recommendation for Internet video is 10 seconds.

All: 12 seconds (360 frames at 30fps), which is enough to cover the vast majority of scenes and result in only "natural" scene-change keyframes, and about as far as we can push it. Fast-forwarding while seeing only one frame for every 12 seconds of video is just barely okay, as a worst-case scenario. Assuming fast-forwarding at 10x speed, that means 6 seconds per minute with 5 keyframes, resulting in 1.2 seconds on-screen per keyframe. Jumping to arbitrary points in the timeline definitely feels sluggish as well, but not too painful, even at 1080p, and it will get better in time with faster computers.


Joel

  
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Weird compression artifcacting problem
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