Bitrate settings/Image quality in WMV.

[Mylenium]

[Eric Steinberg] “1. Can someone explain the “Image Quality” slider under bitrate settings when you render a Windows Media Video? The slider has a range of 0 – 100, and the pop-up help text states: “Allows encoder to favor motion over frame quality and vice versa.” Which value favors what? I’m guessing a value of 0 favors motion, and 100 favors frame quality, but I’m not sure. I ran some tests with different settings, but I honestly couldn’t see any difference between a setting of 0 and a setting of 100. I’m trying to tweak my settings, since I’m not 100% happy with the quality in certain areas, so I was hoping tweaking “Image quality” would help. Which brings me to my second question:”

You’re pretty much spot-on. The slider is supposed to indirectly affect the quality of the motion vectors calculated for the compression blocks. On normal footage with reasonably high bitrates it has barely any effect, it’s more for streaming files.

[Eric Steinberg] “2. The areas I’m having trouble with are reveals/transitions of still images (photos), where I’m doing the following: I’ve duplicated the still image, and set an “add” transfer mode on the top layer. Both layers just appear instantly (“pop in”), and then I do a 1-second fade-out of the top layer so just the bottom layer is left. These transitions are the only areas of my 2-minute presentation that look bad. They look highly compressed/blocky. Maybe I would get better quality if I rendered uncompressed and used a tool like Sorenson Squeeze to encode to wmv?”

You are introducing a “disturbance” into your compression stream. Since there is no previous frames to build on for analysis, the encoder needs to start from scratch and may at that point not have enough bandwidth left to accomodate the many extra small blocks required for maintaining detail. Hence it will use larger, more compressed blocks. You would have similar problems with MPEG-II or MPEG-4. The only way to fully eliminate this is to use a smoother transition. For the time being you should use 2 pass VBR. This will to some degree even out allocation of blocks across the entire stream. Of course other tools may do better also, but beyond ProCoder I do not have much experience which works best.

Mylenium

[Pour Myl

[Eric Steinberg]

Thanks for your reply!

I will try 2-pass VBR!
🙂

[Roland R. kahlenberg]

I believe that a setting of 0 pushes the encoder to leave more bits for a smoother video while a sestting of 100 will push the encoder towards a cleaner image. Kinda difficult to see the benefits. MOre obvious is the keyframes per second setting.

If you’re ging to be playing the presesntation on a computer then I’d suggest that you lower the frame rate to 25fps. I really see no benefit in going 50fps. You’ll also want to set a bitrate setting that is high enough for your computer to playback in realtime and with good quality image after compression.

I’ve not worked with WMVs at the screen resolution that you’re using so I can’t provide any realworld experiences but I suggest that you look into at least 1.5Mbits per sescond for your datarate.

HTH
Roland Kahlenberg
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