I have some videos I recorded with my cell phone that I want to crop using Avisynth's crop(). I am using x264vfw with VirtualDub which asks for a target bitrate. I usually just get the bitrate from the vid's properties in Windows Explorer and plug that value in for the target bitrate for the new encoded video, but, since I am cropping it, I don't want to use the full bitrate. Ideally, I would be able to find out the bitrate of only the region I am wishing to crop. After all, if the original video is 1920x1080 with a total bitrate of 30 mbps (30,000 kbps) and I want to crop it down to a 1080x1080 region, I don't want to use the bitrate of the full, uncropped video.
asked Sep 19, 2015 at 3:45
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1 Answer
You can only estimate the bitrate as proportion of your crop. If you have 30,000 Kbps for 1920*1080 pixel, then for 1080*1080 pixel this would mean 16,875 Kbps. If there is more complexity inside the crop region than outside, I would even increase the target bitrate further.
However, if your videos are not that long and not intended for streaming I wouldn't invest too much thought into saving some disk space. The video quality will degrade anyway with the encoding process and a high bitrate will help you avoiding too much loss.
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That's what I was afraid of. It's not easy being a pedant trying to find the absolute perfect bitrate to yield the best quality/disk space ratio. I'll probably just use original bitrate in the new video for good measure, though using the proportional calculation would probably be just as good. Commented Sep 20, 2015 at 0:12