I want to reduce the size of video which is 1280x720 and 10mbps. I'm going to reduce the bit rate to 1.5 mbps, but I don't know which size would I re-encode it. Should I use 1280x720, 960x540 or 640x360? I would prefer 540p, but would it loose more quality against others 'cause it's 75% of the original size? That would be like sharing 4 candies to 3 children, or am I wrong?
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You either want to encode video stream for web and limit high bitrates , is that what that 1.5 Mbit means? Then you just use low CRF and limit --vbv-bufsize 1400 --vbv-maxrate 1400
If you plan to encode 1.5Mbit average,this will make it absolutely clear for you. Do not start with bitrate but rather with quality and check bitrates. Resize to 960x540 or 640x360 or not, encode some CRF number (10 seconds), normal and "action" scene, check bitrates quick with bitrate viewer. This will give you ballpark where you are at, how to change, CRF up or down, depending what you see in bitrate viewer. If you video has not scene extremes you can even reduce bitrate with --vbv-bufsize and --vbv-maxrate and run 1pass render for your complete video.
After this quick analyses you might even do as a final step 2pass. The whole thing sounds difficult, but it is actually not, because you just encode very short samples. 2pass you have to run as a whole thing, movie , video, to get results, you might spend 1 day encoding and you get this wrong anyway.
Maybe I was too unclear with my question. I'm using Avidemux to encode with average bitrate mode set to 1500 kbit/s. I have found that a good balance between filesize and quality. I only wanted to know if resizing the video affects to the quality or should I just leave it to the original dimensions although I reduce the quality that much? Also I'm not too familiar with the encoding process so those tags don't say too much for me. I have resized some 1080p videos to 540p and I haven't seen too much quality drop (10 times bigger file size =//= 10 times better quality). Maybe because my computer just isn't full hd capable.
There is no linear relation between size and bitrate but for sure you can't expect that for example reducing size twice bitrate will be also twice lower - you can assume that for example reducing size twice bitrate reduction will be around 30 - 25%.
However once again - bitrate is directly related to source complexity and encoder quality - perhaps 1500kbps can be enough in case simple 1080 content or for fancy encoder - you need to try set quality limit for yourself - perhaps you will be required to do multipass encoding to achieve better results.
Reducing the frame size will reduce the bitrate requirement. But you are trading off resolution in order to get fewer blocky artifacts.
If you arbitrarily pick, say, 960x540 and 1.5 Mb/s some movies will look terrible, others will be larger than they really needed to be. This is why "constant quality" encoding was invented. You pick the quality you want, the encoder uses whatever bitrate is required to deliver that quality for that particular video.
Once more 'quality' is subjective.
If you downsample to a smaller size you will lose sharpness, but you should also have less grain/artifacts.
I definitely prefer that. If it's missing some information ... and if you reencode to a smaller bit rate it will be ... but is otherwise clean, your eye will fill in what's missing. If you don't downsample and you get more grain/artifacts, which you will, your eye can't adapt, which I think is subjectively much worse.
Myself, I think downsample when encoding to a smaller bit rate very effective.
Which resolution is best is, as mentioned, hard to say. There's no one answer. The best thing would probably be to edit out a small portion of the original and try different settings. I've found scenes with rain or fog or smoke work well, especially backlit night ones.
Checked on few clips (some of them dynamic - MTV like montage) looks fine - seem it should be possible to fit 960x540 in 1500kbps
@set x264opts="level=41:bitrate=1500:vbv-maxrate=20000:vbv-bufsize=4500:qpmin=4:cabac=1:colorprim=bt709:transfer=bt709:colormatrix=bt709:fullrange=off:overscan=show:pic-struct=1:force-cfr=1:aud=1:threads=auto" @ffmpeg -dump_attachment:t "" -i %1 -c:v libx264 -preset veryslow -tune film -profile:v high -crf 4 -x264opts %x264opts% -x264-params %x264opts% -vf "scale=out_range=auto:force_original_aspect_ratio=decrease:'if(gt(a,30/17),960,-1)':'if(gt(a,30/17),-1,540)':sws_flags=bicubic+accurate_rnd+full_chroma_inp+full_chroma_int:sws_dither=a_dither,pad=960:540:(ow-iw)/2:(oh-ih)/2,setsar=sar=1/1" -c:a copy -aspect 16:9 -f mp4 -y -shortest -movflags faststart %1_.mp4
The clarity of the source and the viewing distance also make a difference. Some Blu-ray discs aren't very sharp to start with. So downscaling to 960x540 may not make a difference (especially if you use a sharpening resizer). With a sharp source, if you sit 6 feet away from a 50" TV you're more likely to notice the loss of resolution from 1920x1080 to 960x540. If you sit 10 feet away from a 32" TV you might not even notice the difference between 1920x1080 and 640x360.