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Video encoding, why change the resolution?

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Nobody99

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Jun 29, 2016
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If the compability of 1080p resolution is not a problem (which in most cases it isn't) why change the resolution to 720p, I don't see any reason to not just change the bitrate if you want lower size. Why do people follow the model "lower bitrate, lower resolution and higher bitrate, higher resolution"?
 
The larger the resolution the more bitrate is required to keep a certain quality per frame. You will get major compression artifacts if you lower the bitrate too much on a large resolution, this can be compensated by decreasing resolution as well.
 
But this doesn't happen unless you have really low bitrate, 2Mbps for 1080p is enough bitrate (especially with h.265) but some people are doing 720p encodes with 4 Mbps and higher.
 
Who uses 720?
 
Nobody99 said:
But this doesn't happen unless you have really low bitrate, 2Mbps for 1080p is enough bitrate (especially with h.265) but some people are doing 720p encodes with 4 Mbps and higher.
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I don't know what kind of video you're encoding but 2mbps is not nearly enough for 1080p if you want minimal compression artifacts.
 
2-5 Should be fine
 
I'll reduce to 720 to save on file size. I don't pick a bitrate, I use constant quality encoding, so reducing the resolution will automatically reduce the file size with it. I don't have a big enough or good enough screen to notice the difference anyway.
 
Video encoding is really complicated without doing some research.

Most encoders compress information by taking a reference frame and then use motion vectors to estimate certain blocks of the next frame. The difference between the previous frame and the estimated frame is encoded instead. Some even take information from the next frame...

Now, a frame itself is encoded in some colour space. A very naive colour space would be RGB. The three values per pixel contain the same amount of information.
We know a pixel next to another pixel is very likely to be the same in colour and we humans see better black/white (luminance) picture than colour itself. Thus it would be wise to encode more in the black/white picture than in colour.
For example YUV colour space. Y contains the picture in black/white, while U and V (chroma) just add some colour.

You often see a colour space Y'CbCr and then a number: for example 4:2:0 when encoding. This is sub-sampling This means in a block of 2 lines of 4, you'll get info of all 4 Y pixels in each line, 2 CbCr pixels on the first row and 0 CbCr pixels on the second row. You'll have quite high compression, but still high image quality.
As you see with this subsampling method, you need to have a fixed amount of pixels for it to work.

Now, everything is tied to blocks/resolution. A common way to lower bitrate, is by lowering the resolution, as you'll keep your 'optimal' parameters of the rest of the system (block size, motion estimation etc...) for the specific content of the scene.

I agree it does not need to be resolution that gets lowered, but it is the easiest way to do it.

My apologies if I got anything wrong, it's been a while since I've worked with this :)
 
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My upload is 0.8mbps so 720p is the best p.
 
hat said:
I use constant quality encoding
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Same here, it's very consistent and doesn't require more than 1 pass. It's the superior method imo, you just have to find the quality setting that's right for you.
 
I used Freemake Video Converter and it works fine. Now for Screen Recording I used ACTion! Which worked nicely!
 
I see why some encoders use lower resolution now, I suppose to each her/his own.

I don't like constant quality, firstly the compression is not as good as two pass and it is also harder to estimate the end size. And with VBR you get higher quality if you have higher bitrate so you have this quality scaling. Constant quality does a similar thing, the lower the factor the higher the bitrate relative to the source.
 
Actually I have been encoding TV shows with 720p all the time because they had a low bitrate. 2Mbps is my mark for 1080p.
 
All my movies are 720p cause all my tv's are 32" to 55" 720p is fine. I stream through plex so the encoder works great with my films
 
What program are you using?
 
Nobody99 said:
Handbrake.
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Good program but i found it very limiting? been awhile since I used it. DVD FAB is the bomb though, should give that a try?
 
Handbrake is good converter, it also has advanced tab where you can put in your parametrs. I don't use DVD Fab, maybe I would have to if RedFox (Slysoft reincarnated) wouldn't exist but since they do I use AnyDVD HD to remove protection and when I am converting I use Handbrake. I don't like those all in one programs and AnyDVD HD is the first to crack the new protection when it comes out.
 
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