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Bit rate required when changing resolution

Oct 25, 2013 9:46 PM

I'd like some suggestions as to what bit rate should be used when changing the size of a clip so as to not degrade the quality. Let me make up some variables:

 

 

B1: Bit rate of orginal clip. e.g. 1 Mbps

 

S: Scale factor (double, half…)

 

B2: Bit rate of scaled clip.

 

Is there a relationship between B1, S and B2 so as not to lose quality? For example, if a clip is scaled up by 2.0, should B2 be twice B1? Or does it go up as the square to accommodate the pixel increase, so if you double the size the bit rate should increase four times?

 

I have done some tests with a jpeg image in Photoshop to see how jpegs operate. Starting with a tiff image and altering its resolution (0.33, 0.5, 1.0, 2.0, 3.0), I saved as jpeg with three quality settings and then looked at the file size. The relationship between resolution and file size was somewhere between linear and square (power around 1.3). Results have been normalised to the original resolution. The three quality settings gave similar results:

 

Resolution     File Size

0.33               0.27

0.5                 0.42       

1.0                 1.00

2.0                 2.71

3.0                 5.00

 

 

Might the same apply to video clips? i.e. if I triple the size I don't need nine times the bit rate (as the number of pixels would indicate), but possibly only about five times.

 
Replies
  • Currently Being Moderated
    Oct 25, 2013 10:25 PM   in reply to Guy Burns

    IMO

     

    Changing the bit rate will not have any positive affect (regarding quality) on uprezzing or down rezzing.

     

    Less recompression via codec choice would be  better "solution" but uprezzing is an unavoidable  quality hit.

     
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    Oct 26, 2013 10:02 AM   in reply to Guy Burns

    Is there a relationship between B1, S and B2 so as not to lose quality?

     

    There is no correlation there.  The only way not to lose quality from the transcode process is to transcode to a lossless format.  This applies to all transdode situations.

     

    Of course, there are other ways to lose quality, such as scaling up, that are unavoidable no matter what you do.

     
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    Oct 26, 2013 7:12 PM   in reply to Guy Burns

    You asked the question in an odd way, and both of the answers previous to mine are correct. However, I don't think they answered the question that you meant to ask. Maybe, but I have my doubts.

     

    So, let's spell this out a little better.

     

    If you take video to over 100% of the original size, no amount of increasing the bitrate is going to improve your quality. That is the issue shooternz is pointing out. The higher the bitrate, the more it will look like what you see in Premiere Pro on the timeline. But you can't increase it to make it look better than what you see on the timeline.

     

    You are always going to lose one generation of quality when you export to a lossy codec. That is what Jim is pointing out.

     

    If, on the off chance, I am correct in assuming that you asked the question in a way that didn't really address your real question, let me answer the question that I think might have been what you wanted to know.

     

    If you have an HD sequence at 1920X1080 and you want to export to use it on a web site, and, let's say the original was captured on the camera at 50Mb/s. (That option is available on my camera.)

     

    If you export to 960X540, you could theoretically use a bitrate of one fourth the original (just under 13Mb/s), and get approximately the same quality as you see on the timeline. Not quite, but generally close enough to make most users happy if you use a decent codec like H.264. You have dropped the number of pixels that the data has to support, so the amount of data required is reduced. Therefore the data rate can be reduced.

     

    Drop the size to 480X270 and even less bitrate is required to communicate the same video since there are so few pixels that have to be communicated.

     

    So, yes, there is a correlation on the side of scaling down, but scaling up? Not so much. You can't make up pixels where there were none before and expect a great result.

     
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    Oct 27, 2013 1:12 AM   in reply to Guy Burns

    There must be some way to work out an approximate bit rate to use so that when a clip is scaled the visible quality is not degraded. Is Bits/(Pixel*Frame) a suitable parameter?

     

    Bit rates have nothing to do with pixels.

     
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    Oct 27, 2013 1:33 AM   in reply to Guy Burns

    They are not discussing scaling of pixels.

     

    Simplified...in any "video size"...eg SD or HD...one can use what ever data rate  one chooses but the pixels are fixed.  ie Fixed resolution.  

     

    The data (bit) rate will determine the "smoothness " in  playback of the movie.  Higher data rate = smoother playback of the moving images.

     

    Pixels are just pixels and are an element of frame / image resolution.  The source pixels is fixed in resolution.

     

    One can have low resolution (pixellated) with high bit rate or low bit rate.

     

    One can have high resolution with high bit rate or low bit rate.

     

    High resolution with High bit rate ( data rate) = best quality.

     
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    Oct 27, 2013 9:00 AM   in reply to Steven L. Gotz

    If you export to 960X540, you could theoretically use a bitrate of one fourth the original (just under 13Mb/s), and get approximately the same quality as you see on the timeline.

     

    Those are entirely arbitrary numbers, Steven, and I believe misleading to readers.  There's no direct correlation between resolution and required bitrate.  The quality of an encode depends on many interrelated variables.  It is entirely possbile to have a higher birtate with less quality, and conversely, a lower bitrate with greater quality.

     
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  • Currently Being Moderated
    Oct 27, 2013 9:07 AM   in reply to Guy Burns

    I probably should have said: "so as not to visibly degrade quality"

     

    It wouldn't have helped much, because what's 'visible' is very subjective.  I see artifacts in broadcast, cable and satellite feeds quite frequently that drive me nuts, but are quite 'invisible' to most of my friends and family.

     

     

    What I haven't worked out is the optimum bit rate to use

     

    You will have to experiment, using your own eyes as a judge.  There is no mathematical formula here.

     

    Having said that, you may want to wait until the October update hits.  It will bring Photoshop quality upscaling to video.

     
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    Oct 27, 2013 10:58 AM   in reply to Guy Burns

    Hardware players have features to upscale video that the software alone can't compete with.

     

    There is software available that is designed to upscale to HD that does a better job than Premiere Pro.

     

    If you shoot low quality on a GH3 you get what you deserve. Shoot using all I-Frame at the highest resolution using RAW and not JPEG. Then you can edit in Photoshop using the Camera RAW filter recently added to Photoshop. But since it is already 1920X1080 there is no real point in doing so unless you have a need to change it because you shot it wrong, in regards to shadows or highlights, etc. Or you need effects that are not available yet in Premiere Pro.

     

    Vimeo recommends different bit rates for different sized video for exactly the reason I already stated. I understand why Jim disagrees, but I have studied this issue in depth (work related requirement in the test equipment field). The more pixels, the higher the bitrate has to be to keep the quality up. If you want 8 bits per pixel, you have to multiply the pixels by the number of bits per pixel to get the bitrate required to support it.

     

    But you can't get great interpolation with Premiere Pro. You can do better with hardware, and you can do better with other software designed for the task. Instamt HD is one example.

     

    Your best bet? Export to as high a bitrate as you can reasonable use. For example, 25Mb/s is usually playable by most players. You may not be happy with the results, but in most cases, going higher just isn't going to help.

     

    Test it out for yourself. Don't rely on anything we have to say. Export to various bitrates and see what you can live with. Compression is more art than science. Back in the days of exporting tiny videos to the web to save storage and bandwidth requirements, we used to say that you should export to as low a bitrate and as small a frame size as you can live with. Reduce the bitrate until you hate it, then raise it back up a bit.

     

    In your case? Just export to the highest bandwidth you can play on your player and live with it.

     
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