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Let's say I want to download video at a bitrate of 4-5 mbps (that's bits, not bytes). Will I get a better quality video if I choose 720p or 1080p? Assume I use a good codec like H.264, VC1, webm, etc.

One might argue that, of course, 1080p is better, but on the other hand, it's also possible that this bitrate is too low to preserve the additional quality of 1080p video, and all I have is overhead, so I'm better off with 720p. See http://www.zdnet.com/blog/ou/dont-believe-the-low-bit-rate-hd-lie/959

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Unfortunately, this question is not one that can be answered in a constructive manner because it is likely to result in debating. Stack Exchange sites prefer questions that can be answered objectively; see the FAQ for more details. Voting to close. – DragonLord Sep 8 '11 at 14:37
4-5Mbps is nowhere near enough for reasonable quality 1080p. There's more than twice as many pixels as 720p. – James Sep 8 '11 at 16:20
I don't understand why this question can't be answered objectively. Maybe a test was done? Or may be some people reading this have the "specific expertise" that the notice below talks of? – Kartick Vaddadi Sep 9 '11 at 3:06
As per my answer, there is no definitive answer to this question. I work in video quality research and in order to answer this, you'd have to set up a very specific test. Maybe I can dig up some papers on this, but I'm not sure there are that much. – slhck Sep 10 '11 at 10:05
You might want to read this paper though: Comparison of HDTV formats using objective video quality measures. From reading through it quickly, it seems to address what you want, i.e. a comparison between different HD resolutions at different bitrates, measured by different objective video quality metrics (see page 9). – slhck Sep 10 '11 at 10:19
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closed as not constructive by DragonLord, Gareth, techie007, slhck, Nifle Sep 8 '11 at 16:27

This question is not a good fit to our Q&A format. We expect answers to generally involve facts, references, or specific expertise; this question will likely solicit opinion, debate, arguments, polling, or extended discussion. See the FAQ for guidance on how to improve it.

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What one could describe "video quality" is made up of many factors. Bit rate and frame size are only two of them. Here's a short list:

  • Bit rate — this is one of the huge factors that come into play when considering file size or available streaming bandwidth, but it's not that important to "video quality", since the effective bit rate is a result of other quality aspects, especially the QP and GOP size (see below).

  • Frame size — this is a major aspect of video quality, but the difference between 720p and 1080p can be minimal to nonexistent when other quality parameters are set accordingly. You can also sharpen a 720p video, which would look better, or blur a 1080p video, so there's less grain (which some users could describe as "better" quality). It's very subjective.

  • Quantization parameters — without going into technical details, this is one of the most important factors that stems from image processing, although not really very widely known. Setting the QP accordingly, you can make a 1080p video look worse than a 720p video. QP will essentially play a role in defining the overall bit rate, and its setting is very critical.

  • GOP Size, Picture types — you can choose to use different types of frames/pictures in a video (like i-Frames). The more i-Frames a video has, the more size/bitrate it will take, and the better it'll look.

TL;DR: You can't compare the quality of two videos just by looking at their bit rate and frame size.

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Well, let's assume all factors above are the same between two videos, except the bit rate. Then does the 1080p video look better than 720, given a bit rate of 5mbps for both? – Kartick Vaddadi Feb 7 at 5:58
Have you read the paper I linked to you above? Using objective quality metrics, it seems that 720p at a given bitrate easily outperforms 1080p. I can't say whether subjective tests would confirm that, but under the assumption that the amount of coding artifacts is reduced when using 720p, and good upscaling algorithms exist, then 720p will look "better" to most people, when shown at 1080p, using the same constrained bit rate. – slhck Feb 7 at 8:24
I looked through the paper twice, both when it was posted on this thread and again, just now, but most of it seems to discuss objective metrics like PSNR rather than subjective metrics. Since you seem to know this far better than me, I will go with your assumption and use 720p. Thanks for taking the time to help me out. – Kartick Vaddadi Feb 9 at 1:37
As I said, I'm not aware of any specific subjective test regarding the difference here. I would assume though that under high bitrates, humans are not even able to spot a difference. If you have the time, I'd suggest to do a small subjective test yourself and see what personally looks better – unfortunately that's the only way to try in most cases. If you have any questions, feel free to ping me. – slhck Feb 9 at 8:07
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