H.264 – Relationship between video bitrate and quality


I have been given the task to convert an uncompressed movie to H.264 format. The original movie takes up 80G. I am hoping to bring it down to 10G without losing much in quality.

It appears most 2-hour long BD movies that are in H.264/AVC format run at 24 MBPS bitrate (video) and take up about 20G to 30G disk space.

I am wondering if the relationship between the bitrate and the video quality is linearly proportional. In other words, if I reduce the bitrate to half, will the video quality go down by 50%?

Is there a formula to calculate appropriate bitrate for an action movie while keeping the quality to about 80% of the original?

Best Solution

It's a lot more complicated than that. Not only is quality is subjective, but the particular encoding options have an effect on quality that isn't related to bitrate. Holding encoding parameters constant, you can compare bitrate to SSIM (an approximate measure of quality) but the result is not very predictable. Have a look at the graph here: http://goldfishforthought.blogspot.com/2009/05/hd-video-standard.html

Unfortunately, I think the only way for you to determine the best bitrate for your project is trial-and-error.

Hope this helps!

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