Re: [rtcweb] Changing video size (Re: use case:)

[Cullen Jennings <[e-mail]>]

On Jun 20, 2011, at 1:30 , Harald Alvestrand wrote:

> On 06/17/11 21:38, Aron Rosenberg wrote:
>> On Fri, Jun 17, 2011 at 11:09 AM, Cullen Jennings <[e-mail]> wrote:
>> 
>> Video Size Change.
>> 
>> Alice and Bob are in a video call in their browsers and have negotiate a high resolution video. Bob decides to change the size of the windows his browser is displaying video to a small size.  Bob's browser should be able to regenerate the video codec parameters with Alice's browser to change the resolution of video Alice sends to match the smaller size.
>> 
>> 
>> Changing compression due to a user change in window size is usually not done since it has a bad long term effect on video quality. In almost all the major video calling systems, video resolution is a function of current bandwidth (which changes as the rate control system detects differences) and/or constrained by the physical device, but not a function of a user dragging the window size. At an equivalent bitrate, it is better to compress a larger resolution and display it smaller (higher PSNR), then compress a smaller resolution if you are within the codec's effective bitrate for that larger resolution.

Consider two different video flows using the same codec ...

Stream A is 640 x 480 at 1mbps which is this scaled to 320x240 and displayed

Stream B is 320 x 240 at 1mbps which is displayed at 320x240. 

My experience has been with modern video codecs that stream B will look better than stream A. As well as looking better, it will typically also have a better PSNR. There's a bunch of reasons why which are probably not worth going into here but give it a try and you will see what I mean. The key point is both streams where 1mbps. If stream B was sent at 256 kbps, then A would look better.