Changing resolution doesn't change size

Mar 21, 2014 at 5:05 AM
Just curious about this...

I'll setup a video to convert, and I'll either set a bitrate for the video or finished size. If I go to the audio tab and change something like the bitrate, or switch from 5.1 down to 2.0, if i switch back to the video tab, the target size or avg bitrate will change... makes sense, since I changed how big the audio track will be.

I noticed though, if I switch to the picture tab and change the dimensions of the video, say 1920x1080, down to 1280x720... when I switch to the video tab, nothing changed.

Being a smaller image size, shouldn't that change something? Like, if my target size was 3GB, then the bitrate should jump up?
Coordinator
Mar 21, 2014 at 6:30 AM
If you have a target size of 3GB it's always going to have the same bitrate, no matter what picture dimensions you choose. That's why I like Constant Quality; it just makes sure the picture looks good then tries to save on space.
Mar 21, 2014 at 7:13 PM
Strange... if I pick a target of 3gb and change the dimensions of the image, going from 1080p down to 720p should change the bitrate... If I pick a bit rate of say 3000kbps and the file is 3gb, and I drop the image down to 720p, then the bitrate will stay the same but the file size should drop down.

Smaller picture takes less space/memory.... Which is why I'm a bit lost.

I keep the frame rate constant and set an average bitrate, I don't use the constant quality option because I can't see how big the end file is going to be.
Coordinator
Mar 21, 2014 at 11:44 PM
Bitrate x time = file size. If your bitrate stays the same and the length of your video stays the same, your file size is not going to change. If you shrink your picture dimensions it's just going to fruitlessly throw more data at a smaller picture.

What do you need to know the file size for? Are you burning these onto a CD or tiny flash drive?
Mar 22, 2014 at 3:39 AM
I have a finite amount of space so, I tend to rate my collection and based on that rating I determine the size... for example, The Hobbit, pretty good, good effects, and about 3h long so having a 4GB file with stereo and 5.1 in my media server is fine...

...but, for some B-Rated horror movie from the '80s that my sister wants on the server, no way I'm wasting 4-6GB of space on that, so it gets dropped down to around 750mb-1GB and stereo only.

I always figured if for the 1GB movies I dropped the resolution down to 720p, because the images are smaller they'd have a bit less detail but the video would be less jerky.
Coordinator
Mar 26, 2014 at 10:27 PM
If you're using Constant Quality you can target a lower quality and lower the resolution to save space. Of course the encoder is always doing the best it can to save space and works best if you just tell it what picture quality you're happy with. If you specify bitrate you are directly controlling how much space the movie takes up and are tying the encoder's hands behind its back. It can't help you when a movie with low amounts of motion comes along, it's just going to throw the same amount of data at it as everything else.

Give the B-movies an RF of 22 or higher and check the savings.
Mar 27, 2014 at 3:15 AM
Be nice if, when using the constant quality option, there was an estimated size calculated... I can't look at a file, the size/length and resolution and mentally calculate that if I want a file Xgb in size I need a quality setting of Y.

I'm thinking using constant quality will result in my having to re-encode the same movie 1-3x to get the desired size.
Coordinator
Mar 27, 2014 at 5:55 AM
Do whatever you want to do, but I find it quite useful to not really care about the exact size of my files and just let the encoder do whatever compression it can.