Is rendered file size variation normal?

Rich Parry wrote on 12/5/2022, 10:33 PM

I’ve rendered two project multiple times, one of them 6 times and get 6 different file sizes, sometimes just a few KB difference but sometimes as much as 2 MB difference. I tried both Sony AVC/MVC and Magix AVC/AAC MP4 mp4 with same results. Using “MediaInfo” the only difference I see in the files is the “bit rate”. I'm using software rendering, not GPU assisted rendering, I can't think of a reason why file sizes aren’t identical every time I render, am I missing something?

In case you need to see details from one of the renders, I’ve included it below. I’m using VP20 214B. I suspect this is normal but only now just noticed it. Is there a reason for render file size variability?

Rich

General

Complete name : D:\My Data\My Videos\_Parry Videos\Render Folder\Bosque del Apache IV 2022 v2.mp4

Format : MPEG-4

Format profile : Base Media / Version 2

Codec ID : mp42 (mp42/isom)

File size : 300 MiB

Duration : 2 min 24 s

Overall bit rate mode : Variable

Overall bit rate : 17.5 Mb/s

Encoded date : UTC 2022-12-02 01:33:59

Tagged date : UTC 2022-12-02 01:33:59

Video

ID : 2

Format : AVC

Format/Info : Advanced Video Codec

Format profile : High@L4

Format settings : CABAC / 2 Ref Frames

Format settings, CABAC : Yes

Format settings, Reference frames : 2 frames

Format settings, GOP : M=2, N=15

Codec ID : avc1

Codec ID/Info : Advanced Video Coding

Duration : 2 min 24 s

Bit rate mode : Variable

Bit rate : 17.2 Mb/s

Maximum bit rate : 22.0 Mb/s

Width : 1 920 pixels

Height : 1 080 pixels

Display aspect ratio : 16:9

Frame rate mode : Constant

Frame rate : 29.970 (30000/1001) FPS

Color space : YUV

Chroma subsampling : 4:2:0

Bit depth : 8 bits

Scan type : Progressive

Bits/(Pixel*Frame) : 0.276

Stream size : 294 MiB (98%)

Language : English

Encoded date : UTC 2022-12-02 01:33:59

Tagged date : UTC 2022-12-02 01:33:59

Codec configuration box : avcC

Audio

ID : 1

Format : AAC LC

Format/Info : Advanced Audio Codec Low Complexity

Codec ID : mp4a-40-2

Duration : 2 min 23 s

Bit rate mode : Constant

Bit rate : 320 kb/s

Channel(s) : 2 channels

Channel layout : L R

Sampling rate : 48.0 kHz

Frame rate : 46.875 FPS (1024 SPF)

Compression mode : Lossy

Stream size : 5.49 MiB (2%)

Language : English

Encoded date : UTC 2022-12-02 01:33:59

Tagged date : UTC 2022-12-02 01:33:59

Comments

EricLNZ wrote on 12/6/2022, 12:39 AM

A file's size is directly related to the bitrate. Hence differences in bitrate will give different file sizes.

Rich Parry wrote on 12/6/2022, 12:51 PM

Eric, you missed my point. I rendered a project 6 times and got 6 different file sizes. This makes no sense to ,me.

Musicvid wrote on 12/6/2022, 2:11 PM

I think @EricLNZ got it correct. Variable bitrate encoding is a moving target, not a constant.

bvideo wrote on 12/6/2022, 3:06 PM

Variable bitrate is said to be dependent on the content being rendered, which Eric has held constant. Is it dependent on something else that varies from one render to the next? Maybe there is a deliberately random process in the render process for some reason we might not guess, but how do we find out?

EricLNZ wrote on 12/6/2022, 4:44 PM

@Rich Parry You are correct. I did miss your point. I got confused by you mentioning the bitrates varied.

You are using VBR. Are you doing one pass or two pass renders? If one pass the variation doesn't surprise me but two pass should give much closer results.

With CBR the sizes should be identical.