Preview resolution vs final rendering - any connection?

Aresby

Hi guys

As a result of some things mentioned in support responses over the last few days, I'm wondering if there is any link between preview rendering resolution and the final rendering time taken?

If, for example, I preview in full HD, does VP still do exactly the same process for rendering the final HD video (exporting it)? In which case, are the different preview resolutions only there so that VP can generate the previews more quickly (along with any transitions)?

Just wondering if this is something I can do to speed up export on 4.58 (whilst the new version which does use all available CPU when exporting and will be therefore quicker) is still being tested.

borate

Using the same export settings, a 30 second Mp4 clip took about the same amount of time to export whether preview was set to lowest or highest.

The completed videos were different sizes, so the preview setting does affect export. Doesn't directly answer your query, but it's another piece of info.

Aresby

borate wrote:

The completed videos were different sizes, so the preview setting does affect export.

Thanks Borate. So this is the bit that NCH must answer. Why does preview resolution affect final export size? And does it ever affect time to render? I can't quite get the connection.

My holy grail is full HD export quality but lowest possible size. Along with super-quick export!

I tried using full HD preview but it took so long to generated the new previews that I abandoned it after 30 minutes and reverted to the lowest (16:9) available just so I could get my video finished. My 40 minute video took pretty much 40 minutes to render. Pre-4.58 it would have taken no more than 30 minutes. But 4.58 uses about 40% of total available CPU. Pre-4.58 took 100%. That's why I'm eager for the next release where they have fixed this, allegedly.

borate

For clarity, I should have said that the screen display of final product, comparing output of low and high preview settings, looks to be at different resolutions.

As for file sizes for the 30" 720HD source test:: the preview high-res output is 6.36Mb vs. 5.25 for the low-resolution.

On closer scrutiny, rendering time increased with the preview high-res setting as well. About 17 for high and 13 for low.

So, without changing export settings, which were 320 X 240, it appears that preview settings do have an impact. Perhaps that's as it should be?.

NCH techs may elaborate.

[deleted]

Preview resolution and framerate should have no impact on exported video. The only way in which it should possibly impact is time taken to export (i.e. If the CPU is busy generating the preview, it can't spend all its energy on the export).

My testing doesn't show any difference exporting the same thing over and over again with different preview resolutions. The only time that should happen is if you have something inherently random (e.g. a noise effect) that would affect encoding.

Borate, when you were exporting, were you exporting the exact same thing (same project, no edits, same export settings), with no effects, and just the preview resolution changed?

Aresby

NotADevWinkWink wrote:

Preview resolution and framerate should have no impact on exported video. The only way in which it should possibly impact is time taken to export (i.e. If the CPU is busy generating the preview, it can't spend all its energy on the export).

Good, as I expected. Thanks for confirming this.

borate

NotADevWinkWink wrote:

Preview resolution and framerate should have no impact on exported video. The only way in which it should possibly impact is time taken to export (i.e. If the CPU is busy generating the preview, it can't spend all its energy on the export).

My testing doesn't show any difference exporting the same thing over and over again with different preview resolutions. The only time that should happen is if you have something inherently random (e.g. a noise effect) that would affect encoding.

Borate, when you were exporting, were you exporting the exact same thing (same project, no edits, same export settings), with no effects, and just the preview resolution changed?

Precisely. The source for this test was the Windows Wildlife.wmv file (1280 x 720) with Mp4 export set to the TV NTSC defaults,

The "low" preview was made at the lowest preset resolution (4 x 3); the "high" at the highest (16 x 9). 30fps.

Both outputted at 320 x 240, but their bit, data rates and file sizes were slightly different.

When the files are played from the server the "low" (4 x 3) is letterboxed, the "high" (16 x 9) is full frame.

[deleted]

Oh, I see what's happened here (TL;DR at the end).

Minor technicality here, the preview aspect ratio (not resolution, although it's determined by resolution) does have a (very) minor impact on export.

If the preview resolution is 16:9 and you export, VideoPad will do all the rendering in a 16:9 aspect large enough to accommodate the export resolution, and then (if it needs to) apply a letterbox or crop to get the video to the export aspect ratio (in this case, 4:3)

Example 1: Preview at 320x240, export at 320x240

VideoPad just exports straight to 320x240

Example 2: Preview at 1920x1080, export at 320x240:

If you choose "Crop edges" when exporting:
VideoPad does it's processing at 427x240 (16:9 like preview)
VideoPad crops the edges off, down to a 320x240 resolution and exports that

If you choose "Resize to fit"
VideoPad does it's processing at 427x240 (16:9 like preview still)
VideoPad adds black bars to the top and bottom, shrinking the video so that it fits in a 320x240 resolution (i.e. shrinking the video to 320x180 with bars on top/bottom)

TL;DR VideoPad will use the preview aspect ratio to export, and then make it fit your chosen export aspect ratio. This will lead to a change in bitrate as the video encoded changes.

Aresby

So as long as I preview in a 16:9 format and export in same format (although at different resolutions), the preview setting will have zero impact on the export performance, bit rate, time taken etc?

[deleted]

Correct.