Vegas does not truly smart render MPEG2

PeterDuke wrote on 1/7/2015, 8:20 PM
In this thread JonnyRoy said, “ I know of no tool that will trim M2T files without re-rendering so it must be hard to do or someone would have done it.”

I always thought that Vegas smart rendered m2t files (HDV), meaning that the GOP(s) where a cut or cuts had been made would be re-rendered but the rest would be passed through unchanged. Certainly VideoRedo does just that. Did JR mean something else?

Also, Laurence said, “Theoretically, a smart-render of HDV and XDcam done at the 1440x1080 dimensions and a 25Mbps bitrate should look the same. That is to say, the bulk of the smart-render should be identical and there should be some second generation loss around the edit points and in to the next I-frame. (etc.) In actual practice, in an HDV smart-render, the degradation around the edit points and to the next i-frame is clearly visable, even to a strongly spectacled individual such as myself. “

I have only done limited testing, but I could not see any obvious degradation at the trims. Either my eyesight is worse than Laurences’s, or I don’t know what to look for (or more likely, both the above!). The nature of the scene may also be important.

I then took a HDV clip and trimmed a few frames off each end and smart rendered it out to a new clip. I then loaded the source and rendered clips into the Vegas timeline, with the source at the top and the rendered clip underneath, and aligned the latter to account for the trimmed frames. I then used the compositing tool to give the difference between the two clips.

Generally the difference view was very dark, but by straining my eyes I could see some noise in the re-rendered GOPs. What surprised me was that I could also see noise from time to time during the “unrendered” section!!!

I then rendered the original clip to a SD MPEG2 file and repeated the exercise with it. Once again I could see noise in the re-rendered GOPs but the noise during the “unrendered” portion was quite blocky.

I trimmed the HDV and SD clips with VideoRedo and in both cases I could see difference noise in the GOP sections, and in both cases no noise during the unrendered sections. (For some reason the HDV clip trimmed with VideoRedo was two frames shorter at the start, but that is another matter.)

So VideoRedo does smart render and Vegas does not, despite the “No recompression required” message and the rapid processing.


Rob Franks wrote on 1/8/2015, 6:45 AM
I can't recall the exact conversation but years ago I remember a thread on why Vegas could never properly 'smart render'. It had something to do with color space. Vegas was restricted to working with rgb or something like that while hdv is natively YCbCr 709

I can't remember the gist of the conversation but you may want to check out that part of the road.
riredale wrote on 1/8/2015, 11:55 AM
This should be easy to check out if someone goes through, say, ten generations.

All I know is that when I tell V9 to re-render an mpg of my project, it says it's smart rendering and really rips through the frames perhaps 50x the original render rate. So this implies to me that if it's changing those frames, it's not doing much changing.
John_Cline wrote on 1/8/2015, 1:11 PM
This should be pretty easy to check, place the original video on one track and the rendered video on another track, apply the Invert filter at 100% to the top track and set that track's opacity to 50%, the scopes and the preview window will now show the difference between the two tracks.

Except for re-encoding some video around the cuts, I'm certain that Vegas is just passing the rest of the video unaltered during a smart render.
Arthur.S wrote on 1/8/2015, 1:47 PM
'Smart rendering' seemed to get broken slightly after V9. With V11 and now V12 it varies quite a lot between "no compression" and rendering little bits. It was also much faster in V9.
PeterDuke wrote on 1/8/2015, 5:34 PM
"I can't recall the exact conversation but years ago I remember a thread on why Vegas could never properly 'smart render'."

Vegas DOES truly smart render DV AVI. Since there is no GOP there is also no re-render near the cuts. The difference function looks completely black all the way through.


See later posts. Version 9c also smart renders MPEG2 properly.
PeterDuke wrote on 1/8/2015, 5:42 PM
"Except for re-encoding some video around the cuts, I'm certain that Vegas is just passing the rest of the video unaltered during a smart render"

Then why do I see noise (quite dark I admit) when I view the difference? VideoRedo output is completely black as far as I can see.

I agree with riredale that Vegas isn't doing much because the "smartish" render is very fast.
PeterDuke wrote on 1/8/2015, 6:17 PM
"'Smart rendering' seemed to get broken slightly after V9. With V11 and now V12 it varies quite a lot between "no compression" and rendering little bits. It was also much faster in V9."


With Version 9c I do not see any noise in the difference effect except at the ends, as expected.

With Version 10 I do see some noise, although very faint (more obvious at the ends).
PeterDuke wrote on 1/8/2015, 7:54 PM
"It was also much faster in V9."

Correct again.

I took a 5m 46s HDV (.m2t) file as captured from my camera, trimmed 7 frames off start and end, and rendered to another HDV file using Vegas 9c to 13. In all cases, the words "No recompression required" appeared. The render times were,

9c 53s
10 3m 19s
11 3m 20s
12 3m 9s
13 3m 9s

So 9c is about 3.7 times faster than the later versions, (about 0.15 versus 0.56 times real time).
riredale wrote on 1/8/2015, 11:45 PM
Wow. That is a remarkable difference. Wonder what happened after 9?

As it so happens the version of V9 that I use is "c." The release notes show a "d" and "e" version, with lots of fixes, but I've never upgraded to them--yet. I wonder if the fast renders would work on those versions also. Presumably so.
PeterDuke wrote on 1/9/2015, 1:26 AM
I use 9c because it is the last version that "smart" renders AVCHD. I use quotes because the smart is a bit dumb at times. I just did a quick test using the compositing difference function and it would appear that there is very little difference despite not smart rendering when I thought it should have. I haven't checked it much yet.

If I installed 9d or 9e it would overwrite my 9c so I don't have them installed.
MadMaverick wrote on 1/9/2015, 1:30 AM
The problem is I edit with Vegas Pro 10 (I like the video stabilization feature that seems to be exclusive to it). Would it be possible to do all my editing with Vegas Pro 10, then open the project up in Vegas Pro 9 and smart render it?
PeterDuke wrote on 1/9/2015, 1:40 AM
"Would it be possible to do all my editing with Vegas Pro 10, then open the project up in Vegas Pro 9 and smart render it?"

As far as I know, you can't open a v10 project in v9, so the answer is "no". You could install ProDAD Mercalli 2 in v9.


I forgot the key point: Once you have stabilized you can't smart render. Smart render is only possible if there is no change to the video. You could have mixed stabilized and unstabilized clips on your timeline, and Vegas would smart render through the unaltered bits.
PeterDuke wrote on 1/9/2015, 2:06 AM
In the cause of science, I installed 9e and it took 48s, so it is similar to 9c for MPEG2 renders. (Codec made by Main Concept. The AVCHD renders are different because the codec is made by Sony.)
farss wrote on 1/9/2015, 7:53 AM
The problem your seeing may having nothing to do with Vegas changing the video data and might have something to do with how Vegas is decoding the before and after videos.
It's been a while since I seriously played around with HDV but I recall an issue with Vegas doing this No Recompress thing where although the video content was exactly the same as what was on the tape the file header and/or file extension was different. That can cause Vegas to use a different decoder or even have the same decoder produce a slightly different result e.g. a slight change in levels.

PeterDuke wrote on 1/9/2015, 4:48 PM
Would a header change also explain the 3.7 times change in render speed that I reported? It looks rather excessive to me. How did you know that the video content was exactly the same?
Laurence wrote on 1/9/2015, 5:19 PM
There is an option in the preferences to enable long GOP smart-rendering. Make sure that is on. Also make sure that your render format is exactly the same as your source format.

When video is being smart-rendered, it will be really obvious during the render. The screen will go black instead of showing the rendering video, there will be a white text header telling you that the video is being smart-rendered. If you don't see this, it isn't smart-rendering. Also, you will see the progress bar suddenly shoot up in speed to what you would expect from a file copy while the screen is black. It will go from showing the rendering frames at the beginning, to black during the smart-render, back to showing the rendering frames during each transition, back to black during the next smart-rendered parts, etc.

There is no doubt when it is working which parts are being smart-rendered.
PeterDuke wrote on 1/9/2015, 7:27 PM
Laurence, what you say is the established wisdom, but you have missed my points. In fact I have learnt more since I started this thread, particularly the differences between Vegas 9 and later versions.

I did try to force recompress with the setting you mention, but it made no difference. I realize now that it must have been Vegas 9 that I tried, because it does make a difference with Vegas 13, but not much. The flag is actually just labelled "Enable no-recompress rendering" in Vegas 13. With the flag unticked (i.e. force recompress) with Vegas 13, the render time was about 3m 23s. This compares with 3m 9s with the flag ticked. There is some difference, but not much. With the flag ticked, the message “no recompression required” appeared, and when unticked , the message did not appear.

With Vegas 9, however, this flag made no difference. (In fact the render time was slightly shorter at 46s with it unticked, but this is probably just variability due to other processes going on). The message “no recompression required” appeared despite the flag setting.

To summarize my findings so far:
Smart renders MPEG2 if the settings are appropriate. On my computer, HDV takes about 0.15 times real time.
A message “No recompression required” appears, and is believable.
Unticking “No-recompress long GOP rendering” makes no difference. (See Note)

Vegas 10-13
Claims to smart render MPEG2, if the settings are appropriate, but does not really do so. On my computer, HDV takes about 0.56 times real time.
A message “No recompression required” appears, but is false.
Unticking “No-recompress (long GOP) rendering” slightly increases the render time to 0.59 times real time, and the message “No recompression required” does not appear.

The files are the same length but a binary file compare shows a difference. The video content is identical, however. MediaInfo reports one difference: the matrix is custom in one case and default in the other.
farss wrote on 1/10/2015, 6:00 AM
[I]"How did you know that the video content was exactly the same? "[/I]

The best way I can suggest to test this it to render to exactly the same thing that does smart render e.g. have Vegas render to one of it's own codecs then smart render a copy of that video and compare them as before.

PeterDuke wrote on 1/10/2015, 6:15 AM
Well since the so-called no-recompression render time is almost the same as the recompression render time in Vegas 13, and a long way longer than the time taken by Vegas 9 to do the same job, I'm convinced that Vegas 13 is recompressing when it says that it isn't.
Arthur.S wrote on 1/10/2015, 9:11 AM
Yes, a smart render on my m/c takes approx' real time. The last version of V9e I used would really rip through it. Maybe a quarter of realtime (bare in mind how old the machine was too!) Just out of interest, does anyone here remember Ulead Media Studio Pro? That used to really smart render fast...and no need to place the original file on the top track of timeline either!
OldSmoke wrote on 1/10/2015, 10:26 AM

Yes, I do remember MSP and I used it until HDV came along. MSP V8 wasn't stable and had lost of issues. But yes, it did smart render and I had a Canopus Raptor card for it that had a hardware encoder on it. It was really great working with MSP and I had V5 to 7. I remeber that only the parts that where changed would be rendered and already rendered during preview which made for some fast final renders. The only reason I changed was Sony Vegas Pro worked flawless with HDV.

Proud owner of Sony Vegas Pro 7, 8, 9, 10, 11, 12 & 13 and now Magix VP15&16.

System Spec.:
Motherboard: ASUS X299 Prime-A

Ram: G.Skill 4x8GB DDR4 2666 XMP

CPU: i7-9800x @ 4.6GHz (custom water cooling system)
GPU: 1x AMD Vega Pro Frontier Edition (water cooled)
Hard drives: System Samsung 970Pro NVME, AV-Projects 1TB (4x Intel P7600 512GB VROC), 4x 2.5" Hotswap bays, 1x 3.5" Hotswap Bay, 1x LG BluRay Burner

PSU: Corsair 1200W
Monitor: 2x Dell Ultrasharp U2713HM (2560x1440)

John_Cline wrote on 1/10/2015, 10:56 AM
"I'm convinced that Vegas 13 is recompressing when it says that it isn't."

You haven't presented any real evidence to back up that claim.
Arthur.S wrote on 1/10/2015, 12:06 PM
@Oldsmoke, Yep, the reason I jumped ship to Vegas was MSP just got buggier and buggier as it got up to 7 and then 8. Shame, as it had a lot of nice features (some better than Vegas even now). 5.1 mastering was poo though!
PeterDuke wrote on 1/10/2015, 5:05 PM
"You haven't presented any real evidence to back up that claim."

My evidence is,
1. the longer than expected render time (four times!) and
2. the non-zero compositing difference effect between the source and rendered video (dimly seen as noise in the monitor and clearly visible in the scatter at the centre of the Vectorscope).

What evidence would convince you that Vegas 10-13 does not smart render?

What other explanation is there for the two observed facts above?