Adobe Premiere Video With Alpha Channel Rendering Funny

shane bourgeois

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Sep 21, 2020, 8:03:56 PM 9/21/20

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Hello,

Anyone know the export settings for videos with transparency Adobe Premiere Pro?

Best,

Shane

Ben Hegarty

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Sep 21, 2020, 9:22:01 PM 9/21/20

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I know in Final Cut you have to export in Apple ProRes 4444.  Not sure about Premiere, sorry.

micpool

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Sep 21, 2020, 9:47:45 PM 9/21/20

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With some difficulty!

Try Quicktime/Apple ProRes 4444/render at maximum depth/32bit

That worked for me when I tried it just now , although it's a lot easier to do in After Effects.

Some people say that you have to have nothing on Video 1 Layer for this to work, but this may not be the case.

Mic

shane bourgeois

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Sep 22, 2020, 12:11:07 AM 9/22/20

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I now have after effects what should the setting be ? ..... i'm not trying to go against the flow this is just new to me

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shane bourgeois

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Sep 22, 2020, 8:53:49 PM 9/22/20

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 you guys are the best thank you !

On Monday, September 21, 2020 at 3:01:41 PM UTC-4 micpool wrote:

Video Codec Apple ProRes 4444

Channels: RGB+ Alpha

Depth: Millions of Colours +  (This is the same as 32bit i.e 8 bits per channel + 8 bit alpha)

Color: Straight( Unmatted)

David Bengali

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Aug 12, 2021, 2:19:08 AM 8/12/21

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Has the guidance for straight vs premultiplied alpha changed in QLab?  I am no longer able to get videos with straight alpha to work correctly; now only premultiplied alpha seems to work (using QLab 4.6.10)

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Sam Kusnetz

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Aug 12, 2021, 8:13:56 PM 8/12/21

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Hello all

We haven't recently changed anything about how QLab handles Alpha channels. macOS might have changed, or the tools you're using to encode videos might have changed?

I'm watching this conversation and will strive to build the best understanding that I can so that the manual can accurately reflect best practices.

I encourage anyone who shares footage or who is discussing encoding trouble to please specify what encoding tool they're using, and what version. For example, I'm using Compressor version 4.5.4.

Best

Sam

Both Straight and Premultiplied working correctly for me in 4.6.10. Do you want to post a few seconds  of your footage (5MB upload limit on this group).

micpool

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Aug 13, 2021, 7:40:33 AM 8/13/21

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My general rule of thumb is to render premultiplied from  a black composition background, and if that doesn't work, try straight. Rendering out from AE CS6 I've never had a composition that wouldn't render correctly in QLab using one or other of these methods, but I've never got round to doing a test workspace that shows a ProRes 4444 RGB + Alpha  failing to render  in QLab correctly, using  one of the output options,  while the other option derived from  the same composition works correctly.... until now.

You can download a test  QLab workspace with content  here

You can use this workspace to see if the files render correctly on your system using QLab, and then use the video files themselves in other programs to see if they work there.

2 compositions were made in AE CS6,  and then rendered out using Straight and Premultiplied Alphas, with the composition background colours being black (RGB 0,0,0)

In QLab the straight alpha versions of these particular files do not render correctly. I can't find another program that  misinterprets them in the way  QLab does,  (in which they are unusable). Some programs, e.g VDMX, and reimporting them into AE, render them identically, others, e.g Resolume Arena, render them similarly, but the premultiplied versions are darker in the feathered areas at the edge of the shapes.

I do have other examples of Straight Alphas exported for AE CS6 which work fine in QLab, so I don't think it's the case that QLab will only handle one method or the other.

As far as I can see, there is nothing in the file headers that gives any indication if a ProRes 4444 file with alpha is premultiplied or straight. If the program is required to guess whether a file with transparency is premultiplied or straight, because  the user can't define this explicitly , it has to have some method of deciding for itself. On the evidence of the attached files QLab seems it may  have difficulty with this.

I can't find much recent material describing how a program guesses the interpretation required, but it does seem to be quite complicated.

Mic

micpool

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Aug 13, 2021, 7:36:58 PM 8/13/21

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Digging deeper, I now find my assumption that After Effects was always guessing whether a ProRes 4444 file was premultiplied or straight was incorrect. The preference setting for a parameter called Interpret Unlabeled alpha as was set to 'ask user'. So because It has never asked me when importing RGB+Alpha files it must have been reading the alpha label. But I have drawn a blank in google searches in trying to find out what this label is and how it is stored in a ProRes 4444 file. Does anyone know?

Mic

micpool

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Aug 13, 2021, 8:33:12 PM 8/13/21

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Hi David

Didn't you go through exactly the same issue in detail with the Isadora folks about 4 years ago

https://community.troikatronix.com/topic/3452/problems-with-straight-alpha

They seemed to say the same thing as Sam, that they hadn't changed anything, but a look at that  thread, shows similar confusions around the voodoo of Alpha interpretation as we are having here.

My conclusions are:

The stuff that comes out of Apple Compressor, Motion and Final Cut  with the default 4444 with alpha settings seems to work fine in QLab.  I can't find a reliable method to determine whether these are premultiplied or straight.

In Programs that offer the option (including Adobe) rendering Premultiplied from a composition with a black background in composition settings (not a solid black background layer in the comp) seems to give the best results most often in QLab..

The golden rule is probably to test in QLab immediately after rendering out  your ProRes 4444 with Alpha  on top of a cue playing a complex coloured background. If it doesn't look good, re-render it with a different setting there and then. (If you are doing long files then render out a 5s test clip first).   Going back to the original composition months later is always going to be more complicated and sometimes not possible.

If you have a video editor or content creator making files for you, make sure, if possible, that  they check them in QLab before sending them to you.

Mic

micpool

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Aug 13, 2021, 10:48:58 PM 8/13/21

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OK, I've finally found a method to sort out most of what is going on!

Isadora selects the alpha interpretation by means of a hard switch, which means the output of other programs can be compared to Isadora's output to determine what they are doing.

Example 1: Footage rendered out of Motion Compressor (AE identifies as straight alpha)

If you look at the edges of the stars carefully, you can see that Resolume Arena, and Isadora set to straight both have a black shadow around the star. This shows that the footage is not as AE detected it (i.e straight) but that it is premultiplied. QLab has interpreted it as premultiplied and it renders correctly. Arena  is therefore rendering all footage with Alpha  as straight

Example 2: Same as 1 but Isadora now switched to premultiplied

Stars now rendered identically and correctly in QLab and Isadora confirming footage is premultiplied.

continued in next post (please can we up the file size limit!)>>>>>>>>>>>>>>>>>

micpool

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Aug 13, 2021, 10:50:49 PM 8/13/21

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<<<<<continued from previous post

Example 3 Rendered from AE with straight alpha

Isadora switched to straight and Resolume render the footage correctly as straight. QLab renders very strangely.

Example 4: As Example 3 with Isadora switched to premultiplied

This confirms that the strange rendering in QLab is purely because it is interpreting a straight alpha as premultiplied.

My observation in a previous post that QLab was doing something strange and unique was not correct. It was just that the other programs were always interpreting footage as straight. As the consequences of rendering a premultiplied alpha as straight are comparatively subtle, where this happens it is easy to overlook that the alpha is incorrectly interpreted. But when a straight alpha  is misinterpreted as premultiplied  the result is obvious and unusable. This is probably why some other programs have opted for a default straight interpretation, and Isadora went one step further  and changed to a user selectable interpretation so any anomaly can be corrected on the fly.

So the outstanding questions are:

Is it true to say that QLab ALWAYS  interprets any Prores 4444 with Alpha as premultiplied?

What are the 'labels' that AE is using to supposedly detect the alpha type of a video  on import, and why does it give the wrong result when importing a clip from a modern version of Compressor?

Do the modern Apple apps  even have the ability to render straight alphas?

Do we ever need to use straight Alphas with ProRes Files for QLab.

Is there any video playback software that is capable of detecting the alpha type, or do they all just pick one or the other (and are the majority straight alpha?) or allow this to be set manually.?

And one other thing that has been bugging me, how  do .png files avoid  this problem by only having a transparency channel  (not really  alpha?) with no options?

Going for a lie down now!

Mic.

micpool

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Aug 13, 2021, 10:57:59 PM 8/13/21

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On Friday, August 13, 2021 at 4:50:49 PM UTC+1 micpool wrote:

 But when a straight alpha  is misinterpreted as premultiplied  the result is obvious and unusable.

I should have written   the result is OFTEN obvious and unusable. Sometimes it is no more than a slight white halo around  the image, which is rendered against a light background will be invisible. This is why I may have previously erroneously believed that some straight alpha renderings worked perfectly in QLab. Perhaps this is never the case (and never has been)?

MIc

micpool

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Aug 15, 2021, 4:08:34 AM 8/15/21

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On Thursday, August 12, 2021 at 2:13:56 PM UTC+1 sam kusnetz wrote:

I'm watching this conversation and will strive to build the best understanding that I can so that the manual can accurately reflect best practices.

I've had another good look at this today, on a variety of OS up to Catalina and  versions of QLab 4 up to the current version, exporting from Adobe and Apple Programs.

I've also searched through previous discussions and found this has  comes up regularly, over the past 5 years, and in all cases where I have had a look at the problem, I have always  come down to the same conclusion,  that premultiplied alphas (with a black composition background in AE), which are as far as I can see what automatically comes out of the latest Apple programs, work every time in QLab and straight alphas always have a problem, which is sometimes subtle, but more  often  makes the footage  unusable in QLab.

I personally think that it's safe to say that QLab currently requires:

Apple  ProRes4444 RGB+Alpha  at a minimum depth of Millions of colors+ with an alpha premultiplied with black ( which means, in AE,  black composition background to be set in composition settings)

My understanding is also that premultiplied is also  the most efficient option for a GPU to process,

Does anyone have an example of any footage in this format that QLab interprets incorrectly, to disprove this?

Mic

Taylor Glad

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Aug 15, 2021, 2:23:49 PM 8/15/21

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Not saying I have an example that doesn't work. Just wishing there was a "like" option here to show appreciation without making a pile-up in the comments. But I'll add just this one!

Your thoroughness is continuously impressive and always appreciated!

micpool

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Aug 17, 2021, 12:23:32 AM 8/17/21

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My earlier observation that Resolume Arena was using Straight Alphas, was incorrect. It allows selection of the alpha interpretation and was in fact defaulting to Premultiplied, but  neither premultiplied or  straight are actually rendering correctly where there is feathered transparency. Straight has a black halo like premultiplied and Premultiplied has a darker black Halo.

Don't know what's going on there, but far more interesting is VDMX5 as you have complete access to all the modes of the Core Image system.

VDMX5 in it's default setting requires straight alpha which it renders correctly  in Open-GL Over Mode. It can use Premultiplied, but to use this you have to select  the blend mode from the Core  Image set . Source Atop or Source Atop-VV work well.  If you feed straight alpha into core image blend modes you can produce many variations of the incorrect output that QLab produces. However, Source Atop produces almost identical output to QLab. With Premultiplied alphas the rendering is correct and with  Straight Alphas the output is incorrect and very close to what QLab outputs with the same footage. I can't find any setting in the Core Image set which correctly renders a straight alpha.

I'm probably overlooking something or completely incorrect, but wonder if QLab is using Core Image Source Atop to render footage with Alpha?

QLab 4 straight alpha footage :

VDMX5 straight alpha footage rendered in  Core Image Source Atop mode:

QLab 4 Premultiplied alpha Footage, rendered correctly :

VDMX5 PM  alpha footage  in  Core Image Source Atop mode, rendered correctly:

micpool

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Aug 17, 2021, 12:48:30 AM 8/17/21

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I'm now almost completely confident that ProRes 4444 Premultiplied  (with a black composition background in AE) will always render correctly in QLab4.

For completeness here is a selection of QLab rendering footage with straight alphas, from obvious artefacts to the extremely subtle.

Motion Graphic produced in AE by rotating a red layer with a star shaped mask on it:

Green Screen Footage keyed in AE using Keylight:

3D animation rendered directly to QuickTime  from Cinema 4D:

This is extremely subtle but if you look carefully at the end of the marbling there are artefacts, similar to the white halo in the green screen example, present that are not there with premultiplied alphas:

David Bengali

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Aug 18, 2021, 5:08:28 AM 8/18/21

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Thanks for this very thorough look at things.  It is true that I had a similar conversation with TroikaTronix regarding Isadora a few years back, and the result was the addition of that user-selectable switch to identify the alpha mode, which is very helpful and if it were not too difficult to add to QLab, it would be really great.  I am currently finding that rendering premultiplied from After Effects is working in QLab, as you demonstrate here.  One challenge is that as far as I can tell, Adobe Media Encoder does not currently have a way to specify alpha way After Effects does, and I think what it generates is straight alpha, which means it is currently not usable as an encoding tool for QLab. This is an unfortunate QLab-specific limitation for workflows that center on that encoder.  Also some other media servers out there work better with straight alpha, and it may be the case that a show gets ported between media servers, so having a way to use either straight or premultiplied would be great.  This would also help in case things in AVFoundation or in MacOS or on different hardware change unpredictably over the years, so that bringing an older QLab show back online can always be done.  I seem to remember a time in the recent past that alpha was interpreted as straight in QLab, so premultiplied would show up with a black halo, or a darkening in semitransparent areas, so it is conceivable that things could shift again.  And in any case, a switch or checkbox would take all the guesswork out of things.

David Bengali

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Aug 18, 2021, 5:22:08 AM 8/18/21

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sorry, i didn't specify encoder version - media encoder 15.4 - does not seem like there is an option to render as premultiplied with black as there is in after effects

also, as to the PNG question from micpool, some info at the links below.  The PNG format definition is that it uses only straight (non-premulitplied) alpha, so any image viewer or PNG decoding library just knows what to do because there is only one option.

micpool

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Aug 18, 2021, 7:35:53 AM 8/18/21

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On Tuesday, August 17, 2021 at 11:08:28 PM UTC+1 dben...@gmail.com wrote:

 I seem to remember a time in the recent past that alpha was interpreted as straight in QLab, so premultiplied would show up with a black halo, or a darkening in semitransparent areas,

It depends what you mean by recent!

I thought you were mistaken about this, because I had checked it back to QLab 3.2.15  and OS 10.10, but then I found a post of mine on this subject from 2015 where I said only Straight Alphas worked with QLab.

I don't know when the change happened in QLab, the only currently  downloadable version of  QLab 3 is 3.2.15, but I found a copy of 3.0.21, which is what I would have been using in 2015, and lo and behold, only straight alphas work correctly with this version. The change may possibly have happened with QLab 3.1 as there were considerable changes to video  compositing between 3.0 and 3.1.

(QLab 2 only worked correctly with Straight Alphas as well.)

Mic

micpool

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Aug 18, 2021, 7:57:36 AM 8/18/21

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And just to illustrate how long this has been a confusing issue, exactly  8 years ago last week we were discussing the same thing  on the Google group.

On 12 Aug 2013, at 08:18, Patrick Spadrille wrote:

I have 2 video cues. One is a background in full screen and the second one is a title in ProRes 4444 with alpha. There seems to be a kind of problem with the way Qlab handles the alpha channel.The title should fade out at the end for 28 frames. Instead it changes color to black in 14 frames then stays black for some frames and then fades out for the rest. I have tried to superpose those 2 clips in Premiere and there everythings works fine BUT in Final Cut Pro 7, it is exactly the same issue as in QLab. So it seems that QLab and Final Cut Pro 7 shares the same (bad) way to handle alpha. I have put in attachment the video from Final Cut Pro 7. Does it seems like something you can solve?

On 12 Aug 2013, at 13:03, micpool wrote:

I can confirm your findings.

The solution is to use a straight alpha not  premultipled  (presumably with black)

I don't know if this is a bug as such. There is a huge variation in how video editing and compositing programs handle alphas.

Your example of white text with a fade even plays differently in QuickTime Player and in the finder quick view.

With the premultiplied version you see the fade correctly

With the straight the text is visible at 100 percent through to the end of the fade

So it's worth knowing that straight alphas are best for Qlab but if you open clips encoded this way in quicktime player you might no see the true result.

Unless you put the alpha type in the filename it's pretty difficult to see which type it is as it's not reported in most get info panes.

Mic

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Source: https://groups.google.com/g/qlab/c/ppjGbNZ-fqc

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