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Independent RGB channel from the alpha issue
Hi,
I currently have a big issue regarding a project i have right now.
I did a keying of a person. Then, I applied a glow on that person via outer glow. Now, I need to export that video to a engine. But a problem came up.
I need the clean keying in the alpha chanel, and then on my rbg channel, I need my shot with the glow apply on it.
I tried many differenc way , but at the moment I replace the alpha channel of my composition with the clean one(with no glow), it cut’s it beacause it’s outside the mask.
Unfortunatly, for my engine, there is no work around, i need the export this way, so the caracter’s glow (which will be in additive mode) will actually transfer on the background of the scene.
Thanks for your help in advance
Frederik-Andre Savard
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22 Replies
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So if I’m getting you right you want to have an image (with Alpha) displayed differently than what is stored in the Alpha Channel. Sorry if I’m mistaking or if I got you wrong, but this is literally impossible. You’re trying to store two different variations of a channel in just one channel and then even have the image displayed with the first one but the software access the second one.
You could try and use openEXR if you bother… Also why can’t you just export a seperate matte and use that for your engine? Yeah I know that’s a stupid question…
I hope I didn’t disappoint you too much..
Max
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I don’t know what you mean by “engine”. Nor do I understand what you’re asking for.
If you export your video clip with the alpha channel of your keyed character, there is no way to have anything show outside of it. The alpha channel will cut everything off.
Also, there is no way to have blending modes (such as add) be in a rendered video. Once you render it, it’s all going to be the same.
You can render the video with transparency including the glow, but it won’t be blending with an additive blending mode to the footage below.
Now, if your “engine” has the ability to use blending modes, you could render out your person and the glow seperately.
– The Great Szalam
(The ‘Great’ stands for ‘Not So Great, in fact, Extremely Humble’)No trees were harmed in the creation of this message, but several thousand electrons were mildly inconvenienced.
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When you render out a clip with premultiplied alpha, it mixes the background color into the RGB data of partially transparent pixels. Transparency is stored across all the RGB and alpha channels.
When you render out a clip with straight alpha, transparency information is only stored in the alpha channel. The RGB data are not modified at all to store transparency.
Does that help to answer your question?
Walter Soyka
Principal & Designer at Keen Live
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By engine, i mean a game engine, like the unreal engine.
Once render, what goes pass the alpha channel will be in additive mode. That information will not be embedded in the video, i know it’s impossible. But i also know that in other compositing software, we can do what i want (like autodesk combustion), but my production pipeline does not let me use another software.
In photoshop, you can draw in you rgb channel, an then draw anything you want in that channel. They are simple channel, that it’s. So there must be a way to do the same thing in photoshop. In 3D compositing, it is one of the simpliest task to do. You map a channel, that’s all. So that’s why my question might sound wierd but it is suppose to be actually a very simple process.
And unfortunatly no, i can’t render a second pass for the glow. I already have a -not so great fps-, so adding another layer will simply kill it.
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Don’t worry, no question is stupid.
i Cant use a separated matte in my engine cause of my memory. My project need top optimisation. The process should be simple, in photoshop you can draw a alpha channel, and still have your photo in the rbg channel. But in after effect, he remove everything that pass my alpha. Every compositing software is suppose to do that, it’s simple, you say to the program, that layer is now the alpha (wich im able to do in after effect with channel selector), but i also want him to keep my rgb channel untouche. -
Unfortunatly no it dosent help,
when i render my keying with my glow, i do not have any backgroud. I do understand the principle of the premultiplied alpha.
I’m able to map my alpha channel with the good one. I just want after effect to leave rbg channel as it is.
Like the openEXR, you can store zdepth channel or matte id channel, and it dosen’t affect your rgb. i think to solve this problem we need to focus on the principle that works in photoshop.
You have your rbg channel, with a person that glow’s. but when you look in the alpha channel, you see that the alpha cut’s at the person, so it dosent include the glow. It is very possible to have this in a picture, like a targa 32 bits.
But now it’s to tell after effect the same thing. i know i could do a photoshop script, but once again in my production pipeline, the option is not a option.
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I’m not sure I fully understand what you need…
You have a keyed subject, you created a glow around the keyed subject and it sounds like you can render that out as rgb+alpha.
Is the issue that the areas of the fill (the rgb portion) where the alpha is zero are black?
If so, I can’t think of a way to have ae not render that way…. You could try to find a utility that would take two separate renders (rgb and alpha) and combine them as a single file without first applying the alpha to the fill.
If you could live with the alpha not being completely zero, you could use something like alpha levels to increase the alpha slightly for the keyed subject. Then render from ae as straight (unmatted). This should retain all the rgb values from the frame, but the alpha for the subject would not be totally transparent (though it would be nearly transparent).
Kevin Camp
Senior Designer
KCPQ, KMYQ & KRCW -
https://i.imgur.com/elbRo.jpg
https://i.imgur.com/K1nFc.jpgOkay you can see better what i want here,
The glow is in the rbg channel, but the alpha actually stops at the glow. It quite simple to do in pohoshop. But in after effect, at the moment that my alpha channel stop, nothing appers. So my glow is there yes, but dosent render because of the alpha. In composite of autodesk, you simply have to say to the program render the rgb channel apart from the alpha, So the program dosen’t take care of the alpha while rendering it, but still store the alpha channel in the image sequence.
Does that make sens or im still too vague ?
It seem quite complicated to explain but the problem is actually very simple.Thank in advance for your answer !
Frédérik-André Savard
VFX Artist – https://www.fasavard.comWhen you are not practicing, remember, someone somewhere is practicing, and when you meet him he will win – Ed Macauley
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https://i.imgur.com/elbRo.jpg
https://i.imgur.com/K1nFc.jpgOkay you can see better what i want here,
The glow is in the rbg channel, but the alpha actually stops at the glow. It quite simple to do in pohoshop. But in after effect, at the moment that my alpha channel stop, nothing appers. So my glow is there yes, but dosent render because of the alpha. In composite of autodesk, you simply have to say to the program render the rgb channel apart from the alpha, So the program dosen’t take care of the alpha while rendering it, but still store the alpha channel in the image sequence.
Does that make sens or im still too vague ?
It seem quite complicated to explain but the problem is actually very simple.Thank in advance for your answer !
Frédérik-André Savard
VFX Artist – https://www.fasavard.comWhen you are not practicing, remember, someone somewhere is practicing, and when you meet him he will win – Ed Macauley
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I see… ae should be rendering the alpha of the glow in the alpha of the composite.
What type of file/codec are you rendering? And are you choosing to render the alpha in the file (ei, rgba file) or separate rgb and alpha files?
One last thing are you choosing th render by file>export or composition>add to render queue?
Kevin Camp
Senior Designer
KCPQ, KMYQ & KRCW
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