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DPX and Alpha Channel???

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melissa kern
DPX and Alpha Channel???
on Jul 28, 2010 at 2:44:36 am

HI.
one more thing. question. I am outputting a title sequence for a film.. and they requested a DPX Seq.
when I choose DPX Seq.. the option for RGB + Alpha is grayed out?
so how do I out put a DPX Seq with an Alpha Channel from AE CS5???

ok. thanks again..
m


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Matt Silverman
Re: DPX and Alpha Channel???
on Jul 28, 2010 at 4:12:13 am

You can't. Render the matte as a second DPX pass.



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melissa kern
Re: DPX and Alpha Channel???
on Jul 28, 2010 at 4:04:15 pm

hi.
well. first- thank you for the help.
just to be clear-I'm a bit new to compositing for film- I should make a Quicktime .mov with RGB+Alpha.. and then do a "second pass" as a DPX Sequence. correct?
cool
m



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Walter Soyka
Re: DPX and Alpha Channel???
on Jul 28, 2010 at 4:43:12 pm

[melissa kern] "I should make a Quicktime .mov with RGB+Alpha.. and then do a "second pass" as a DPX Sequence. correct?"

Not quite. You'll render two DPX sequences: one with RGB channels selected in the output module, and the other with Alpha channel selected in the output module.

Walter Soyka
Principal & Designer at Keen Live
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
RenderBreak Blog - What I'm thinking when my workstation's thinking
Creative Cow Forum Host: Live & Stage Events


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melissa kern
Re: DPX and Alpha Channel???
on Jul 28, 2010 at 8:11:21 pm

Hi again..
so 2 strange things occured..or at least I find them strange.

the Alpha Channel Sequence is shorter than the RGB. though the render que says its done.. and when I check the output module- it says the correct length. about 30 sec shorter.
and finally, when I import the DPX Sequence with Alpha channel- the screen in completely white?
-not sure if this okay.. both seem odd to me.



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Walter Soyka
Re: DPX and Alpha Channel???
on Jul 28, 2010 at 9:21:39 pm

[melissa kern] "the Alpha Channel Sequence is shorter than the RGB. though the render que says its done.. and when I check the output module- it says the correct length. about 30 sec shorter."

Sorry, but I don't quite understand what you're saying here. Does the alpha sequence contains fewer frames on disk than the RGB sequence?

[melissa kern] "and finally, when I import the DPX Sequence with Alpha channel- the screen in completely white?"

It sounds like your sequence isn't actually transparent -- perhaps you have a fully opaque solid or a piece of footage in your comp that fills the frame. If you click the Toggle Transparency Grid button under your comp viewer, do you see a checkerboard pattern?

Walter Soyka
Principal & Designer at Keen Live
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
RenderBreak Blog - What I'm thinking when my workstation's thinking
Creative Cow Forum Host: Live & Stage Events


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melissa kern
Re: DPX and Alpha Channel???
on Jul 28, 2010 at 9:37:14 pm

I made first the RGB sequence.. and then imported back into AE. from that sequence I attempted to make theAlpha sequence. and yes- when it was all done it had at least 700 less frames and the screen image was completely white- I tried pressing the checkerboard icon= and nothing happened.
so. I went back to the original comp(with all the multiple layers)...and made another DPX Sequence with only Alpha selected. this- when it was done- had the right amount of frames but then the screen was completely black= again clicking the checkerboard icon did nothing.
I'm now rendering from a pre-comp- only Alpha.. and I made doubly sure to have the checker board icon
clicked before rendering- though I don't believe AE renders the bg color ever-I just wanted to make sure.
sorry, that I am such a dunce about all this. I usually just output RGB+Alpha to a Quicktime. I really have no idea what I am doing..but this is what the studio asked for. I'm not even sure if premultiplied is the right setting????
anyway, thanks so much for your help. its really amazing that you do this. I really appreciate it.
m



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melissa kern
Re: DPX and Alpha Channel???
on Jul 28, 2010 at 9:51:30 pm

okay. so the last render came out black as well. I was using no color management. this time I am going to try using Adobe RGB Linearize.. should that make a difference?



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Walter Soyka
Re: DPX and Alpha Channel???
on Jul 28, 2010 at 10:03:21 pm

Just to be clear on what you will output, you'll have a sequence with RGB color data, and a grayscale sequence showing the alpha channel. Black areas are fully transparent, white areas are fully opaque, and gray areas are partially transparent.

[melissa kern] "I made first the RGB sequence.. and then imported back into AE. from that sequence I attempted to make theAlpha sequence."

You have to make the alpha render pass from the same comp that you used for the RGB render. In the render queue, just duplicate the RGB render and change it to alpha.

The RGB sequence that you re-imported has no transparency (because it's RGB, no A), so when you tried to render alpha from it, you got all white (fully opaque).

[melissa kern] "I went back to the original comp(with all the multiple layers)...and made another DPX Sequence with only Alpha selected. this- when it was done- had the right amount of frames but then the screen was completely black="

Is it black all the way through the entire sequence? If your comp starts as fully transparent, the alpha sequence will start as completely black, but once the action begins, you should see some grays and whites in the alpha sequence.

[melissa kern] "I'm not even sure if premultiplied is the right setting????"

See this quick explanation on straight and premultiplied alpha:

http://podcasts.creativecow.net/multimedia-101-podcast/what-is-a-straight-r...

In my work, I usually render with straight color unless I am asked to do otherwise.

[melissa kern] "anyway, thanks so much for your help. its really amazing that you do this. I really appreciate it."

You're welcome! I hope this is helpful.

Walter Soyka
Principal & Designer at Keen Live
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
RenderBreak Blog - What I'm thinking when my workstation's thinking
Creative Cow Forum Host: Live & Stage Events


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melissa kern
Re: DPX and Alpha Channel???
on Jul 28, 2010 at 10:22:22 pm

wow. that really helps! I'll listen to the podcast as well.
what I am rendering is a title sequence- just white lettering that goes over the film image-except for a few instances- where it appears over black. when I said it was all black- I meant except for the white lettering.
I expected-as is the case with RGB+Alpha Quicktime .mov- that I would see the transparent as transparent. which I guess in this case- I would not. and now I am rendering from the same comp as the original RGB render so all should be well. I am rendering as premultiplied. hopefully, that is correct.
I'll go listen to the podcast.
awesome. super super awesome.
thanks
m



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Walter Soyka
Re: DPX and Alpha Channel???
on Jul 28, 2010 at 10:45:01 pm

[melissa kern] " when I said it was all black- I meant except for the white lettering."

That sounds like it's working, then. Your alpha output should be a grayscale sequence, just like an After Effects track matte. Aharon Rabinowitz's What Is An Alpha Channel might be useful for you, too.

Your client will use the alpha (or "key") sequence to knock out the transparent areas from the RGB (or "fill") sequence when they composite it over the film.

Walter Soyka
Principal & Designer at Keen Live
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
RenderBreak Blog - What I'm thinking when my workstation's thinking
Creative Cow Forum Host: Live & Stage Events


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