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Help: Alpha Channels for Soft-edges or particle effects

Posted: Fri, 8th Nov 2002, 2:43pm

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moebius

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Here's a problem that completely baffles me. For the love of God, I can't seem to create decent alpha channels for animated images with soft-edges or where the object splinters into tiny particles. I'm left wondering how some of the plug-ins (e.g. Vampire Dust) were created.

For example, let's say I film an animated sequence against a greenscreen. I then import it into a compositing program e.g. AE or Commotion, and proceed to key out the green parts. Given that a lot of 'bleeding' occurs due to the translucency of the 'soft' edges, I'm forced to rely heavily on feathering so as not to create very 'hard' edges after the keying is done. I usually have to exploit the "spill killer" feature as well.

This usually creates a black halo around the remaining object, which completely ruins everything.

I've tried other plugger options like "Black Transparent" but the results are always unsatisfactory.

So my question is: how does one go about creating alpha channels for 'soft-edged' or particle effects without creating a black halo in the process?

I believe this is a huge problem facing plug-in makers that has to be addressed, along with other perplexing issues such as keeping filesizes small when dealing with 32-bit TIFF images.

Any help would be greatly appreciated! biggrin
Posted: Fri, 8th Nov 2002, 3:23pm

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b4uask30male

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I think some of the plugins such a smoke and fire were made via software, It sounds like you want to film something then change it a plugin.

I have used cinema 4D for smoke and it saves the file as a tiff with alpha, so all the small bits are kept in tack.

I do know what you mean, and i couldn't get round it, (eg: my clouds were realy good with fluff hanging off, but when i came to make the tiff, i lost the fluff cry and had the black rings around them ) cry
I would be interested to know this answer ( i'll offer a reward +1 )


if anyone finds my fluff can i have it back please cry
Posted: Fri, 8th Nov 2002, 3:56pm

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malone

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Ok, im not sure if this is actually your problem (like b4 says it might just be cause your having to greenscreen it), but I've seen people make this mistake often enough that I should point it out.

Its very easy to end up with a black halo effect round your image because you have effectively alpha'd it twice. Once when compositing it with your background color, and once when applying the real alpha channel too it.

Check out this example of just a simple red dot image



On the left ive got my nicely translucent red dot. This is how I want it to appear in AlamDV.

In the centre Ive put my translucent dot on a black background to make the plugin. Ive also created an alpha channel for this image based on how translucent the dot is.

Now the image on the right is what it will look like in AlamDV. Its got a nasty black border round it and doesn't look right at all. Its quite hard to explain why this happens; Go back to the middle picture and look at a pixel on the edge of the circle. The pixel is almost black so AlamDV knows it has to draw and almost black pixel on your video, AlamDV then checks the alpha channel and sees that it should be fairly transparent. So it ends up drawing a fairly transparent almost black pixel. This effect gives the transparent blackness all round the edge.


The proper way to do it is like this



On the left I've got the same translucent red dot.

In the centre Ive put my translucent red dot on the black background and made the alpha channel based on the transparency just like before. But because Ive got the transparency information now stored in my alpha channel I no longer need it on the red dot. So I've made all parts of the red dot completely opaque.

Now on the right it looks how I expect it too, and this is how it will look in alamdv.


If it helps think of it in 2 steps;
1) What color do you want the pixel to be? Forget about transparency, just think what color it should be. If the color is red make sure you have a fully red pixel on your image. If its yellow, make sure its fully yellow etc
2) Then decide how transparent you want it. If you want it semi-transparent set the alpha channel so its semi-transparent, but don't alter the color!

Take a look at the vampire dust images without the alpha information.


They look very strange, but they work because all the transparency info is in the alpha channel.


I hope that makes it a little clearer. Basically you want all the transparency information to be contained in the alpha channel not in the actual image.
Posted: Fri, 8th Nov 2002, 4:14pm

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moebius

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Thanks for the tute, Malone biggrin Problem is, the technique you outlined probably works best with stills or very short and simple animated sequences (given that you're probably using Photoshop or Paintshop Pro).

I think huge problems will still arise when dealing with 'objects' like frantically swirling clouds biggrin

I'll try to work it out when I have time smile
Posted: Fri, 8th Nov 2002, 4:47pm

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malone

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Im pretty sure it should work for all images. I just chose a simple one as an example. At the end of the day, plugins are just a sequence of stills anyway smile

Sure its a pain in the ass if you have to do 100 frames or more by hand. But the trick is to stop your greenscreening program from compositing the image with a black background in the first place. Ive never used AE or Commotion so I have no idea how you'd go about doing that.

Then again, prehaps its a completly different problem smile Could you post an example tiff so we could have a look.


And I meant to mention before about large tiffs. I find the best thing you can do is just crop all wasted space round the edge and make sure you use LZW compression. Other than that theres not really anyway to make them much smaller
Posted: Fri, 8th Nov 2002, 4:57pm

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sidewinder

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MECHA SEZ:

Or a really easy way to get the alpha channels w/o anykeying or hard mumbo-jumbo is to export you plugin as a TIF sequence directly from the program you're making it in - TIFs have built in alpha channels. TIFs are smart.


Look in the export setting and stuff for alpha channels - set em to RGB + Alpha or the closest thing to it. Sidewinder and I are doing a TON of alpha channels stuff with our next movie. Almost every scene in our movie uses alpha channels to some scale . . .
Posted: Tue, 10th Dec 2002, 8:12am

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billy3d

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does anyone know how to get lzw compression while exporting from AE??
billy
Posted: Tue, 10th Dec 2002, 10:07am

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moebius

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billy3d wrote:

does anyone know how to get lzw compression while exporting from AE??
billy
Hey Billy, if that isn't possible, maybe you could do the compression in Photoshop. I've created a custom action that I use to compress my images automatically. The cool thing about it is that I can batch process all the images automatically, so that I'm spared from clicking on the mouse for 1 hour lol
Posted: Sat, 21st Dec 2002, 7:35am

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billy3d

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i found out a way to do it in ae, thanks anyway, i hope my export as TGA from illusion and covert to tiff in ae methods working smoothly?
billy