Alpha Channel from Particles

[Caroline]

I'm trying to get my fire with a flickering light - I've looked through the forum and found that if I render the fire to a tga alpha channel I could apply it as a gel to a klieg light ( )

 

However, I've tried various ways - the simplest is rendering a ball with the flame material as described in TaoAM Chapter 16. I get the alpha channel for the ball, but the flames do not show in the alpha channel.

 

Am I doing something wrong, or are particles not supposed to show up in the alpha channel?

 

I was reading this thread on Compositing Particles, and, while it is all a bit too hard for me, it does seem to imply that you can get an alpha channel from particles.

 

I tried an EXR sequence with the ball, but the flames only appeared on top of where the ball is - all of the flames that come out of the ball were not there.

 

(I'm sure that there are other ways I could simulate a flickering light, but I did want to see what could be done with alpha channels too.)

[youngman]

Hi Caroline,

Would you be able to attach a screen dump of your pws or attach your project file.I have been messing around with smoke a lot for my re-do of excersise 6 and might be able to figure it out,if not I'm sure someone else will be able to.

 

Jay

[Caroline]

Project attached I hope. It's a very simple project just to work out how to do this, just a sphere with the flame material on it. The sphere renders to the alpha channel, but no flame.

 

Thanks for looking, Jay.

Project1.prj

[rusty]

Caroline, Jay --

 

I'll assume that you are using the lastest version of V13.0.

 

If you find a way to get alpha channels and particles to work can you please share this with me? I have not tried this in a while however, to my knowledge this has never worked. If this has changed or there is a work around I'd sure like to know.

 

Thanks,

Rusty

[youngman]

Hi Caroline/Rusty

Well I'm stumped a little on this one!

I think it might have to do with the fire image used,the reason for this is if you look on your Hash cd you will see under the project file folder in the sprites section a project called smoke 2.

 

 

See above

 

When i rendered a frame of this out i was able to composite the image to a stock photo with no problems.

 

 

See above

 

I know this is of smoke particles but if they are substituted for a flame image then it should work?

Hopefully someone more experienced in this area can come to our aid on this and restore some of my grey hair.....he he.

 

Jay

 

 

P.s.Yes i am using the current version of A:m v13 o

 

 

P.p.s.Have a look at this thread and download the project file,i think this might work.

 

http://www.hash.com/forums/index.php?showtopic=19672

 

Here's a quick composite i done from it.

 

[rusty]

The smoke looks good.

 

Now that I think about it, fire (sprite fire/explosions, uses almost same images as smoke) has see through qualities that play hell with trying to isolate them with alpha channels. In both smoke and fire use black as the final color before the fade and this causes black outlines when alpha'd against a light color. Probably you need to create fire/explosions in a slightly different way to work well with alpha. But again, I have not played with this in over a year -- the workaround was render against black and turn the black into alpha with some soft edges to smooth the transition.

 

More recently, trying to use an alpha channel with lens flares was a big problem -- this did not work at all. There was no workaround I could find except to put those lens flares in front of... where you wanted them to be in front of.

 

Rusty

[Caroline]

Good find, Jay - when I take that model and put it into the chor next to my model, it renders the alpha perfectly on that model (not mine). So it should work. The only difference I can see so far is compared to my model, that smoke model is really really tiny. It still works when I resize it though.

 

Screen capture of alpha channel:

 

Mine is on the right and does not show the particles, only the emitter. On the left the emitter is transparent and shows particles. (N.B. It doesn't work either when I make my emitter transparent)

 

I don't think it's the tga, as when I changed the model to use that tga, it still didn't work.

 

I shall experiment more later....

 

(Yes, I'm using 13.)

[johnl3d]

In 2005 the tga files for this fire sequence were made available not sure if they are still around I know I have a copy.

 

http://www.colins-loft.net/free/fire%20example.mov

 

I know they had an alpha channel

[johnl3d]

I started a project turned off everything in options except sprites made sure alpha buffer on then rendered a tga sequence of the explosion material on the cd from 0-30 using a small black model

 

I then used the used the sequence of alpha tgas as a rotoscope on 2 kleig lights and a sprite sequence in another chor and got this

 

expl.mov

 

project file ..sprites on cd ..then after rendering from first chor (adjust as per above) you get the image sequence for chor2

alpht.zip

 

 

alpha sequence

explalph30.zip

 

 

I'm probably missing something but I tried

[Caroline]

EUREKA (I think).

 

I'm doing Exercise 16. I took the three tgas from the tutorial, as specified, and in the emitter Color Method, I turned the Additive Color to OFF instead of ON.

 

I also had to put an alpha channel in the tutorial tgas (thank you John), so that I didn't get the black surround on the particles.

 

I now get an alpha channel when rendering. (However, the renders aren't nearly as nice as with the Additive Color ON, but I'm still working on that....)

[Caroline]

And looking at John's very nice explosion, 2 of the emitters had OFF and 2 had ON, and so you only get the alpha channel rendered for the ones where the emitter is OFF. I checked, and turned the other 2 off and got the full alpha channel rendered.

 

Is this the way it's supposed to work?

[Bruce Del Porte]

Hey Caroline,

 

I think you are on the right track. A couple of tips:

 

1. For your gel, move the emitter out of frame and let the sprites move from bottom to top.

 

2. Reduce the Number to reduce the density. Generally what makes good fire is too dense to be a good gel. Really all you are trying to do is make shadows for the kleig, the gel doesn't have to look like fire.

 

3. I generally render a gel to a 32 bit .mov using the Animation codec although tgas should work fine. Example

 

B

[johnl3d]

Caroline

 

If it works its correct..I didn't check for the additive colors just used the material as per the one on the cd...will have to remember that the additive should be off

 

 

thanks

 

Johnl

[Caroline]

Hooray - I got a moving klieg light gel! The tip there is to add it to the shortcut in the choreography, not, as I discovered after a lo-ong time, the object.

 

It doesn't look much like I thought it would yet, but I'm now going to follow Bruce's tips.

 

(Ungrateful person that I am, Bruce - I 'finished' my fire last night having given up on gel lights, I went for animated intensity. Then I saw your movie, and thought Bother - I'm going to have to redo mine. What a brilliant movie!)

[zandoriastudios]

If you are just making a light gel, then your sequence doesn't need an alpha channel--since the colors define the color and intensity of the light, black just means zero light.

So if you just render the flames against a black background, you will have what you are wanting for a light gel.

[Caroline]

You're right, William, I have a knack for complicating things.

 

I was going to put flickering fire on the back burner as I don't think I'm ready for the whole light thing yet, but today's a new day. We'll see what it brings.

 

(Did you know that if you look at your blue Q in your sig, then look at a white part of the screen, you get red Qs!?)

[youngman]

I would like to say a big thank you to caroline for asking for some help on her problem regarding alpha channels.Although i couldn't figure it out (Big thanks to John) i must say i have never had so much fun messing around on this.

 

The picture below was a quick composite after finding out about turning the additive colour off for alpha rendering.

 

 

I then done a quick 3 second render of the fire.The one on the left is additive colour off,the one on the right is additive colour on.

It seems that the way to to a decent fire for alpha rendering is to have good quality decals (I need to do some work on mine) but that something for later.

 

fire_render_2_versions.mov

 

Here are the decals i used,very basic.

 

 

Big thanks again to Caroline and John.

:D :D :D :D :D

 

Jay

[rusty]

Cool Beans! This has totally made my day! I'm DEEP into testing hair rendering right now but this item was just a few more items down my list!

 

Thanks!

Rusty

 

PS Now can someone please do this for lens flares (I think/hope volumetric lights and alphas work... don't remember... 4 down on my list)? :-)

[Caroline]

That's a nice fire, Jay.

 

Thinking aloud here - do you think that coloured decals are necessary rather than greyscale? You could have several greyscale ones emitting different colours?

 

I'll put that on the lengthening list to try out.

 

Lens Flare? I haven't come across that being used - is it a good feature? And in what way doesn't it seem to work?

[johnl3d]

I think Rusty is looking for the flares with Alpha channels

to composed into his projects

 

 

http://www.virtualmediastudios.com/index.html