Alpha's and Lens flares

[Johnno1972]

Is there a way to render lens flares with an Alpha channel? Or do I have to composit in Layout to render the lens flares into the final comp?

I'd rather have all my layers in After FX.

[BeeVee]

In Image Filters add Flare2Alpha

B

[spiroz]

You might as well want a straight alpha when compositing
(instead of the default premultiplied) so check the 'fader alpha mode'...

[Johnno1972]

Thanks guys. I used the 'Effects/Processing Menu' and added the Alpha2Flare filter.

Since I already render the sequence out over night, what does the 'fader alpha mode' do? Does it improve the quality because it's ended up darker than the first render that had the elements rendered in one pass (I hope you understand that).

The reason I'm compositing is that all the elements haven't been done as I had to re-render the entire sequence when one object had to be replaced!! So I desided to do seperate layers this time.

[spiroz]

for some clarification of the fader alpha mode
http://forums.cgsociety.org/archive/index.php/t-108101.html

[Johnno1972]

I did a test render and the flare is way better with 'Fader Alpha Mode' checked! Thank's for that tip, I'll set it up for another render pass, luckly it won't take too long to render the flare only.

[NanoGator]

I'd like to offer an alternative method. It requires a few more steps, but it saves having to save the Alpha buffer. It's also a little more accurate. Here's how to do it in Photoshop:


Save your image as normal without any image buffers etc. The alpha channel should only represent solid objects. You'll need to do this in two seperate layers. (Note: Because of Photoshop's handling of Alpha channels, you may have to literally save two seperate images. Sorry about that.)
Put your Alpha layer right above the BG Layer.
Invert the Alpha Layer so that 'solid' is black and transparent is white.
Set the transparency mode for that layer to Multiply. Basically what you're doing here is your cutting out the shape of the mesh from the BG Layer.
Take the RGB layer of your image, and place it in a layer above the Alpha layer.
Set the transparency mode to Screen.
Look at the pretty image you just created. :)


So what did we do here? In the simplest terms, we additively* projected these two images together, then used the alpha channel to cut the 'solid' parts of the image out of the background. Since a lens flare isn't 'solid', it is projected onto the BG the same way any other lighting pass is.

Of course, use whatever works best for you. I just found this little exercise to be quite helpful in understanding how compositing works. It opened a lot of doors for me, so I thought I'd share it. :)


*Although the effect of using Screen Transparency is similar to Additive, mathematically Screen and Additive aren't quite the same.

[SCS5]

I use post_Alpha2 all the time. Look for it on Flay. It composite all the alpha data in the render. Both Pre & Post processing effects..Works GREAT!! Better than any of the solutions that come with Lightwave.

[arrow1234]

In Image Filters add Flare2Alpha

B

ok this is exactly what i need, soup is ON !!!!

thank you soo much :P

ps. err...it may not be a good thing to post a reply because this is like a 3 yrs old thread :3