Cycles Smoke Simulator Will Not Let Me Use Transparent Background

When using the Cycles rendering engine to render out smoke and fire with 2.71 I cannot get a transparent background:

(yes, I did enable “transparent” in the film tab, and tried disabling “use environment” in the render layers)

transparent view(during and after rendering):

transparent view off(during and after rendering):

I’m assuming this is a bug, but I thought I’d post the problem here before reporting it.

See the little image type icon in the bottom right? Make them the same ie. choose the Image + alpha, same as the top image.

Oh, sorry if it was unclear, both the images are rendered with “transparent” checked. they are the same image but with different image views selected.

Make any progress on this? I can’t get a transparent background on smoke/fire either. Can’t composite the renders into anything without it so whats the point.

Seems to work fine here:


Perhaps you could upload a simplified scene file that still shows the problem to pasteall so that we can check your settings?

Some volumetric effects in cycles appear to be completely transparent with a transparent background, but it seems to be just a display artifact. If you overlay it on a solid background you should be able to see it again.

You have to composite it over something using alpha over. It looks like something is wrong, but it’s not. If you want to composite it later you have to save your images as OpenEXR. It was reported by me and some other people, and this is what Sergey wrote: “It’s not bug with cycles, it’s a specific of visualizing premultiplied image in viewport and image editor. Viewport is like compositing the render result on the checker, image editor converts image to straight alpha first (because ogl is set up in a way it expects straight alpha in blender atm).”
“In order to make it look in more expected way you actually need to alpha-over it on some background.”

Yeah that’s what I was trying to say. :stuck_out_tongue:

i’ll have to look up what the differences between premultiplied and straight alpha are… and how to use openEXR lol.just been going with .png so far. does the combined pass hold enough info to composite this shot later with openEXR. I don’t see anything specific to volumes in the render passes. i’ll play around with it.

thanks!

Attachments


Yes, combined pass is enough.

would you happen to know of an easy way to output to png files. the end result would be the same as rendering out png with a black background and then taking the image into gimp and doing the convert-color-to-alpha filter on black, which actually works really well.

compositing with alpha over black with the exr image did show the flames. can I just…have the flames and the alpha and not the black.

my after effects guy doesn’t even want to mess with exr images right now cause hes a noob to lol.

I just started with a new scene and I can see the smoke in the .exr without using any alpha-over compositing now. I think there was something messed up before. just wasn’t acting right.

Can you please post your .blend file. I can’t solve this problem either.
Thanx a lot :slight_smile:

“It’s not bug with cycles, it’s a specific of visualizing premultiplied image in viewport and image editor. Viewport is like compositing the render result on the checker, image editor converts image to straight alpha first (because ogl is set up in a way it expects straight alpha in blender atm).” :slight_smile:

Yes I can get the Flame and Smoke to show in the Video Editor by using OpenEXR and setting the Alpha Mode to Premultiply but have you tried to use any of the Video Editor Fade In and Out on these images. They work POORLY at best. I’m sure there are a number of workarounds on this issue as well. Why not just correct Cycles to work with PNG files?

TC

work around for fading an alpha is to mix between comp and non-comp versions of a shot.

3pointEdit: What?

TC

Guys, I know it’s an old thread, but since I’ve found that rendering fire in cycles with transparent background is still problematic (if not impossible), I thought I’d post a simple compositing workaround here. Maybe It’ll help somebody.

The trick is to render fire with a 100% black background (I like to use a black plane behind it, so other render layers can still be rendered on transparent BG if I’m rendering to OEXR). Then in compositor use the nodes as shown below. The left picture shows rendered image (fire on black BG). Middle one is the final effect (after “Set alpha” node, the last in the green frame). To check if the result is correct, I the used alpha over black (last visible node in the tree on the right), and the right image is the effect. As it looks quite the same as the pure render, I assume that using nodes in the green frame gives the same effects as rendering fire on transparent BG if it was giving proper results.