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View Full Version : Need Help: Matte objects behind transparent objects. Why is alpha channel messed up?



4dartist
02-01-2019, 02:16 PM
In LW 2019, if you load a sphere, 100% transparent and render it. You get an alpha that is all black. (good)

If you take another sphere, place it behind your transparent object. Maybe slide it over so only it's only behind half of the transparent object. Set the back object's properties to have Matte Object ticked on, and black. Then set Alpha Channel to Constant Value, 0%. You would think, with the same transparent sphere you'd still get a black alpha. I mean all you changed was to add an alpha 0 matte object behind it.

What am I doing wrong.. :(

Render output is set to 32bit png. Tried straight matte and pre-multiplied.

In my picture, I have a little shockwave, when it goes in front of objects set to matte black/alpha 0, the alpha channel gets all screwed up.

Edit: I loaded up LW 2015, and it works as intended. If you place a 100% transparent object in front of a matte black alpha set to black object, you get an alpha black alpha, no weird white where the object is. There must be something in 2018 / 2019 that I'm missing or it's a bug. I'm going to keep hunting..

4dartist
02-02-2019, 02:16 PM
I'm going to work on a bug report. I couldn't figure this out. This'll be unfortunate when trying to render cars and such as a separate layer with the windows being in front of roads/buildings set to matte black and alpha black. I can't help but feel I'm missing something but for the life of me I can't figure it.

I've attached a scene/object file that shows what I'm talking about and what I submitted with the report.

jwiede
02-10-2019, 02:35 PM
I'm going to work on a bug report. I couldn't figure this out. This'll be unfortunate when trying to render cars and such as a separate layer with the windows being in front of roads/buildings set to matte black and alpha black. I can't help but feel I'm missing something but for the life of me I can't figure it.

I've attached a scene/object file that shows what I'm talking about and what I submitted with the report.

Appears they didn't verify all the edge cases while fixing LWB-4569 (pBSDF transparency yields incorrect alpha buffer rendered output).

4dartist
04-15-2019, 07:05 PM
Well we are on LW 2019.0.3 and this bug still exists.. I have to render a car driving down a road, and now my car layer with everything behind it as matte black/alpha black is jacked up. The windows need to be transparent in the render layer. As of now, they render white in the alpha. This is killing me!

lardbros
04-16-2019, 06:26 AM
Yes, this seems to be an issue.

What are you needing from this? An alpha of the transparency that is correct?

Could you plug your transparency output for the 'blast wave' into a 'custom buffer' and output that instead, and then using it as your alpha channel in comp?

It's a workaround, but may work? :)

4dartist
04-16-2019, 08:36 AM
Yes, this seems to be an issue.

What are you needing from this? An alpha of the transparency that is correct?

Could you plug your transparency output for the 'blast wave' into a 'custom buffer' and output that instead, and then using it as your alpha channel in comp?

It's a workaround, but may work? :)

Well I made a work around for the blast wave effect by just using masks and such. This new project with the car is screwing me up so bad. I need the car on it's own layer because I do some fx on it in After Effects where I do some things to highlight the car and need it on it's own layer. But I cannot for the life of me figure out how to render it. It's on a terrain that's all bumpy and the wheels don't ride on it perfecty. I really need the terrain to just be matte black / alpha 0...

I updated the bug report to say that it's still an issue on the latest version, and indeed the ticked was still open. I'm going to spend today figuring out how to work around this. I tried to do a transparency render buffer, but it's got the same issue. We'll see if I can find something.

MonroePoteet
04-16-2019, 08:44 AM
I don't know if it'll help, but even in real cinematography windows / visors / windscreens are problematic due to unwanted reflections of the camera / film crew or refraction issues. Very regularly the glass, reflections, dirt/smudges, etc. are composited in later.

You might try putting green-screen type windows in the car which are matte and opaque, 100% luminous and 0% diffuse, and then do a separate rendering of the windows (reflection, refraction) and the interior of the car you need with the same camera motion and composite them in post.

Just a suggestion. I'm not suggesting the refraction issues of transparent surfaces in LW shouldn't be fixed / better, but as a workaround, it might work for you.

mTp

Ma3rk
04-16-2019, 04:45 PM
I took a look at your scene 4dartist.

You simply didn't have the alpha channel enabled in the Render option Buffers. A common Gotcha when they decided to scatter related settings all over the place.

4dartist
04-16-2019, 04:55 PM
I took a look at your scene 4dartist.

You simply didn't have the alpha channel enabled in the Render option Buffers. A common Gotcha when they decided to scatter related settings all over the place.

I really hope that's all it is. I'm going test it out shortly and see if I can find what you're talking about. Thanks for taking a look at it!! That's awesome.

Ben

4dartist
04-17-2019, 04:09 PM
I took a look at your scene 4dartist.

You simply didn't have the alpha channel enabled in the Render option Buffers. A common Gotcha when they decided to scatter related settings all over the place.

I can't seem to get it to work. Can you elaborate more? I tried turning on the Alpha in the Buffers tab, I tried making the buffers 32bit in the Output tab, I keep trying anything and everything, but I cannot get the alpha channel to be all black. I'm getting desperate. Please if you have some time tonight I'd really really appreciate some help.