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Thread: Need Help: Matte objects behind transparent objects. Why is alpha channel messed up?

  1. #1
    AeroAnimator 4dartist's Avatar
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    Need Help: Matte objects behind transparent objects. Why is alpha channel messed up?

    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..
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    Last edited by 4dartist; 02-01-2019 at 02:33 PM.

  2. #2
    AeroAnimator 4dartist's Avatar
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    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.
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    Last edited by 4dartist; 02-02-2019 at 02:32 PM.

  3. #3
    Electron wrangler jwiede's Avatar
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    Quote Originally Posted by 4dartist View Post
    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).
    John W.
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  4. #4
    AeroAnimator 4dartist's Avatar
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    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!

  5. #5
    Not so newbie member lardbros's Avatar
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    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?
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  6. #6
    AeroAnimator 4dartist's Avatar
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    Quote Originally Posted by lardbros View Post
    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.

  7. #7
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    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
    Last edited by MonroePoteet; 04-16-2019 at 08:53 AM.

  8. #8
    Curmudgeon in Training Ma3rk's Avatar
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    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.
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  9. #9
    AeroAnimator 4dartist's Avatar
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    Quote Originally Posted by Ma3rk View Post
    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

  10. #10
    AeroAnimator 4dartist's Avatar
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    Quote Originally Posted by Ma3rk View Post
    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.
    Last edited by 4dartist; 04-17-2019 at 04:19 PM.

  11. #11
    I've been using Alpha channel renders for years to create renders of models on transparency to use in After Effects and never had a problem in LW 2015. A couple of years ago I upgraded to LW 2018. This week I needed to render a model with an Alpha transparency, and it will not work anymore in After Effects. The Alpha channel render looks correct, with a white model on a black background, but it will not work in After Effects. I can import renders that did before in LW 2015, and they still work fine in After Effects. Has something changed since LW 2015 regarding how the Alpha channel is rendered?

  12. #12
    Frequenter
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    hard to tell... Colourspace maybe? can you confirm it is 8 bit of depth? When you check with a colour picker, what are the values you get?
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  13. #13
    Quote Originally Posted by fishhead View Post
    hard to tell... Colourspace maybe? can you confirm it is 8 bit of depth? When you check with a colour picker, what are the values you get?
    Thank you for your response, Lorenz. The Alpha render has RGB values of 0,0,0 for the black and 255,255,255 for the white over the entire frame. I checked it in Photoshop.

    I've tried rendering in .jpg, .bmp, .png and .rla, and none of them work as an Alpha in After Effects. In LW 2015, I could use any of those formats for an Alpha.

  14. #14
    Not so newbie member lardbros's Avatar
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    You need to render as a 32-bit PNG in order for it to render with an alpha. JPEG's have never rendered with an alpha, unless it's some specialised bespoke version of JPEG. In LW though, they never have.
    If you're checking the render using the image viewer after a render, this is not the format that you have just exported. It is the final render buffer straight out of LW, with all the HDR goodness and floating point values, including the alpha channel.

    Just make sure your output file format is a 32-bit file and you should get an alpha channel that you can use in AE.
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