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I am rendering out some bubbles for use in a composite Photoshop image, so I need to render as a transparent png with the bubbles semi-transparent.

I tried setting render output to RGBA and checking the transparent option under film, but while the resulting png is transparent the bubbles keep the environment background colour.

The bubbles are definitely semi-transparent, because if I include a coloured object in the scene behind them, it is visible through the bubbles in the render, but I can't find a way to allow the semi transparent material to have any alpha transparency in my png render.

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You can use a Holdout shader:

enter image description here

The holdout shader will render as actual image alpha, as opposed to giving the color of the environment on the other side of the object.

Or you could use a Transparent shader, which will allow other objects to be visible through the transparent object:

enter image description here

Note that transparent in Render settings > Film must be enabled, otherwise the holdout shader will appear black and the transparent shader will show the environment behind it. You did mention that you already did this, so you should be good.

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  • $\begingroup$ Great! thanks for the holdout shader tip. I got as far as realising that the Glass shader does not allow for alpha, but I was having trouble finding a decent alternative $\endgroup$ – Finglish Jun 10 '14 at 5:33
  • $\begingroup$ is using the holdout shader going to correct the semi-transparency problem in cycles viewport? $\endgroup$ – Pow Jun 11 '16 at 21:39
  • $\begingroup$ @Pow Not sure what you mean? What semi-transparency problem is there in the cycles viewport? $\endgroup$ – gandalf3 Jun 11 '16 at 22:07
  • $\begingroup$ i asked it here, if you do know a work around please share blender.stackexchange.com/questions/55692/… $\endgroup$ – Pow Jun 11 '16 at 22:22
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    $\begingroup$ Keep in mind that PNG uses unassociated (straight) alpha which does not cover the subtle transparency effects associated (premultiplied) allows. A clear glass or a bubbly reflect the environment with little to no occlussion of the background, therefor unassociated alpha won't produce a realistic effect. Considered the method described above as a hack: the holdout portion of the shader will occlude the light from the background layer, which is something that a bubble or a glass won't do in reality. $\endgroup$ – Gez Feb 22 at 21:22

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