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Hope you can help me with some issues I've been dealing these days.

As I'm rendering products, most of the time I need to render some transparent materials in order to do a post-pro in Photoshop, change the backgrounds, superpose images, etc.

I've been looking for an answer but yet I've couldn't get a solution for this problem.

I need to render materials with transparency and no background in Cycles.

I've found that there are two ways to do this:

  • Render with transparency in the materials.
  • Render and get two renders, the original and another one only with the alpha channel to edit in Photoshop.

For the first option, I've achieved to render a glass-like material, as PET plastic using this node tree:

enter image description here

For the second option, I haven't found a way to get the alpha channel in cycles that includes the object with transparency and, if possible, the shadows in the background| but without the background.

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  • $\begingroup$ You're up against some tricky situation. Blender is perfectly capable of rendering out transparency on the alpha channel, the viewer for the compositor is not capable of displaying it. You don't state what format you are using to save the image, but if you are using PNGs you might not be able to do what you want. You would need to export the image as EXR (with proper associated alpha). Read this related port: blender.stackexchange.com/questions/56950/… $\endgroup$ – cegaton Apr 10 '17 at 16:45
  • $\begingroup$ A different option is to do the compositing directly in blender. $\endgroup$ – cegaton Apr 10 '17 at 16:46
  • $\begingroup$ Thanks Cegaton, it was really useful, but I'm still not getting the results I need, the composition has to be in Ph and I need to put the same render over various different backgrounds (thats why in Ph). Also I need to get the render with transparent shadows but with transparent materials too, in order to have a good composition. Thats why I don't know which is the better way and neither how to do it. $\endgroup$ – Luciano Ungarelli Apr 10 '17 at 19:52
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Newer builds of Cycles (post 2.79, currently available from the buildbot) include a native functionality for this:

example of glass suzanne over alpha channel

This is activated by enabling the new "transparent glass" option in the "Film" panel. The normal "transparent" option must be activated as well, or "transparent glass" will be greyed out. There is also a "roughness threshold" control, which allows you to have only clear glass transparent, with frosted glass blocking alpha. Alpha transparency will only be enabled for glass with roughness below the set value. Above a certain roughness, you're usually better off getting blurred refractions from whatever environment/HDRI you're using for reflections. The default setting of 0.1 is pretty decent cutoff point to start with.

transparent glass checkbox location in the "film" panel

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  • $\begingroup$ hey that's cool! just what I need : blender-2.79-e0a436f-win64.zip $\endgroup$ – Blender Sushi Guy May 11 '18 at 0:22
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Sounds like what you need is a Shadow Catcher material, unfortunately that doesn't exist in Blender at the moment, as of version 2.78.

It is currently being developed and may be ready for release for the next 2.79 version.

Until then you may risk playing around with it with a Development unstable build from Graphicall.org

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  • $\begingroup$ Thank you Duarte Farrajota Ramos, hope this new release come soon then. For the moment I've found that you can generate a Shadow Catcher Material by moving the element to another layer with a Shadow Only Property, yet I can't still do what I need as the shadows only appear to extract with the alpha channel if they are generated with lamps (I use emissive objects for the render that Blender doesn't recognise for the shadows extracted with the alpha channel). I've seen that there's another way, by using a Green Screen, and extracting this in Ph, but I couldn't reach good results yet. $\endgroup$ – Luciano Ungarelli Apr 11 '17 at 13:45

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