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I have rendered an object using hdri and lamp lights. Using render layers i want to have object and shadows only. I show you my nodes. I need to know how to transform white pixels to alpha channel using nodes. enter image description here

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  • $\begingroup$ No it is not. I want to know how to do it using render layers in order to have an alpha channel. Just i need to transform white pixel to alpha. $\endgroup$
    – jagoji3d
    Apr 30 '15 at 18:57
  • $\begingroup$ Jerryno, I need a pgn image in order to use it as image web site. Just, the object and shadow only. $\endgroup$
    – jagoji3d
    Apr 30 '15 at 19:01
  • $\begingroup$ You cannot transform white to alpha, you can transform black pixels to alpha but that won't give you shadow. You can transform white pixels for CMYK mode into alpha. $\endgroup$ Apr 30 '15 at 19:01
  • $\begingroup$ related:blender.stackexchange.com/questions/5158/… $\endgroup$
    – user1853
    May 1 '15 at 2:48
  • $\begingroup$ @jagoji3d That question has the answers you want, you just need to output the transparent image and not add it onto the background image. $\endgroup$
    – VRM
    May 1 '15 at 12:44
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One solution would be to enable a shadow pass, invert the colors and use that as alpha using a Set Alpha node.

enter image description here

The Dilate/Erode will help with the antialiasing of the shadow pass.

EDIT

Now... if you hate the previous option and absolutely want to use the layers you have set up, you can try this:

From the background layer you can indeed subtract the shadow layer, creating a black and white image that holds the shadow information only. But use difference instead of subtract (also, you don't need the invert node because you can invert the image with the curves node, but you do need to get rid of the color information). Next, combine that image with the alpha channel of the sphere, and use the resulting image as alpha channel on the mix of the shadow and the sphere...

enter image description here

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