Shadow on transparent plane?

Hello. I need to bake a shadow on a transparent plane. But Cycles doesn’t bake transparency. So I guess I need to bake an alpha mask or map in Cycles, so I can bring it into Gimp to get my plane with transparent shadow? How do I do that? The compositor in Blender is a bit alien to me.

Or is there a better or easier way?

Thank you.

Bake a greyscale image and then use Multiply blend mode in Gimp?

Thanks. Could you describe how to do that?

In Blender, render your “shadow” onto a white background. Then in Gimp Multiply that layer where you want the shadow.
See gimp layer modes here… search for “Multiply”.
Edit clarity: The core principle here is that provided you use a Multiply layer, alpha doesn’t enter the equation - just know white=nothing, black=shadow.

Ok thanks. But I’m baking. Does it make a difference?

Completely unrelated. Application of technique is up to you. You have the core principles, now run with it!
Believe in yourself! YOU make the difference! :+1:
Good Luck!

Thanks. But I would need more than that. :slight_smile:

Like, how to set the bake or the node tree. Like I said, new to this. :wink:

I recommend this course by Grant Abbitt for beginners who are interested in the basics of sculpting. Baking textures is part of the course. By the end of the course you should have the fundamentals down to be able to tackle this problem.

Best of Luck.

You could also use any renderer – such as EEVEE – to calculate where the shadows would fall on “any plane that is located there.” Send the data to a MultiLayer OpenEXR file. (Do the same with your Cycles renders.) You’re interested in the “shadow” layers. Now, use the compositor to insert the shadow into the final scene, now that you know pixel-by-pixel where the shadow is located and how intense it is.

Always bear in mind that “a picture is captured in the camera, but it is made in the darkroom.” The final image that you present does not have to originate from any single render output – and, it rarely does.

Update. I have made some progress on this using the compositing features within Blender. I have a shadow on a transparent plane. But the problem is the plane is not 100% transparent. It’s almost. I think this is because I can’t make the plane fully white beforehand. Unless I totally overexpose it. So the plane is light gray at the start of the compositing. If I throw enough light at the plane to make it white, the shadow looks wrong, Because everything is overexposed. I have tried using a principled BSDF shader loaded with a white texture and also just making the material on the plane white. I can’t get a fully white plane to make the compositing work. Using an area light in cycles. But tried other lights as well. Any ideas how to? Thanks.