How to bake ambient occlusion of a plane with transparency?

Question

I'm new to Blender and 3D in general.

I have a single object (mesh), a plane, and no lights and I've set an environment (world?) for my scene. Now, I want to export the scene to GLTF and want to bake the shadows on the plane (the very soft shadows of the object resulted from the environment lights).

I've enabled transparency in Film section and have also made the plane a shadow catcher:

When I bake the ambient occlusion for the plane (note that it seems to be important for the color space of the image node to be Non-Color), it results in black and white image. The black is good. I want the white to be transparent:

And here is the result GLTF in the browser (using Three.js library):

So, again, how can I have the plane with only its shadows and the rest transparent?

Answer 1

Since we're using the glTF exporter, the "Invert" node is not going to fix the problem. Instead, manually invert the baked ambient occlusion image, using Blender's image editor:

Be sure to "Pack" or "Save As..." your image after this. It will look quite odd, with the shadows white and the empty parts black, that's OK for now.

Next, set up a node graph that uses this manually-inverted image as the "alpha" input, paired with a fully black "base color" setting. Set metallic and roughness both to 1.0. Make sure the "color" output of the image is being used (as this image contains no alpha of its own), as shown below.

Finally, one last important step! Before exporting to glTF, we must set the "blend mode" to "Alpha Blend." To do this, you must temporarily switch your rendering engine from Cycles to Eevee, and make the change in Eevee's material settings. You can switch back to Cycles as soon as this change is done, and Blender will remember that this happened even though it's gone from the UI.

The glTF exporter will find this blend mode, regardless of the selected rendering engine at the time of export.

So now, you should have an all-black material with an opaque (white alpha) alpha channel in places where the shadow should appear, and transparent (black alpha) everywhere the shadow should not appear. It's set to alpha blend mode, because otherwise glTF will ignore the alpha entirely.

Hopefully if you get these steps correct, the export should work. Good luck!

Answer 2

use a principled bsdf node and place the backed texture into the alpha and base color input of the node, insert a ramp in between the alpha and texture and invert/adjust the contrast until you get the transparency you want and ensure you set the roughness to 1 so their isn't any reflections.