How to: Render only where spotlight hits plane, but rest of plane is transparent?

I have a block with a logo cut out of it. I’m shining a spot light through it so that parts of the spot light show through the cut out and illuminate the plane underneath. I’d like to render the plane as transparent, except where the spotlight hits.

I’ve seen several tutorials on how to render only shadows on a plane, but not just the light. I’ve seen something on how to render only reflection on a plane, but it doesn’t seem to work here, so reflection may not be the right term for this.

Here is an image of one of the first frames:


I’d like the blue plane to be transparent, except for where the light shines through. The color of the plane/light hitting the plane can be any color really, probably white would be best.

Thanks for any help!!


Rendered separately, composited using the shadow pass for alpha


Rendered separately, composited using the value component of hsva for alpha which makes the lit pixels transparent.

JA12 - thanks for the suggestions. I think I’ve used your first suggestion and gotten pretty close. I can now render only the light hitting the ground plane, with the rest of the plane being transparent, which is probably good enough for what I’m doing.
Your second example would be better, but I’m not quite sure how to go about doing it.

I’m posting a screenshot of what I did for your first suggestion. I’d love any feedback. Or if you have a screenshot of your nodes in your second example.

Thanks much!!


separate hsva node for the layer, v used as alpha for set alpha node.

Didn’t save the file and not doing that again.

The most popular way of asking questions is to limit the information to only what is written, remove information from images by cropping them, and certainly no .blend file to get to that information and help replying. After answering with a request for more information a couple of thousand times, providing a tutorial on how to post better, and users not answering in their own thread no matter how extensive the information they receive, I’m clearly doing something wrong. Perhaps the correct way to reply is with the same format, amount of information, and in the same amount of time they were asked.

I’m getting the impression I broke some protocol, or did something wrong asking here. I appreciate you taking time to share your expertise. I’m just trying to learn blender and the ins and outs of this community.


Top branch: with transparency, bottom branch: without.
Bottom left: with transparency, bottom right: without

untitled.blend (94.9 KB)

No I just sometimes put my frustrations into words, which I shouldn’t do. It’s not wrong to ask, that’d be weird since this is a support forum, it’s that most suck at it.

I could write all the reasons why it’s frustrating when it’s expected that new users don’t know how to ask questions, and why it’s a problem, but that’d be a waste of time. The information that might have some value to you is in the tutorial linked in my signature.

Although should mention that it’s not personal. One big reason it’s frustrating is because this is the support forum, yet the forum doesn’t support its users. New users don’t get information about how to open up when asking questions, and anyone browsing through and answering them has to deal with that. Blender bug tracker has given posting instructions for ages now, but seems to be too big of an obstacle here.

I have this exact same need, however, when trying to render the setup above when switching to Cycles, it loses the transparency. Anyone ideas how this could work using Cycles Renderer?

Much appreciated.

I’d save to a MultiLayer OpenEXR file and then go back and look at what data layers the file actually contains – and what they look like. It sounds like the Alpha information is different . . . or missing.