C4DtoA render problem



So, I hit a wall with this. I’m new to the Arnold render world and quite happy with the results it produces. Let me get to the point.

Okay. I have a camera move backplate that needs to feature a car on it. I worked on the car and while it still needs some kinks worked out I wanted to see it on the backplate that is provided. I used a similar hdr to light the car and used it for my initial renders as a background but for my final sequence i want the background hdr (skydome light) turned off. I don’t want it to show up in the alpha. I thought that a simple switch can help me but it turns out it doesn’t function like that. Is there a way to turn the hdr in the background in a way that it still affects the reflections but the cars glasses are transparent and not show the bg?

I added a render with the hdr and the backplate in a mockup to illustrate the point. The left side is the hdr skydome and the right side is just a quick mockup of the backplate that should be in the final composite. The glass surfaces still show the hdr in the bg. I need to have the backplate showing up.

A quick resolution would be favourable, I will take any advice at this point.




Use an Arnold Tag and set the visibility like the C4D Composite tag.


Thank you for the response.
Sorry, not sure which tag and what parameters should be manipulated. Can you elaborate?


The Arnold tag can be found by right-clicking on your object in the object manager, and choosing C4DtoA Tags > Arnold Parameters.

The Arnold Parameters tag will adapt its settings to the kind of object you place it on (mesh, light, camera, etc) and is essential for manipulating objects in the context of the Arnold renderer.

For instance, you can set an object’s level of render-time subdivision in the tag (and potentially avoid using the subdivision surface object), or set the level of displacement.

In this context, you will want to adjust settings in the main tab to either prevent or enable an object (like a sky object) from being seen in reflections.

You should also probably add tags to your glass objects and disable the “opaque” setting, which is essential in Arnold to get realistic transparency.


Thank you. I understand that part but when I add the tag it doesn’t handle primary visibility like for a mesh. I’m very confused and I’ve been searching for a solution for two days. Either it’s staring me in the face and I’m a total idiot or not simple at all. My deadline is approaching and because of this I’m unable to send the scene to render.


Sorry, you don’t need to add a parameters tag to objects that are already “Arnold” objects, like the Skydome light; they are redundant.

In this case you want to adjust the sliders for (most probably) the transmission and specular contributions of the skydome light. Make them “0” if you want the HDRI not to show up through transparent objects or in reflections.

But this will cut off those qualities for all objects in the scene, which you may not want.

Instead, add tags to the geometry of the reflective and/or transparent objects (like the car windows), and disable the features you don’t want (specular, reflection, diffuse, reflection, specular transmission, diffuse transmission). This may take some experimentation.

Also, you should look at AOVs workflows to see if you can get what you want by rendering alpha channels.


Thank you for the info. This turned out to be more difficult then it has to. This is a non issue with the built in renders of Cinema 4D and I’m not sure why it has to be so complicated with Arnold. AOV are too complicated for me now. Don’t know anything about them and I’m not sure how to solve it with them. I succeeded in adding opacity to the AOV thingie but the pass still includes the background which I don’t want.

At this point I would pay for a 1on1 session with someone for the help.


Its nothing complicated at all…

Lets say you have a cube and you have a Skydome providing the HDRI, and another Skydome providing the background.

Just go to Skydome A parameter and turn the camera setting to “0”, and then in Skydome B parameters, turn diffuse / specular / transmision / SSS / Indirect / Volume to “0”

If the background is provided in an Sphere object, just add an arnold tag to the sphere, turn on primary visibilitym and turn off diffuse, specular.

Hope that helps.


Hi @luisRiera

Yes it put me closer to what I want. I have a clean alpha around the car now but I cant get the clean alpha without the hdr background. The camera value change helped. What do I need to do to remove it from the windows. And also we are going into a new territory of owing you big time if we can fix this. A couple of Portuguese wines and some codfish :slight_smile:



I think you are looking for the Opacity setting that used to be in the Specular tab, but in Arnold 2.0 im not sure were is it.


(Its similar to the Alpha settings in Cinema 4D material)


you can also set the Exclude options in your Arnold Light Project tab, and exclude the glass from the HDRI, but not from the final background, basically this:“include” the glass in one skydome , “exclude” it from the other.

edit… Just excluding should work… including means “this object only”, that is useful when you want the HDRI to only affect the glass.


Final note, if you want the alpha to cutout the windows, thats also easy to do… just set an AOV pass for the windows and the car, and cut them in After Effects

Follow this to set the AOV:



alternatively use the arnold skydome light object and disable seen in refraction.


A lot of options changed in Arnold 5.

I don’t think there is that option in the skydome ( I think that was the option of the old Arnold Sky, which we shouldn’t use it anymore)… although, it feels strange for me to tell you this, because, as far as I know, you are like Neo when it comes to Arnold.



Thank you for your continued support, it really means a lot.
I’m not sure why I need the second skydome light. I don’t want to use another hdr. I have a footage that I need to compose this on. The setting of the camera value is now giving me a clean alpha outside of the object. Now I need to find a way to remove it from the refractions in the car window. The problem is that the support page on Solid Angle hasn’t been updated yet and I cant find the alpha settings for the material. There is a new tab called alpha in there but not sure how it works.

If anyone else has a clue on how to fix this please let me know.

Thank you

UPDATE: With Transmission turned to 0 I have now no background behind the glass surfaces. The only thing left is to figure out how to manipulate the alpha settings for them.


I think you mix up the alpha from c4d with the one from arnold. What you afe looking for is either the opacity or transmission aov that would give you appropriate masks for refractive shaded objects or opacity mapped shaders.

Ha, Luis thanks for the praise:) Although I must admit and hope there are many users and artists already that bypass my humble knowledge about c4dtoa. After all its a perfect example of userbased improvements on the development. I enjoy helping everybody achieving neo level though, as much as I can:)


Thank you for the info. I’m starting to grasp the whole thing now. But it still doesn’t make sense that the render doesn’t give you glass transparency values in the alpha (or opacity) by default. In the new uber shader the only thing that affects the alpha is the new Cutout Opacity under the Geometry menu. If you change the value from white to black the object will become transparent and all the shading goes with it. Now, I know that I can use that to mask out a the parts that i need but that also means that I need to send my render 2x to the render farm and that costs money. I must be missing something.



Please make the ‘camera’ parameter to 0.(in your arnold lighting object)