Compositing mysteries


I am pretty new in Blender but now I have to create some animations involving fluids and fluids interacting with objects. So Blender is the perfect choice.

However now I have a problem. I need to composite this render footage into an existing picture.

Here’s the situation:

I have an photograph of a person holding a bucket. Into this real bucket I need to composite a Fluid stream that splashes into it. For that I have created a simulation in Blender and the fluid splashes into a stand-in collision object.

How can I just render out the stream with an alpha that also cuts out the parts that are obscured by the collision object?

Thanks in advance

When you render and save the image you have to also rendering
with the Alpha option turned on and save into a formal which
supports alpha such as PNG or TARGA and NOT JPG.

give the collision object a shdeless material with 0 alpha without ztrans or raytrace transparency on

you could also just turn off render on that object

Or move the object to a different renderlayer, enable shadow and AO pass for that layer and then use the nodes to composite the shadows and occlusion on top of your footage, and then on top of that you can add the fluid. If you want to dig deeper into the topic of 3d integration into footage: