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Hi Guys,

I have a 3d model of a car and am curious to know how to combined the model with and hdri - I know how to light the scene with the hdri
but more info about getting the car to "blend" into the background - Do I render the car on its own with just the hdri then render a shadow and alpha pass
where I then compose the car into the background image? any help would be good!

Thanks

Precisely. Although, I don't think you'll need an alpha pass.
Render full pass with hdri, then seperate shadow pass. Overlay shadow pass in post with multiply layer attribute and mask out any unwanted occlussion from the shadow pass depending on how you setup your shadow pass.... Usually just a plane matched to camera that your object is sitting on with any other dummy objects that would receive shadow pertinent to hdri scene.... Sounds like you're on the right track with the process. You could also render out a diffuse layer and reflective layer for even greater "blending" control. Ultimately, the more layers you have to work with in post, the greater control you'll have over the composited image.

The first time I played around with compositing in this manner, I did it first with basic primitives into various hdri/mxi environments with diffused and reflective materials to see what kind of problems I would run into before facing any failure with a more time consuming image operation. This way (with a few experiments under your belt) you'll notice things that you may not have thought about going into the operating room.

Trial and error is key to success! Hope some of this hot air was helpful....

@Rusteberg - Will take your advice mate! Ive lately seen alot of your work around the forum! (Thumbs up) - Thanks for the info on what process
to take will start small using basic primitives as you mentioned and build on that!

Thanks

why on earth is there no option switch to tick for this?
you would imagine a tickbox "do not show in render but show shadow that falls on this" then you would tick this for the ground plane and your done, no need for photoshop and seperate passes. anyway my 2 cents