HDRI background & Alpha channel question

obloquy

Hey, sorry for the simple question - I wanted to add an hdri background to my character render in Nuke using the alpha channel as described here. The problem is I've (stupidly) rendered the character against a 50% grey backdrop (pic1) rather than just making the backdrop invisible so the character would be against black. So basically the alpha channel is totally white as the backdrop is seen as solid geometry now.

Because the backdrop has been included in the render theres a subtle gradient from dark to light because of the scene lights (pic 2). I just wanted to know if I would still be able to use this render to add an hdri background if I made a new alpha (without the backdrop rendered) or would the rendered backdrop thats got a gradient be too much of an issue to remove as its not 100% black?  Hope that makes sense and many thanks to anyone who can help

Eric Chadwick

If you try to use the existing render, and retroactively tack on an alpha, you may get some fringing, especially on heavily antialiased areas like the hairs. Why not just re-render?

obloquy

Yeah I came to that conclusion sadly so I just re-rendered it and made the backdrop invisible to get the white alpha for comp (I was being impatient lol).  Thanks for replying Eric

.Wiki

In arnold you can turn off "camera ray visibility" for the backdrop. It will still contribute to all calculations but it wont be visible to the camera.

Edit: You might need to add the "arnold properties" to the object to get this setting. I don´t know how its handled in maya, i only used arnold in softimage and houdini.

obloquy

Thats exactly what I did .Wiki!  Just turned off primary visibility of the object  in the arnold tab (even though I was using vray) and kept all the shadows and diffuse/spec reflections and hey presto I had the alpha!