How to render alpha in mirror reflection?

Support Compositing and Post Processing
#1

http://blenderartists.org/forum/%3Ca%20href=%22http://photobucket.com%22%20target=%22_blank%22%3E%3Cimg%20src=%22http://i220.photobucket.com/albums/dd201/dwarfedfilms/BlenderquestionAlpha_01.jpg%22%20border=%220%22%20alt=%22Photo%20Sharing%20and%20Video%20Hosting%20at%20Photobucket%22%3E%3C/a%3Ehttp://i220.photobucket.com/albums/dd201/dwarfedfilms/BlenderquestionAlpha_01.jpg
If I want to render the monkey reflected in a mirror - but I want the background reflected behind the monkey to be transparent with alpha 0 (so that I can composite a different background into the blue area of the shot later on without having to Chroma Key it).
How would I go about doing this?

Thanks in advance,
Shane

#2

Dwarfed Films,

The blue that is being reflected is the world color. You could:

  1. change the world color to black
  2. create an image plane behind Suzanne - a simple plane with a material and image texture.

Best of Luck!

#3

Hi there and thanks for the advice, :slight_smile:

I know the blue is from the background reflected in the mirror. What I’ve done before in a previous film is - like you’ve mentioned - put a cube or plane with UV mapped background on it, directly behind the character/monkey head so that the mirror reflects that. I could’ve also chroma keyed the blue section and composited in whatever I need.

But what I’d really like to find out if it’s possible to make the reflected part totally transparent? Is it impossible?

Shane

#4

Real mirrors don’t have alpha channels so your going to have to fake it. This should save a lot of rendering time, so it’s a GOOD thing!

  1. Render the reflected object(s) only form the mirrors pov with an alpha channel.
  2. Apply the resulting sequence to the mirror plane with alpha and re render from the camera view with alpha. No raytracing needed.

Now you can composite anything you like.

Pappy

#5

With raytracing and mirrors, i remember something about increasing the bounce factor from the default 2 so that the photons properly are mapped. BUT, as said above, the blue is from your world; delete your world if you dont want it.

#6

Just copy the suzane head and position it where you want the reflection.

Forget the mirror.

#7

Thanks for the responses, everyone.

#8

Use a key node in the compositor to quickly create an alpha channel from the blue area.