View Full Version : Rendering an alpha channel for a reflection

01-23-2007, 03:41 PM
Hi all!

I'm doing a job wich involves rendering a product placed on a reflective surface. I plan to create a number of images with different backgrounds with photoshop, therefore I'm using multipass rendering with the product assigned to it's own object buffer channel.

Nothing unusual at all there, but my question is: Is there any way to render out an alpha channel for the actual reflection so I don't have to create an alpha manually in photoshop?
Or a solution that would give me a useable, similar result?


01-23-2007, 04:07 PM
Ok, so naturally, the defunct blob of grey matter in my skull started working after I posted, and I figured out I can assign a white luminant material to the product and use a comp tag to make it unseen by the camera. Problem solved...

01-23-2007, 04:29 PM
Your other option, which i do quite often, is to mirror the actual geometry for the reflection, and just render the scene in passes. I do this so that I don't have to worry about the kind of surface the reflection will live on.

Doing a multipass render with Reflections activated is great, but those reflections don't look to hot when applied to a white surface (i've tried and tried without success). It's a little time consuming to set up, but in the end I really like the results since you have really great control over the falloff of your reflections in post since they possess a true alpha.

Rantin Al
01-23-2007, 04:55 PM
I had the same reflection problem with a white floor.

The solution is to darken the floor and render out multi-passes using a higher than required level of reflection. I then substituted a white background in Photoshop and used a bit of masking and transparency to control the reflection depth and fall-off.

In this case the floor was a white tile grid effect. Worked a treat once I figured the dark floor trick.

Cheers, Alan.

01-23-2007, 05:13 PM
Yep, mirroring would be another solution, but I rendered my reflections on a floor with black color and 100% reflectivity, which works out great for me.

01-23-2007, 05:36 PM
Yeah the dark floor trick works well, but I find for animations I prefer having a true Alpha for my reflections so I don't have to worry about odd artifacts in the reflection when I use Screen or Linear Dodge when applying to various colored backgrounds...just less masking and "knock outs" that need to be done.

But hey, this is all compositing and we all have our special recipes and black magick techniques to get the final output to look stellar. :)

See sample below:

note: I know that the floor reflection isn't showing up in the image on the right side, but that is easily resolved by rendering passes (I just did this to illustrate my point above)---But this has to be approached on a case by case basis. In the end, the final image defines the path you will take.


Rantin Al
01-23-2007, 06:49 PM
Just to add another aspect to my example. The job was multiple images for a corporate brochure and I had to match the RGB renders to a number of Pantone reference swatches and ensure that the CMYK output was spot on.

What a basket of squirrels that was.:)
Cheers, Alan.

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