Rendered Reflections: possible to get a separate channel (alpha) for this?

[Be.eM]

Hi all,

is it possible to have an additional channel which allows the selection of only reflections on a plane? Similar to a shadow map which is rendered out?

I'm currently struggling with that hyper modern approach of "every item on the title page must stand on a reflecting ground, everybody has this, so let's have this, too". My problem: I can initially render those reflections on a plane without problems, but later I'd need to replace the background with predefined color in the post, in order to match underlying CMYK vector graphics surfaces. This would be easy if I could simply cut out the reflected whatevers by means of a channel and put that CMYK color as a separate background layer in PS. Since reflections are rendered on geometry, would it be possible to render such a "reflection received map", too, somehow?

Bernd

[Be.eM]

To answer this question myself: I've found a way which is not perfect, but does the job. As such, it's close to perfect ;-)

What I'm doing now is:

• Set the whole scene geometry to be a "Reflection object only"
• Set the reflective ground to be completely black, and the reflection configuration to "Full" with 100% white als the reflection color
• Render the scene, which gives me only the (100%) reflection on the black floor, save the image
• Switch the scene geometry to render normally, hiding the reflective floor, save the image

Now I've got two images to open in Photoshop, the reflective Floor including reflections, and the normal rendering without the floor. So in Photoshop I'm doing this:

• Open the reflection image, change image modus to "greyscale", save image separately (to be used later)
• Open the reflection image again, display its (so far useless) alpha channel, copy the complete greyscale image from the previously saved image to the alpha channel of the (still rgb) image of the reflection.
• Switch to the layer view again, make the background layer a regular layer (double click), use the alpha channel as a selection (CMD + ALT + 6), invert the selection, hit the delete key. Only the reflection remains, the rest of the background is gone.
• Open the normal device rendering, make the background layer a regular layer, use the alpha channel to delete the background, only the device remains.
• Drag the device layer over the reflection layer in the other image, so you've got the device including its reflections
• Add a third, underlying layer for the desired background color, and fill it accordingly. You can also do this after converting the image modus to CMYK (while maintaining the separate layers)
• Use the opacity of the reflection layer to control the strength of the reflection.

Done, the image blends flawlessly into the colored (vector) CMYK background :-)

That's it.

Bernd

 

[Tomas Egger]

Ola Bernd!
Im glad you solve it! I was traveling :)
Thanks
Tom