composite improvement?

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kitsu
Posts: 0
Joined: Sun Jun 06, 2004 8:08 am

composite improvement?

Post by kitsu »

I am currently using blender for photo-compositing at work and I have run into a problem. I have created a 3d duplicate of the terrain in the image I'm using, set it to render shadows only and now my objects cast shadows on the ground :) . The problem comes when I try to create reflections. There is water in the scene and my structure (a bridge) spans over the water. So I set up a model of the water's surface, make it reflective and shadeless and ny bridge reflects. The problem is that the rest of the world is reflected as well. This changes the color of the water, makes wierd reflections etc. What I need is an option to render reflection only...

I don't expect anyone to jump right on this, I was just wondering whether this sounded plausible? Right now I'm going to use the sequencer and RGBA to composite the reflection into the background, but it would be nice if I could just render reflections in one go.

ilac
Posts: 131
Joined: Mon Oct 14, 2002 8:24 am

Post by ilac »

Unfortunatly Alpha channel doesn't work through raytraced objects (though I cannot understand why - at least rudimentary. Blender is an artistic tool not a technical tool)
The (not always reliable) work-around I've managed to use on some occasions is to make my base material a chroma key colour and key it out in post-production. ie make your River bright green or something suitable and key out the green after! Hopefully most of your reflections will remain intact.

Alpha in raytraced materials would be nice though...

kitsu
Posts: 0
Joined: Sun Jun 06, 2004 8:08 am

Post by kitsu »

I just did a test and it seemed to work as I expected:

I went to the render buttons and changed to RGBA and sky to key. Rendered to png and saved somewhere handy. Then go to sequencer and load background, then load png, and finally add alpha over to them. Render with do-sequence and all is well...

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