Composition: How do I reflect composited objects?

Hello, I have a question.

How do I make composited objects reflect correctly.

an image of my problem is at the bottom of the post.

As you can probably see the reflection at the bottom of the screen is reflecting the original image instead of the composited one. So how do I fix the reflection?

Any help would be greatly appreciated. :slight_smile:


Once solution would be to make a “mirror pass” of the object using a duplicate of it mirrored in Z and placed & animated to follow the object’s motions as if it’s the reflection. Then render it out to use as an element in creating a matching glow on the reflection (more Compositor work, or add it to your current noodle)

chipmasque, can you provide a bit more details for a lamer that I am?

Sorry I missed this earlier.

The idea is to use the model itself to create a duplication of its reflection in the mirror, then render it as a separate element to be used in the Compositor to add the glow effect to the real reflection.

My assumption is that Render Layers was used to separate the object & BG so the glow effect could be generated and applied in the Compositor – that’s SOP. The object essentially provides its own matte for the effect. But of course a reflected image can’t be separated out that way.

So instead, make a duplicate of the object and Mirror it in Z (assuming Z is “up” in the image posted), then adjust the IPO curve(s) for any motion such that its LocZ makes it match the reflection’s apparent position in the image. If a rendering of the object’s motion & reflection has been made this can be loaded into the camera view as a BG image and used to help position the mock reflection in Z, rotoscope-style.

Then render ONLY the reflection object. To simplify matting, it can be shaded to be solid white against a black BG (since most mattes deal only with silhouette of the object) using the Shadeless option, or if a graduated-density matte is needed, render it with lighting.

The images produced in this “reflection-pass” render can then be used in the Compositor to generate the glow for the actual reflection in the original footage, because if done carefully, the mock-reflection object should exactly match the motion of the original reflection.

Because the real object & its reflection overlap some, it would be necessary to use the real object’s matte (its Render Layer Alpha channel, most likely) to mask the areas of overlap on the mock-reflection matte. This can be done with a Math node.