compounding a model in a photograph

[JamesMK]

You're in luck (me too, since I'll save a lot of keystrokes) - read this article: http://www.cgnetworks.com/story_custom.php?story_id=2585&page=1

[Marius Jonsson]

thanks a lot, its a nice article but i dont really understand more about the compounding part. Maybe i dont understand the article right, or you're missunderstand what i'm wondering about or maybe i failed to explain it properly. I'm going to compound a car in a photograph. The pavement will be wet, and will need to reflect the car + the shadows will be very smooth.. not sure how to compound that. I've never compounded anything, think i need things in detal :). i tend to model my scenes, but im not doing that this time,

thx

[JamesMK]

Ooookay, that requires a slightly more lengthy reply... but I'm off to bed, so I'll be brief :D

Basically you would model some simple (or not so simple) geometry to match the parts of the photo that need to get the fake (i.e. 3D rendered) reflections and shadows. You obviously also need to match your 3D camera to the background plate, either manually or using some sort of camera matching software.

When everything lines up properly, you start adding lights to match the lighting conditions of the photo. Be careful finding the proper black point, since it usually will not be entirely black in the photograph. One way to match the black point is to use a low energy ambient light to achieve this.

Since the car itself probably is reflective as well, the ideal thing would be to have an HDRI from the location. If that's not an option, you could use the available photo to stitch up a spherical environment map. Not as good as HDRI, but you can get away with it depending on the purpose of the image.

When it's time to render, you basically just want the car plus the reflections and the shadows from your matched geometry (i.e. the sidewalk and so on). Depending on what package you are using there are several different ways to achieve this... anyway, render out one shadow pass and one reflection pass of the sidewalk, then add these to the photo in your compositing software (shadowpass in multiply mode, reflection pass in add/screen mode)...

A long story, but I hope this brief version can help.

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