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A few weeks ago, I posted a video covering the process of relighting rendered CG elements in a compositing tool like Shake using a surface normals pass.

Since that covered normals-based relighting of rendered cg elements, I thought I'd write an article that takes the concept a little farther and should stretch the reader's mind on what's possible with this sort of approach.

The earlier tutorial covered something that naturally can be accomplished by going back to the original scene in your 3d application, adjusting lighting and re-rendering: but I think there are times when we want to adjust the lighting primarily in the compositing application but want more than some masked-off gamma correction, lifts and crushes. So it's handy, then, to be able to apply an effect more similar to real-world lighting and that's what a rendered surface normals pass will enable us to do.

But what about real world imagery? Can this same technique be applied to live action footage just as it can be on a CG element?

Absolutely!

You'll want to start with a few elements like the following:

A full-color image like the above picture of a farmhouse. Under ideal conditions, you'd have an image with very diffuse lighting: something with a lot of bounce light or an exterior on an overcast day. The idea here is that there are relatively few real shadows in the image. The reality is that you won't have that very often and you'll have to use what you've got. You may decide that the shadows that do exist need to be painted out before you continue. I haven't done that for this example, but you'd be hard pressed to find a high end effects project that relit a real world environment and didn't remove the existing shadows at some point in the pipeline.

The above image is a matte of the farmhouse pictured above. You'll probably need to generate something like this for your scene if you haven't already.

A surface normals pass of a polysphere with its shading set to be as unsmooth as possible. I prefer this because I have a better feeling of where these surfaces are facing than I do straight from a sphere. It makes it like a special kind of color palette so I can perform the next step:

Next, I roto out a super-simple normal map of the surface in the image. This can be more or less complicated, depending on how much detail you want in the relight. If the surfaces are softer, you may want to blur the resulting image some: this will soften the transitions. Pick colors for each of your surfaces from the faces of the polysphere normal pass, making sure to keep parallel faces the same color as you interactively create a normal pass for your real-world element.

Then, I set up the OpaqueWhite OneLight Plus node as described in the normals relighting video here. Shake users can download the tool for free here.

It'll look similar to this:

Over on the right, you see where I've created the interactive surface normals pass. Those are groups with a few quickshapes and Mult nodes layered to create the interactive normals. In this case, since I didn't build out the ground and the barn next to the farmhouse, I chose to use the alpha from the interactive normals as the matte for the OW_OneLight+ node instead of the matte extraction I showed above. A smoothly blended surface for the ground, and a couple blurred normal shapes for the trees could make this quite convincing.

On the left, the fade leading into the over and the mix node to mix the whole thing back together is just one way of tweaking the levels of relighting that you're doing.

I posted a really short render of the light source moving across so that you can get an idea of what can be done. I kept it fairly subtle, and keep in mind that a little more work to create normals data for the ground, barn and trees would be required for a real project.

Moving footage naturally requires animated roto work to generate the interactive surface normals pass (keep the colors the same unless the element itself is moving!), and more complex objects with broader camera moves can be relit if you:

1. Build 3d geometry to match the visible elements
2. matchmove that geometry to match the footage
3. render a surface normals pass of those elements
4. use that rendered surface normals pass to relight the live footage.

In a future installment, I'll describe a method to generate surface-normals data by painting a bump map onto an image. In the meantime, I'll leave it as an exercise to the curious reader: how could you convert a simple greyscale height map into surface normals data in Shake?

I noticed the other day that my own 2d crowd generation software tool (DynaCyc) is ranking higher on google than Massive Software's listing for their incredible 3d crowd sim software.

In fact, if you google "Crowd Generation" there are two hits for my own websites before theirs comes up.

While DynaCyc is nowhere near as sophisticated for generating realistic behavior (it's more of a dynamic matte painting tool than a crowd generator - I created it for doing plants and trees to algorithmically make forests and such), it's pretty cool that I'm placing so well.

I really need to get the tutorial for my uv remapping tool recorded so that people can start using it. At the moment, though, I'm working 12-14 hrs a day, 6 days a week scripting some fancy animation pipeline tools.

The funny bit is something I've come to realize of the last couple days: while I fully grasp the minutiae of what I do, some of what I'm doing just baffles me in the big picture. I wrote a tool for "recasting" an animation from one space to another. For instance, if there's an object in a shot done during previs, and that original object has a particular animation on it, and there's a camera with a camera angle that we like... but the object itself is given a different animation in our scene because it has to fit in the "world" relative to other objects and be moving at a certain speed, reach a certain place at a certain time... I can take the camera motion from the original animation and transform it to the new shot maintaining the relationship between the camera and the element... while replacing that element's animation.

That's tricky... but I know how to make it happen and I've embodied it in a deceptively simple tool.

Separately, I worked out a method for stitching multiple animated shots containing a common object and many separate cameras together. The object's motion remains constant, and the cameras follow it along the path. Additionally, I developed a method for steering that element so that I can change its motion, adjusting the cameras to follow it.

That last one works using the first tool... so they end up all being connected to each other.

Once that happens, it feels like magic. I no longer understand the big picture: how we now have nearly a hundred separate shots stitched seamlessly together, but with different animation... and still working with the cameras and tracked objects matching to the footage? That starts to get away from me... I can conceptualize it as steps, as stages to reach a goal... I can break it down into its component parts: but the whole big process I understand only in that way: as big steps. I don't hold the whole thing in my head at once, though I use the tools that make it happen several times a day.

What's also funny is how much I'm even animating at this point... and I'm no animator, believe me. Let me write you a few expressions, though, and I can sometimes fake it.

I am officially in crunch...

It's mini-crunch, though, because we're just working to get things together for the trailer. Most of the staff isn't actually working overtime, but there's a few of us... the dynamics/fluid guys, an animator or two, the cg supe and me, piecing together the necessary bits to get the shots ready for the trailer.

I wish I could be a little more clear on what exactly it is I'm doing... When the trailer comes out in a couple weeks I can at least indicate then what segment of the film I'm addressing - I'd rather leave that part as a surprise for when the trailer hits (not to mention that talking about things like that would get me in hot water).

In a sufficiently vague sort of way, I can say that there's a segment I'm working on right now that's around 76 shots long, each ranging from 50-300 frames, that has a common element that's travelling between them. Our tracking team has been tracking that element, and its progress through each individual scene, for each of those 76 shots (and at least that many more as well that I'm not dealing with yet).

The particular task I've been working on is basically a continuity script. It's a MEL script that gathers all of that data: cameras, animated and matchmoved elements, and stitches them together end to end in a master shot. This way, the element that's moving through the scene will maintain an accurate location relative to the CG environment.

To coincide with what's happening in each shot, it's necessary on that big grandaddy master shot to make adjustments: turn something here, nudge something there... and that affects the path of that common element throughout all of the individual shots.

Do you see where I'm going with this?

Basically, if we imagine that I'm talking about a man driving a car across the salt flats in Utah (not at all what's happening in this film), there are features on the horizon that we want to be consistently and realistically moving in relation to the view of the car. If the driver swerves to miss a rock, we want to change the path he's taking across the salt flats (even though in the original plates, that driver would have been sitting in a stationary vehicle in front of a bluescreen, with some hydraulics underneath making the car tilt and sway to add realism). Our view of the car from the camera should stay the same... Car and camera must maintain their relationship... but the position, speed, orientation, etc of the car and camera combined will need to change. If there's a guy on one of those sail-carts out there, we need to make sure his path is realistic and that he's in the right place in the background of each shot...

Remember that the car, the driver, and the camera are often the only real things here... the sail-cart, other things on the horizon, in fact the entire salt flats... all are cg or augmented with cg.

So I've been developing tools for funneling data from all of these shots... everything from camera data to object matchmoves, world-space offsets for each shot, and animation data... stringing it together into a seamless master scene, manipulating the paths and locations of elements in that master scene and transferring that data back into the individual shots. If we need our hypothetical salt-flats driver to swerve this way and that across several separate shots, I'm making sure that it doesn't just "kinda look right" but that we can get that data integrated into the object and camera motion of several live camera plates.

If somebody didn't ask me if I could do it, I'd probably have never thought it could even be done.

If I'm kinda scarce around here in blogland, it's because I'm catching a much-needed nap in between long shifts.