Virtual Environments

Posted Saturday, July 25, 2009, 7:46 PM

The main role for a digital matte painting is to create an entirely virtual environment, which is to say, a completely artificially assembled background for a shot.

They used to be made using paint, often on glass, which is then blended optically, that is practically, and re-photographed. But these days you can use photographs, blend them together seamlessly, and then apply them directly to 3D CG models to give them a small amount of dimensionality.

The art of putting all the elements together for the final shot is that of the compositor's. Though I have to be familiar with compositing as part of my job, it's not quite my forte; I am better at dimensional-related things than I am at colour matching and rotoscoping (clipping around edges). So the compositing for Checkpoint is being handed over to its Director, Rob. My job is just to create the background plates.

One of the pickup shots we filmed a few months ago contains possibly the most creative camera move of the entire shoot - a dolly and pan combination. The camera moves rightwards sideways along a track, simultaneously the camera pans left, while the actress runs up to camera, all in front of a greenscreen, which we set up in Rob's backyard.

The usefulness of a greenscreen is simple in effect, but it's not until I started actually doing all this work that I have come to realise just how wide the potential really can be. We can film anybody in any clothing and doing any activity, and then place them in any environment you can think of - in a French village, a Cafe, on a roof, off the side of a skyscraper, in a sewer, on the Moon... or on a country road near a UK airfield.

What do country roads near airfields look like? Well, almost anything we want it to look like. I decided on a stony road surface, a length of grassed verge, and then, 30m beyond, a line of pine trees. I figured I could put that together, using photographic elements, combined to look photoreal. Then I apply that onto 3D geometry, each plane representing a textured surface. Simple, but it has a limitation.

In my software, the only way for that to be useful is if the virtual camera stays static. The projected image is always oriented from the camera's point of view. That means if I move the camera, the image will move with it. This is no use, as I want the image to stay locked onto the geometry, not slide along in relation to the camera.

There is a solution, which is to check the box that fixes the image, relative to a specified frame of the camera motion. For example, if I set the camera up on frame 1 to point where the image best matches the geometry, and then set it to be fixed there, then, when I move the camera, the image will stay locked at that orientation, even though the camera is in a different place by frame 257. But what that means is the camera then leaves the image behind, it disappears out of view, and now you're pointing at blank geometry.

But there are solutions to that, too. One is to orient it to a different camera, perhaps one with a different frame scale, to give you more space to move your proper camera through.

Another solution is to use the tiling feature, which lets you repeat the image in any axis you wish. But that means your matte painting image has to be seamlessly tileable, and with its geographical features oriented in a parallel fashion to keep the tiling logical.

And this is what I did. I included four horizontal stripes - road, grass, trees, and sky - and blended the edges so they repeated seamlessly. Then, when the camera moved and panned sideways, the landscape continued horizontally along the geometry, and, though it repeated, it wasn't too glaringly obvious.

The repetition can't happen on all axes, so the top of the image, the sky, needed to be blended. I repeated its edge colour infinitely, so the "sky" continued up as a solid shade of blue.

And though that's the crux of this shot's makeup, it doesn't take into account the hand tracking of the three dimensional move (because the tracking software didn't fnd enough points to figure it out), the car model that is sitting on the roadside, the particle-created grass blades in the foreground, and all the other doodads that make up variation in the landscape.

I love creating something out of nothing.

1 Reasoned Responses:

Gayathri said...

I love the amount of details you gave in your instructions through this article. Great example. Looking forward to more from you.

clipping path

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