Introduction, Concept and Purpose:
I created this shot to demonstrate the ease of matching CG elements and plates using HDRI. I'd hunted around several scale model shops for any suitable el-cheapo models i could use to model in 3D, but I could not find anything suitable in scale to my backplate. The smallest I could find was a 1/2400 scale space battle cruiser. No way it'd look the correct scale - in a parking lot! I also did not want realistic models, as I wanted to create a quirky kind of animation, kinda like the pilot was uncertain of the controls.

CG Modeling:
However, as fate would have it, the Star Wars edition of CineFex also had a story on H2G2, absolutely one of my favourite anti-climatic type movies (another example would be Steve Odekerk's Kung Pow). CineFex's story on H2G2 had a rear view image of the HogPod taking off, and I thought that it'd be the perfect scale for the backplate. The flip side of the coin was, it was showing the *rear* of the craft. Hunting for reference footage turned up NIL (I'm hopeless at googling, believe me), apart from two other concept artworks featuring the rear of the craft as well.

Thus, the new HogPod KAI was created, KAI meaning custom, or modified, since I had no reference of the front, and I don't remember what it looked like in the cinemas. This test render used Paul Debevec's Uffizi Gallery Light Probe. You can get it, and lots of others at this URL.

HDRI Creation:
I'd talk about the backplate first, but decided to leave it till the digital matte stage. My method of creating HDRI maps for reflections and Image Based Lighting (IBL) is very much standard - taking pictures of a mirrored ball. In my case, I did this on a very low budget... a silvered christmas ornament. This little baby was mounted on a short rod fixed onto an older tripod. See the pic below. It's just a christmas ornament! If you want something a little more hi-end, check out HDR Shop's FAQ page for a list of places to purchase real chromed spheres from. I'd actually found some in Singapore (look under bearings in the Yellow Pages), but their cost was prohibitive, not to mention heavy.

Note that the way I mount the sphere is via a plastic rod that extends downwards vertically, at which the base is attached to the tripod. This is not really ideal as the reflection of the tripod appears. A better way to do it was to attach the sphere with the tripod offset. If you choose to use it the way I do, try to do it on ground that will be easy to clone out in your paint app.

Since this is an outdoor LDR (Low Dynamic Range) aquisition, you'll do well with a camera with high shutter speeds, and small apertures. My digital camera only allowed me to up to F8/1000, unlike my film slr that I take down to F22/4000. (And oddly, my film slr costs only a quarter of my digital camera.) Hence, I was having an issue in that I *could* not take the under exposed shots.

This could easily be solved by using a Neutral Density (ND) filter to limit the amount of light, in effect similar to a higher shutter speed, but, as things go wrong, I brought the wrong filter out - a polarizer. Polarizers stop about 2 stops of light, but as I am not certain how it would affect the final image, so I decided to skip it, and just rely on using the camera's EV adjustments to get the exposure values I wanted. I'm no camera equipment fanatic, but I read somewhere that EV adjustments adjust only the gain of the sensors, not the actual shutter speeds. Still, it works. In my EXIF data, it records that the leftmost ball (see below) was captured at F8/16000 :-)

As you can see, there are issues with the tripod, and me in the photograph. Tripod removal is easy - just clone the surroundings.

Front sequence, with the tripod removed.

But to get rid of the photographer, I'd to take two series of chromed spheres, where each set was to be taken at 90 degrees to each other. Ensure that the distance between the lens and sphere are equal, as well as the height of the camera's lens in relation to the sphere.

It is best of course to keep the camera as far away as possible from the sphere, so that the photographer will appear small in the picture. But if we were to move too far away, the image captured will be very small. So far, my best results were by taking pictures of the ball with the camera set to it's longest focal length, with a distance as close to the sphere as the camera can focus @ its longest focal length. My camera, at 38mm focal length, can focus at 4cm. But at 190mm focal length (it's longest) this is increased to about 100cm. Thus on my actual day shoot I the camera's lens was about 100cm away from the sphere. Bring along a measuring tape!!! This gives me a chromed sphere about 650 pixels wide on a 2560x1920 canvas.

Once I've got two series of chromed spheres I will assemble them in HDR Shop to get two series of spheres in HDR format.

Next, the assembled HDR spheres are unwraped in HDR Shop into a longitude/latitude format.

Thus, when it is unwrapped, it will be possible to use HDR Shop to create a version of the HDR without any distortions or unwanted photographers :)

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Granted, it's not as good as using a fisheye lens plus a panoramic tripod head, but for the cost versus the results, I really cannot complain!!!

In the 3D software, our camera is at an angle between the front and side positions. If I were to use a reflective sphere (i.e. no fresnel falloff used) to setup the HDR, I'd need to tweak a bit of the U co-ordinates, to "shift" the reflections, so that they reflect properly.

Digital Matte:
The original concept was to just use the original plate for the final shot, but after seeing it on my PC, it was so boring. So I decided a quick sky replacement was in order:

	Original Backplate. With a very exciting skyline.... ladies and gentlemen, it's the top of a multi-storey carpark! Woot!
	I couldn't pull much of a mask from the channels, so I manually cleaned up unwanted areas.
	A new background was dropped in. Not shown is the color correction. note: the final backplate's color correction are different, as I'd done some TV tests. I had to bring down the reds of the roof, and highlights of the foreground.
	A new sky was layered in. Love those white puffy clouds.
	I hope you see *some* difference. I cloned in some photographs, and painted in some small structures. And that's it! More adjustments were done while compositing. Also, the backplate markers, were removed later. I just love 'em :) These were empty 35mm film canisters covered in a red-pink sticky paper. They stand out, excellent for camera matching. And they are able to survive heavy gusts of wind without falling over! Woot!

Camera Matching:
Fooling around with a camera and different focal lengths yielded the a good match to the markers, allowing for the construction of a CG "road". This object is had a matte/shadow material applied to it, allowing it to catch cast shadows.

Character Setup:
Setup was very simple. I had 5 seperate controls, 3 for roll, pitch and yaw, 1 for movement , and another control to adjust the attitude of the nacelles/thrusters.

The advantage of this setup is that it allowed me to adjust each rotation axis individually, with curves that are easy to adjust and read. If I were to use a single control for all 3 axis, not only will the curves be confusing, it'd be so much easier to get into a gimbal lock. (i.e. short of using quaternions/tcb controller)

Animation:
I did the animation of the HogPod in several stages, starting with the gross movements. Once I was happy with how it moved, I then animated the roll/pitch/yaw channels. Finally, the nacelles were animated.

Since this craft isn't "real", I imagined that this baby has an anti-gravity drive (or Minovsky craft system haha) of some sort, to allow it to take off with its engines pushing it the wrong way. Still, I kept a tiny bit of realism, such as during the initial take off, the craft flips backwards, thanks to the propulsion on the front.

Exhaust Flame:
Two modeled mesh objects with a noise modified applied to selected vertices. A mesh smooth was applied on top of this lot to smooth it out. The flames are then parented to the individual nacelles. Midway through the animation, the trail expands and elongates, thanks to a few modifiers. It could have been done easier with a Free Form modifier, but I forgot argh! Still, for a one-off shot, it works :) One wonders how the exhaust flame would look without animation, since there will be a distortion map on it... hmm...

Particle System:
The particle systems of this shot was used to produce a) the heat ripples and b) burn marks on the pavement and c) faint smoke due to the burn.

The heat ripples were simple to generate. They are basically just facing particles with a divergence placed at the four engine exhausts. These are rendered off as a seperate pass, and used as a displacement map during compositing.

The trails on the ground were slightly more trickly to set up. I had to create a system that only generated the burn marks when the tip of the exhaust trail touched the CG ground. This was done by using a particle system that projected a line of particles from the base of the exhaust to the tail of the exhaust. The particle age was set pretty low, so they'd die off if they got away from the exhaust end.

If any of these particles hit the CG plate, they'd first spawn a shape mark (for the burn trails), and more particles to simulate some smoke. You'll see the individual passes later in the compositing stage.

Compositing:
Individual elements will be on the left, and the comp with these elemnts will be updated on the right. Let's go!

We start out with the backplate.
This is the hero's diffuse pass. It was rendered without shadows, into two seperate files, one for the color data, and one for the alpha channel.	Once the seperate alpha channel is applied to the diffuse layer as a matte, it is cleanly seperated, perfect for use on the backplate.
The effects pass for the particles. As with the diffuse pass, there is a seperate alpha chanel to be used as a matte.	Here we have the effects comped in. Some gaussian blur was added to break up the geometrical shapes of the particles.
This is one shadow pass specifically for the hogpod. Another shadow pass, for the cast shadow on the CG environment.	Shadows added to the plate. Since it is possible to control the shadows independantly, the HodPod's shadows are adjusted to be lighter, due to ground bounce. The ground shadow was also tweaked so it isn't fully dark. Basically, these shadow passes are used as *mattes*, which allow the use of levels/curves/hue sat/your favourite tweaking tool to specific, shadow covered regions. This allows you to even lighten shadow areas, or change colours. Works better than just using the shadow pass inverted in multiply mode.
The flames pass.	Here we have them layered on several times in multiply mode, add mode, gausian blurred etc. By layering them on, with each layer a different effect, we can get some specular blooming effects as well as getting the flame brighter.
Yep the boring distortion pass. Just particles.	Unfortunately, I don't think you can see much here :( It's (obviously) much more evident when seen animated.
Lightwrap comp. To simulate light "bending" around the edges perpendicular to the camera's view. This is not a seperate pass, but a comp created using the alpha channels of the diffuse pass. I will not go into detail here, but you can find out how to do it in the book "After Effects in Production", in the 3D Mechanic chapter. Or search the web :)	It's a very subtle effect that helps anchor the cg element to the backplate.
	Some color corrections and shadow tweaks, bit of blooming, blurs to specific elements (e.g. hogpod has a 0.5 pix gaussian blur). I had to test this part between my PC and my TV & DVD player. I'd revisited the backplates again, to create a subtle cloud movement using MeshWarp (in AE). After which, some grain and camera shake added.

Well that's it! I hope you find it interesting.

Alvin