Compositing

The Background Image is similar to the gradient backdrop; however, it is always registered to the camera. That is, it will always appear in exactly the same position/location no matter which way you tilt or move the camera. You set this option on the Compositing Tab of the Effects Panel.

Background images are considered infinitely distant from the camera. You can never have an object behind a background image, nor can you light a background image or cast shadows upon it.

Background images stretch to fit the Camera resolution and frame aspect that you are using. Make sure to use similar-sized background images if you wish them to match.

You often use background images to merge live action and 3D elements, like creating the illusion that a UFO crashed into the Empire State building. An image of the New York skyline would be the background image and the UFO would be a LightWave object.

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Background images are visible only through the Camera View.

If you set Camera View Background on the Display Options Tab of the Preferences Panel to Background Image, you will see the set Background Image in Layout’s Camera View.

Of course, actually seeing the background in the Layout window is optional. The background image will always appear in the rendered frame.

The whole idea behind traditional image compositing is quite simple: take two or more images, and merge them into a new composite image. LightWave lets you do this, but also takes it one step further by letting you throw objects into the mix.

The images can also be a sequence of pictures, so you may use captured video footage as a background for your objects. A simple example would be a modeled UFO moving against a real sky and trees. Images can appear behind all objects, in front of all objects, or a combination of the two with objects showing in-between.

Compositing can be a render time-saver. If you set up a scene with many objects, but only a few are moving, you could render one frame with only the inanimate objects, and then remove all of those objects and render the animated objects against the single frame saved earlier. This is especially useful when the still objects are complicated or involve radiosity, refraction and/or reflection.

Placing an image in front of everything may seem like a silly thing to do. However, some options let you cut parts of the image away so you can see through it. You can also make the image partially dissolved or even envelope the dissolve. Dissolving in a black image in the foreground will produce the common fade-to-black effect, or reverse the dissolve to fade in from black.

You can make holes in the foreground image based on a defined color range in the image. However, the biggest drawback to simply clipping out portions of the foreground image is that you will have hard edges. A color is either clipped or it isn’t, so you see either the foreground image or the background image.

Use the Foreground Image dropdown to set the foreground image.

You may also use a special alpha image to merge the background and foreground images. This type of alpha image is very different from the type you use to set up a transparency surface texture. LightWave composites the foreground image over any objects or background by adding their colors to the foreground image. How much background is added is determined by the alpha image. The darker the area of an alpha image, the more background is added to the foreground.

The pseudo-mathematical equation might look like:

Foreground + (1 - Alpha) * Background

If you used the same exact image for both the background and foreground images, plus a solid black image as the alpha image, you will receive a final rendered image where every pixel is twice the color value it was. This results from the background image being completely added to the foreground image.

Generally, alpha images will be generated when you render a scene to create the foreground images. When you select Save Alpha on the Render Properties Output Files Tab , LightWave will generate and save an alpha image in addition to the normal RGB output. The alpha image will be composed of grayscale values representing the opacity of any objects that were rendered in the scene.

An object that contains no transparent surfaces will be rendered as solid white. Transparent surfaces will render in some shade of grey depending on how transparent they are. One hundred percent transparent surfaces render as black. A 50-percent transparent surface will render as 50-percent gray. Using object dissolve, anti-aliasing, motion blur, and so on. will also give you values other than white in an alpha image. Any background (image or colors) will be black in the alpha image, as will any additive effects such as glow or lens flare.

Since glows and lens flares are additive effects and are assigned a value of black in an alpha image, glows and lens flares in the actual foreground image will simply have the background values added, so they will appear brighter where the background is a value other than black.

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Generally, due to LightWave’s additive compositing method, foreground images are created using a solid black backdrop. This allows the composited background to show through unaltered when it is mixed 100 percent with the foreground.

What happens if you want to composite a foreground element on top of objects or a background, but the foreground image was not rendered over black? In this case, LightWave provides a Foreground Fader Alpha button that can be selected. In this mode, LightWave will ignore any areas of the foreground image corresponding to black areas in the alpha Image. These areas of the foreground image will be faded away to nothing so you see 100 percent of the background instead.

When using Foreground Fader Alpha, glows and lens flares will not be added to the final rendered image (their corresponding alpha is black) unless you are using a different alpha image containing non-black areas in those locations. Additionally, anti-aliased object edges most likely will stand out because they contain bits of color from the original non-black background.

Activate Foreground Key when you want to key out (i.e., not render or see) a color (or range of colors) from the foreground image. Use the same color value for both Low Clip Color and High Clip Color if you wish to key out one color only. Use different values to key out the Low Clip Color and High Clip Color values, and all those in between.

The Low Clip Color is the color value for the darkest color value that will be keyed out. High Clip Color is the brightest color value that will be keyed out.

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Contributed by Richard Feeney