Apple Shake 4 Tutorials Instrukcja Użytkownika Pobierz pdf (Strona 102)

102 Chapter 3 Depth Compositing

Color Correcting Premultiplied Images

To create a more realistic lighting effect, you can color correct the foreground objects

to “reflect” the ambient colors of the background. Use the Color–Add, Color–

ContrastLum, and Color–Brightness nodes to correct the balloons image.

The images in this tutorial are premultiplied, which means that the RGB pixel values are

appropriate for the transparency indicated by the alpha channel.

For example, an RGB pixel value of 0.75 multiplied by an alpha value of 1.0 (a white

pixel) results in the full value of that pixel being passed to the composite. An RGB pixel

value of 0.75 multiplied by an alpha value of 0.0 (a black pixel) would become zero (and

result in 0 percent opacity). Gray values in the alpha result in varying levels of opacity.

For images that are not premultiplied, an alpha channel may be present but there is no

transparency “baked into” the RGB channels.

When performing color corrections in Shake, premultiplication can be an issue when

the correction alters the RGB channels that correspond to semi-transparent areas of the

alpha channel. This can affect the layering operations in your composite.

One way to deal with premultiplied images is to toggle the preMultiply control in the

layer nodes you’re using.

Another technique is to manage premultiplied images with MDiv and MMult nodes

when performing color correction.

Premultiplied: RGB pixel values are multiplied by the alpha pixel values and interpreted as

No premultiplication: RGB and alpha channels exist in the file, but there is no transparency.

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Apple Shake 4 Tutorials Instrukcja Użytkownika Strona 102