Subtitles of the Movie

In this movie, we will learn what premultiplied images are and how best to work with them in our composites. Our images are comprised of red, green, and blue channels with transparency information in an alpha channel. When our images are premultiplied, it means that our alpha channel is multiplied by our color information. This means that our color information is baked into our color information. One way to think of it is in terms of grade school math. Any number times zero is zero. In a premultiplied image, the color values in the RGB channels are multiplied by zero, or black values in the alpha channel, resulting in transparent areas. When color values are multiplied by 1, or white, it's a number multiplied by 1 results in the number itself, so we end up seeing the color. It is possible to have unpremultiplied images where the color and alpha values are handled separately, but why would we want to do such a thing? In Shake, premultiplication can be a curse or a blessing. It's crucial to use premultiplied images when using transform and filter nodes. But it's often necessary to temporarily turn off or unpremultiply an image during color correction. Why? Because keeping the image premultiplied can cause edge fringing, or worse, the color of the whole image might change because of the way a premultiplied alpha channel is being used in the color correction. To unpremultiply a node prior to color correction, select the node and then go to Color Mdiv to divide the image by its alpha. An easy way to remember this is to refer to the alpha as the mask and the Mdiv as the mask divide. Then perform your color corrections. Once your color corrections are finished, attach a mask multiplier to premultiply once again. You'll find this in the color tab - Mmult node. After this, you can do your layering, filters, and transforms. To review, premultiplication means that an image's color channels have been multiplied by the alpha channel. Before adding color nodes to a premultiplied image, apply one Mdiv node, located in the color tab. Then do all of your color corrections. You don't need to do an Mdiv before each color correction, just at the beginning of the color correction chain. Once you've completed all of your color corrections, apply an Mmult node to remultiply your alpha channel by your image. Then you can use your Over node or another layering node, apply your transforms and apply your filters. For more information and examples of premultiplication gone bad, refer to the Shake 4 user manual, found in the Help section. There's a section devoted to premultiplicaiton starting on page 421 of the PDF manual.