The blend mask blends the border between the keep and delete masks. Fluid Mask uses the characteristics of the edge between two image objects to produce an optimal border. If the edge is sharp, the resulting border will be sharp. If the edge is soft, the border will be blended across the width of the original transition. You can paint a thinner blend mask to produce a thinner blend on the border of the cut-out.
See Edge Blending Options for more information about the Edge Blending Options palette.
Fluid Mask has two methods for tackling the blending across the edges of a cut-out border:
Feather: creates a simple even transition across the edge.
Smart: varies the blend according to the characteristics of color and texture variations around and across an edge.
Smart blending typically produces higher quality results. However, if an edge has low contrast or a lot of noise, the edge produced may be uneven. In such circumstances, Feather blending should be used. If you set the Intelligent Blending slider all the way to one side or the other, Fluid Mask will always use the chosen method for blending. If you choose one of the middle settings, Fluid Mask will choose which method to use based on edge characteristics. If the slider is closer to one side, the choice will be weighted toward that method. Fluid Mask makes the decision for each edge along the border of your cut-out.
The following image illustrates the benefits of smart blending:
The inset image shows the original picture. Note the fine hairs against the blue sky. The top of the main image has been cut out using Smart blending. The hairs are preserved and all the blue background has been removed. The bottom part of the image has been cut out using feather blending. The hairs are almost entirely removed and a blue fringe is evident along the blended border.
The next image illustrates a situation in which feather blending is the better choice:
The inset image shows the original picture, which suffers from low color contrast and a fair amount of noise. The left side of the main image has been cut out using smart blending. Note the rough edge along the border. The right side of the cut-out shows feather blending, which can produce a smoother border on low contrast, noisy images.
When you set the Intelligent Blending slider to a position between the two extremes, Fluid Mask will select the type of blending based on the contrast across the edge. Setting the slider toward one side or the other (i.e. 3:1 or 1:4), will make it more likely for that type of blending to be used.
Note: You can view a blended border in a test render or the Output view more easily by choosing a strongly contrasting background color.
You can paint blend masks manually, but you can also let Fluid Mask apply blend masks for you automatically. The Edge Blending options include an Blend Mask Width setting. If you choose any option other then None, then Fluid Mask will create a blend-mask border whenever you paint a keep mask against a delete mask. (To prevent Fluid Mask from adding a blend-mask border, set Blend Mask Width to None.)