This is something of a FAQ: how does one apply a mask (a.k.a. matte) to an effect in Conduit, so that the effect is applied only to certain parts of the image?
There are a few ways to accomplish this. If the effect to be masked is a single node and it has an Effect opacity parameter, you can simply plug the mask image into this parameter input:
But if the effect is more complex, maybe consisting of multiple nodes, you’ll need to use a node to combine the original image with the effect. You could use the Blend node for this purpose:
The Blend node’s Bias parameter controls the blending of the two input images. This parameter only accepts a scalar (=greyscale) image as input (as indicated by the light-blue color), so I’ve added a Separate RGBA node to separate the alpha channel from the mask image.
Alternatively, you can achieve the exact same result by using an Over node:
The light-blue middle input on the Over node is the mask that controls where the top image is composited over the bottom image. In this case, the top image is the one with the Bezier Curve applied.
Whether to use Blend or Over is entirely up to personal preference. (I prefer Blend because it somehow makes more sense to me to think of this operation as blending two images, rather than compositing one on top of another. But like I said, the end result is exactly the same either way.)
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