Mask

Sets the pixels of a target image to a certain value depending on the pixels in some other image. This allows several types of special effects, such as combining parts of two images. The term ``mask'' is from photography, where originally a black cutout was placed over the image, and the film was then double-exposed, to accomplish this.

1. Mask (1 &=2) Each pixel in image 1 is set to 0 if the corresponding pixel in image 2 is 0, otherwise it is untouched. ``Corresponding pixels'' are pixels the same x and y distance from the upper left corner of each image.
2. Inverse Mask (1 &=~ 2) Each pixel in image 1 is untouched if the corresponding pixel in image 2 is 0, otherwise it is set to 0.
3. Add (1 + 2) The value of each pixel in image 2 is added to the value of the corresponding pixel in image 1. This was used, for example, in overlaying the watershed demarcation lines onto the original image in Sec. 7.16.

4. Subtract (1 - 2) The value of each pixel in image 2 is subtracted from the value of the corresponding pixel in image 1.
5. Multiply (1 * 2) The value of each pixel in image 1 is multiplied by the value of the corresponding pixel in image 2.