Using masking

Image Analyst MKII uses special pixel values for masking. There is no separate mask layer. Each pixel is stored as a 32bit floating point number, thus has a numeric value between ±3.4´10³⁸ or as little as 1.2´10^-38. In addition pixels may have infinite or no numeric values, these pixels are considered as masked. Therefore, masking happens by setting pixels to non-numeric values, e.g. by division by zero. This also implies that masking is permanent, cannot be removed from an image.

Properties of the mask:

• Masked pixels show up in black regardless the used LUT, and the 'MASK' pixel value is indicated
• Masked pixels are excluded from:
  • ROI mean, sum, SD, etc... calculation
  • histogram, percentile, Otsu's optimum threshold value calculations
• Most image processing functions work with masked images, and the result of any operation on a masked pixel is mask. This results dilation of the mask when using kernel filtering, or morphological operation on a masked image.
• Some functions, like DFT filtering are incompatible with masking, therefore zeros are substituted into the masked pixels.
• To substitute any value into masked pixels use the Segmentation/Fill Mask function

There are multiple ways to mask an image:

• Use the Segmentation/Mask Image function to mask using a binarized image:
  1. Select all images to mask as Image A, and the binarized image as Image B. Press in the tool bar.
     This function multiplies and then divides Image A by Image B.
• Divide the image by a binarized image using the Math/Image Arithmetic or Math/Image Arithmetic in Place function:
  1. Type: / or MASK
  2. Select all images to mask as Image A, and the binarized image as Image B. Press in the tool bar.
     The MASK operation multiplies and then divides Image A by Image B.
• Use the Segmentation/Threshold function:
  1. Way: Mask Above or Mask Below