Image Algebra - Thresholds and Limits

Limits

In addition to the limits imposed by the data type of the output image, it is possible to limit the range of pixel values that are produced by the algebraic operations. This may be useful if you wish to constrain the output image intensity values to be within physically-meaningful limits.

To limit the minimum / maximum output pixel values, enter the values in the Min and Max fields:

algebra_limits

In the example above, the minimum possible pixel value is zero, and the maximum is 1000. Values computed that lie outside this range would be truncated (clipped) to lie within the range. To remove any limits, remove the text from these fields.

Threshold

In certain situations, for example to avoid applying the formula to background noise, it may be useful to use a threshold on the pixel intensity so that pixels that are below the threshold do not have the formula applied to them, but simply have the intensity in the output image set to zero. To restrict the calculation to pixels above the threshold, type a non-zero value into the Threshold field:

algebra_threshold_50

In the example above, the threshold is set to 50: if the pixels at a particular position in all the input images are above 50, the formula is applied to that pixel position as normal; if not, then the pixel in the output image is set to zero intensity.

Output Image Masking

You can mask the input images, so that the Algebra calculation is only performed for pixels inside the mask; for pixels outside the mask, the output image will have a zero pixel value. Use the standard image masking options to use a mask.

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