**CIGAL Reference Manual, Chapter 3 (Functions): MASK**
===== MASK -- Use a mask to select part of a variable =====
**usage: varb2 = mask(varb1,maskvarb,thresh,compressflag)**

MASK copies a portion of the input variable (VARB1) to the output
variable (VARB2), where the portion is determined by the MASKVARB
argument.  MASKVARB can be any of the following:
  .  - a variable with the same dimensions as VARB1; values in VARB1 that
       correspond to non-zero values in MASKVARB are copied to VARB2 (see THRESH)
     - a number: 
                 0 - use the cursor to enter a box (VARB1 values within the box
                     are copied)
                 1 - use the cursor to outline the region to be copied; press
                     <RETURN> or the left button to trace the outline, any other
                     key when done

The other arguments to MASK have the following meanings:
    THRESH:       If MASKVARB is a variable with many values, values less than or
                  equal to THRESH are considered zeros (default: THRESH = 0).
    COMPRESSFLAG: If non-zero, compress the output array by skipping values not
                  within the masked region (default: COMPRESSFLAG = 0).

The total number of pixels within the masked region is returned in register variable 0 (#0).

**See Also:**\\
[[jvs:cigal:manual:chapter1:iprocs|IPROCS(1)]], 
[[jvs:cigal:manual:chapter3:map|MAP(3)]], 
[[jvs:cigal:manual:chapter2:sketch|SKETCH(2)]], 
[[jvs:cigal:manual:chapter4:vimage|VIMAGE(4)]]

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