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Where

Where[img] returns the positions of all nonzero elements in img.

Where[img, crit] returns the positions of all elements in img meeting crit.

The argument crit may be given in the form of a pattern object or PatternTest.

The argument img may be a list or ImageData expression.

Some common test criteria for single-channel images include the following:

To test for zero, use Where[img,_?(#

0&)] or Where[img,0].

To test for values in a range, use Where[img,_?( 0≤ #<64 &)].

Here are examples of tests on multichannel images in meshed format.

To test for zero on the third channel, use Where[img,{_,_,_?(#

0&)}] or the simpler form Where[img,{_,_,0}].

To test channels one or two for 0, use Where[img,_?(#

To test a composite condition, for example an average value greater than 128, use Where[img,_?(Plus@@#/3 > 128 &)].

See also User's Guide 7.2.

Examples

This loads the package.

In[1]:=

Here we find the positions of all the nonzero elements in a list.

In[2]:=

Out[2]=

Here we find the positions of all the elements greater than some chosen value.

In[3]:=

Out[3]=

Here we load a test image.

In[4]:=

Out[4]//InputForm=

Out[4]//InputForm=

Here we use a test to find all pixels with a large red channel value.

In[5]:=

Out[5]=

Here we use a test to find all pixels with a large cumulative color value (the sum of the three colors exceeds 500).

In[6]:=

Out[6]=

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