Getting around for loops in image processing

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Fri, 05/16/2014 - 10:46 am

I do not know if this exists, but for loops that replace some native built in function in scripting languages tend to be slower than built in functions that do exactly the same thing.

For matrices there exist scalar functions that apply automatically to each index in a pair (or more) of matrices. It's easy to take a cosine of a whole matrix, ect. Does there exist a way to apply if-then actions to each index of a wave individually without constructing a for-loop that runs through the indicies of the wave? I imagine that'd save some time if igor didn't have to re-interprete the if-statement every time it looked at a different scalar in a wave.
Do you mean something like this, which replaces even and odd values by 0 or NaN?

wave1 = (mod(wave1[p],2) == 0) ? 0 : NaN


Look on page 173 of the manual and for Operators in IV-5.

--
J. J. Weimer
Chemistry / Chemical & Materials Engineering, UAHuntsville

May 16, 2014 at 11:57 am - Permalink

Quote:
Does there exist a way to apply if-then actions to each index of a wave individually without constructing a for-loop that runs through the indicies of the wave?


Yes. Here is a demo of what you can do:

// To run the demo, execute:
//      Setup()
//      mat = NaN
//      Demo1()
//      mat = NaN
//      Demo2()
//      mat = NaN
//      Demo3()

Function Setup()
    Wave/Z mat
    if (!WaveExists(mat))
        Make/N=(5,5) mat = NaN
        Edit mat
    endif
End

Function Demo1()
    Wave mat
    mat = p==q ? 1:0
End

Function Demo2()
    Wave mat
    mat = SelectNumber(p==q, 0, 1)
End

Function Helper3(pp, qq)
    Variable pp, qq
   
    if (pp == qq)
        return 1
    endif
    return 0
End

Function Demo3()
    Wave mat
    mat = Helper3(p, q)
End


For details execute:
DisplayHelpTopic "Waveform Arithmetic and Assignment"

The whole topic will be a good refresher. The section "Example: Comparison Operators and Wave Synthesis" addresses your question.

May 16, 2014 at 12:01 pm - Permalink