[Numpy-discussion] numarray: need Float32 abs from array of type na.Complex64 or na.Complex32
jmiller at stsci.edu
Thu Mar 2 07:14:03 CST 2006
Sebastian Haase wrote:
> I just was hunting a very strange bug: I got weird results .... anyway.
> I found that I was using this line of code:
> if img.type() in (na.Complex64, na.Complex32):
> img = abs(na.asarray(img, na.Float32))
> Now I don't understand how this code worked at all ? But I only saw those
> "weird results" after looking at the "dimmest corners" in my image and
> otherwise all images looked O.K.
> The goal here was to get a single precision absolute for either a complex64 or
> complex32 valued image array WITHOUT creating a temporary array.
This idea sounds inconsistent to me. If the array is Complex64 and you
pass it into asarray(,Float32), you're going to get a temporary. AND
you're also truncating the imaginary part as you downcast to Float32...
you're not getting a complex magnitude.
Since abs() transforms from complex to real, you're going to get a
temporary unless you get a little fancy, maybe like this:
na.abs(img, img.real) # stores the abs() into the real component of
the original array. img.real is a view not a copy.
img.imag = 0 # optional step which makes the complex img
array a real valued array with complex storage.
img = img.real # just forget that img is using complex
Note that img is now discontiguous since no copy was made and there are
still interleaved 0-valued imaginary components. So, I think there
are two points to avoiding a copy here: (1) use the optional ufunc
output parameter (2) store the abs() result into the .real view of the
original complex array.
> My thinking was that:
> img = na.abs( na.asarray(img, na.Complex32) )
> would create a complete temporary version of img starts out being Complex64
> Is this really the case ?
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