## OtfToPsf

##### >Psychtoolbox>PsychOptics

OTFTOPSF Convert a 2D optical transfer fucntion to a 2D point spread function.

[xSfGridCyclesDeg,ySfGridCyclesDeg,otf] = PsfToOtf([xGridMinutes,yGridMinutes],psf)

```
Converts a optical transfer function specified over two-dimensional
spatial frequency in cycles per degree to a point spread function
specified over positions in minutes. For human vision, these are each
natural units.
The input spatial frequencies should be specified in matlab's grid
matrix format and sf for x and y should be specified over the same
spatialfrequency extent and with the same number of evenly spaced
samples. Spatial frequency (0,0) should be at location floor(n/2)+1 in
each dimension. The PSF is returned with position (0,0) at location
floor(n/2)+1 in each dimension. This is the form we mostly want to
look at and use.
The OTF is assummed to have the DC term in the center poistion,
floor(n/2)+1 of the passed matrix. If you have your hands on an OTF
with the DC term in the upper left position (1,1), apply fftshift to
it before passing to this routine. The DC in upper left is the Matlab
native format for applying the ifft, and is also the format stored by
isetbio in its optics structure.
Positions are returned using the same conventions.
No normalization is performed. The psf should be real, and we
complain (throw an error) if it is not, to reasonable numerial
precision. If it seems OK, we make sure it is real.
We also make sure that the returned psf is all postive and sums to
1. In some cases, we found that there were small negative values
and after setting these to zero renormalization was needed.
We wrote this rather than simply relying on Matlab otf2psf/psf2otf
because we want a routine that deals with the conversion of spatial
frequency to spatial support.
If you pass the both sf args as empty, both position grids are
returned as empty and just the conversion on the OTF is performed.
[PsychOpticsTest](PsychOpticsTest) shows that this works very well when we go back and
forth for diffraction limited OTF/PSF. But not exactly exactly
perfectly. A signal processing maven might be able to track down
whether this is just a numerical thing or whether some is some small
error, for example in how position is converted to sf or back again in
the [PsfToOtf](PsfToOtf).
```

Optional key/value pairs:

‘warningInsteadOfErrorForNegativeValuedPSF’ - Set to 1 (default

0) to get a warning

not an error if the psf

values are too negative

before they are forced

to zero. Set to 2 for

no warning msg or

error.

‘negativeFractionalTolerance’ - The error/warning is

thrown if the magnitude

of the most negative

value is more than this

fraction of the maximum

(positve) value.

(Default 1e-3).

See also: PsfToOtf, PsychOpticsTest.

`Psychtoolbox/PsychOptics/OtfToPsf.m`