AIPS HELP file for PGEOM in 31DEC22
As of Thu Jun 8 16:38:12 2023
PGEOM: Task to transform an image into polar coordinates.
INNAME Input image (name)
INSEQ 0.0 9999.0 (seq. #)
INDISK 0.0 9.0 (disk unit #)
OUTNAME Output image (name)
OUTSEQ -1.0 9999.0 (seq #)
OUTDISK 0.0 9.0 (disk unit #)
BLC 0.0 4096.0 Bottom left corner of image
TRC 0.0 4096.0 Top right corner of image
IMSIZE 0.0 2048.0 Output image size in pixels
APARM (1)=ref-x (pix),(2)=ref-y,
(9)=spir.rad. ,(10)=zero flag
Use: PGEOM will transform an input image into polar coordinates
and back into rectangular coordinates using Everett
interpolation (bilinear, bicubic, or biquintic). PGEOM is
capable of doing arbitrary transforms of any size image since it
now uses dynamic memory. It can "correct" for inclination and
unwrap spiral objects. Be aware that the interpolation method
causes edges and blanked pixels to expand into the output image.
INNAME........Input map name. Standard defaults.
INCLASS... ...Input map class. Standard defaults.
INSEQ.........First input map seq no. 0 => highest.
INDISK........Input map disk: 0 = any.
OUTNAME.......Output map name. Standard defaults.
OUTCLASS......Output map class. Standard defaults.
OUTSEQ........Output map seq no. 0 => highest unique.
OUTDISK.......Output map disk. 0 => highest with room.
BLC...........The bottom left-hand pixel of the input image
which becomes the bottom left corner of the
input subimage. The value (0,0) means (1,1).
TRC...........The top right-hand pixel of the input image
which becomes the top right corner of the
subimage. The value (0,0) means take the top
right hand corner of the image.
1 & 2 = (x, y) location where the "center" is
located. This need not be the actual center
of the image. It is simply the reference
point from which the radius and angle are
computed. THERE IS NO DEFAULT. These are
the radius zero pixel coords in the input
image (APARM(7) <= 0) or the output image
(APARM(7) > 0).
3 = inclination angle - the angle between the
plane of the object and the plane of the
sky (i.e. 0 indicates the object is parallel
to the sky, 90 indicates the object is
perpendicular to the sky). This parameter
allows for deprojection of objects such as
4 = rotation angle - the angle between the
plane of the object and a horizontal plane.
5 = border size. This parameter specifies the
number of pixels that should be added to each
side of the output picture. Thus, negative
radii pixels can be created for use by the
interpolator. If APARM(5)<=0 it defaults to
APARM(6)/2 + 1.
6 = Order of Everett interpolation formula
[range 1-7: 1=bilinear, 3=bicubic,
5=biquintic, 7=biseptic] 0 => 3.
Higher orders will produce more accurate
interpolations although oddities may appear when
the output cell size is significantly smaller than
the input cell size. See extensive discussion in
7 = inversion flag. Causes the inverse of the
specified transformation to be performed.
Useful to undo previous operation.
8 = spiral flag. Causes the program to unwrap
a spiral with a specified reference radius.
The reference radius is the radius of the
spiral at the rotation angle specified. The
program assumes a linear relationship between
the angle and radius of the spiral.
9 = spiral radius. This is the reference radius
for the spiral. It is the spiral's radius at
the specified rotation angle. Must be greater
than zero (0.0 => 1.0).
10 = zero-flag. If > 0.0, zeroes are used
wherever blanks would normally be created in
the output image.
PGEOM - an incomplete explanation.
(numerical analysis topics below apply to LGEOM and HGEOM too)
On the output headers
The output image (when APARM(7) <= 0, i.e. rectangular to polar)
has radius on the X axis with increment SQRT (x-incr * y-incr)
and angle on the Y axis. If the first two input axes are a sky
position pair (i.e. longitude and latitude), the angle is the
position angle measured CCW from north.
Maintaining header information in this transformation is tricky.
Most AIPS programs will not give correct RA and Decs for pixels
in the radius-angle output image. Header information is not
lost however so long as the input rectangular image has no more
than 5 axes. If your header has more than 5, use TRANS to make
sure that axes 6 and 7 are null (only 1 pixel).
On the Relation of Input Image Size to Output Image Size:
The adverb IMSIZE specifies the dimensions of the output image
more or less. When APARM(7) <= 0 (rectangualr to polar), the
number of rows is IMSIZE(2) + 2*APARM(5) with a default for
IMSIZE(2) of TRC(2)-BLC(2)+1. The number of columns is set to
the maximum of IMSIZE(1) and some side of the input subimage.
If IMSIZE(1) <= 0, the number of columns is the diagonal of the
input image in pixels. One APARM(5) is then added. When
APARM(7) > 0 (polar to rectangular), the number of rows is
IMSIZE(2) - 2*APARM(5) with the same default. The number of
columns is IMSIZE(1) with a default of 2 * (TRC(1) - BLC(1) + 1
-APARM(5)) (note the 2).
A Discussion of Blanking in PGEOM:
Output pixels are computed as a weighted sum of the input
pixels in the neighborhood of the computed location in the
input (the interpolation weights depend on the fraction of the
pixel spacing). If one of the input pixel values in the sum is
blanked, i.e., indefinite in value, then the whole sum, i.e.
the output pixel, is blanked. If APARM(9) is true, then instead
of blank a zero is delivered as the output pixel value. As a
result, if an isolated pixel in the input is blank it will
produce blanks (or zeroes) in a region in the output image the
same size as the interpolating kernel (4x4 pixels for bicubic,
8x8 for biseptic). This is not nice, but all suggested cures
considered so far have other difficulties.
There is a special case implemented for the edges of the input
image. Here PGEOM "reflects" the pixels near the boundary out
into the terra incognita and uses them for interpolation. This
means that pixel 1 is copied into the pixel 0 position, pixel 2
is copied into pixel -1, etc. The effect of this algorithm,
implemented on all four edges, is that first derivatives of the
interpolant are zero at all boundaries. NOTE: "Boundary" in
this case also means the window defined by BLC and TRC.
If the output image is larger than the transformation of the
input window then the extra area will be blanked (or zeroed).
Regarding the Everett Interpolation Formula:
This program computes where in the input image each output pixel
appears. This location is generally at some arbitrary fractional
pixel position in x and y, surrounded by four input pixels. The
program then interpolates in the pixel values surrounding the
position. For APARM(6)=1 it uses linear interpolation, first in
in x (twice) and then in y. This is called "bilinear"
interpolation, and is the default choice.
If APARM(6)=3 two pixels on each side in x and y are used,
sixteen pixels total. A cubic polynomial is fitted through the
four pixels in each of the four rows in x, and evaluated at the
computed x-position. Then a cubic is put through those four
values (in y) and evaluated. This is "bicubic" interpolation.
For APARM(6)=5 we use a 6x6 array, 36 pixels total; this is
"biquintic" interpolation. For APARM(6)=7, we have 8x8, 64 pixels,
Regarding the Numerical Accuracy of Interpolation:
The accuracy of interpolation increases with the order of
the polynomial, but of course so does the cost of computation.
As the polynomial order increases the interpolating function
approaches ever more closely to the sinc function, the perfect
bandlimited interpolator, as a limit. Bicubic is good enough
for many ordinary purposes. Biquintic will probably be good
enough even for high dynamic range data like optical CCD
imagery and self-calibrated VLA maps. The point is, if we
want very accurate results we want the "noise" of interpolation
errors to be less than the observational noise in the data at
all spatial frequencies of interest.
The interpolation is done by computing a set of interpolating
weights which are a function of the fractional pixel position
where the answer is desired. The answer is then computed by
multiplying the weights by the data. This is a convolution.
It follows that an interpolator of a given order has a
corresponding transfer function (i.e. in the terminology
of aperture synthesis, it acts somewhat like a peculiar,
probably "bumpy" taper function in the u-v plane). This
function is always going to be less than unity in the
neighborhood of the Nyquist frequency of the map. The size
of this "neighborhood" where the transfer function is
significantly different from unity is smaller for higher
order polynomials. It follows that maps which are well
sampled, say three or four pixels per beam, are easier
to interpolate accurately.
How to Evaluate the Numerical Errors of Interpolation:
DCW is not aware of any proper quantitative treatment of
these accuracy issues anywhere in the literature. This
situation is not as bad as it seems because for critical
work there are two simple tests which can be used to
estimate the interpolation error:
Test1: compute a simple change, reverse it, then difference
input and output. E.g., transform the input image into
polar coordinates, then invert the transformation. Use COMB
to subtract the second output from the input. The difference
is all interpolation noise. If that noise level (due to TWO
interpolations) is small enough to suit you then you know that
the noise of the first answer, the rotated image, is even lower
(only one interpolation). Actually, this test is excessively
conservative because the transfer function of interpolation is
SQUARED by the double calculation (NOTE: any multiple
application of interpolation to data will square, cube, etc.,
the transfer function of data. Mostly one loses high spatial
frequencies in the data, but errors creep into the lower
spatial frequencies, too.)
Test2: compute a small change with one order of interpolation,
compute again with increased order, then difference the two
results. E.g., transform the input image into polar coordinates
with APARM(6)=3 (bicubic), then perform the operation again with
APARM(6)=5 (biquintic). Use COMB to subtract the the first
answer from the second. The difference is an estimate of the
error of bicubic interpolation for your image. An even more
accurate estimate of the error would be obtained by using
APARM(6)=7 (biseptic) for the second computation. Although this
test does not inspire the degree of conviction which Test1 can
by reproducing the input image, it more than makes up for this
by giving an estimate of the interpolation error of a SINGLE
NOTE: in performing tests such as are suggested above it will
make sense to use BLC and TRC to select a small region of the
input image (say, 100-square) which contains interesting
structure whose details need to be preserved. This will save
time and make it reasonable to perform both Test1 and Test2,
and to use biseptic interpolation for Test2. For example, one
could perform Test1 using biseptic interpolation in order to be
convinced that it's results are accurate. Then perform Test2
for bicubic and/or biquintic and compare them to the biseptic
See EXPLAIN LGEOM for a discussion of Virtual Memory in PGEOM