Help for GRIDCOMP

PURPOSE
     GRIDCOMP is a VICAR applications program which composes two
     GEOMV grids into a single grid so that a single GEOMV can
     be performed in place of two.  This avoids double resampling
     of the image pixels in the overall operation.  High geometric
     accuracy can be maintained by using high resolution grids
     (say, 500 x 500 or more).
     
     The resulting grid is in a four column IBIS file with the
     normal column order (1,2,3,4) in double precision.  The inputs
     can be single or double precision, or integer.
	   
CALL
     gridcomp (GRID_A,GRID_B) GRID_OUT ACOLS=(C1,C2,C3,C4) BCOLS=(C1,C2,C3,C4)
  WHERE:
     GRID_A         is the first grid in GEOMV format.
     GRID_B         is the second grid in GEOMV format.
     GRID_OUT       is the output grid in GEOMV format.
     ACOLS          are the columns used in the first ibis file.
     BCOLS          are the columns used in the second ibis file.
     
     The GEOMV format of the warp grid will now be described.

     The IBIS-file form of the grid contains four columns of
single or double precision tiepoint records.  The columns in
order are (newline,newsamp,oldline,oldsamp); or the column order
can be user specified.  NAH and NAV will be calculated from the
grid and the grid must be rectangular.

     The TIEPOINT-GRID is a set of points describing the relation
of the output file to that of the input file using the keyword
TIEPOINT and optionally NAH and NAV which are ignored.
   NAH=nah  NAV=nav
     tiepoint=(nl1,ns1,ol1,os1,nl2,ns2,ol2,os2,...
                   ...nlk,nsk,olk,osk)

where the grid is rectangular in the output image space, nah is
the number of grid cells horizontally (across the top in the sample
direction), nav is the number of grid cells vertically in the output
image space, the point pairs (nli,nsi,oli,osi) are line-sample
coordinates in the output (new) and input (old) spaces respectively.
the number of pairs k must equal (nah+1)*(nav+1).  The grid must be
perfectly rectangular in the output image space (the rows and
columns must be perfectly horizontal and vertical respectively).
Each direction must be evenly spaced as well, but all values can
br fractional.  There are no keywords NAH and NAV, the true
values are calculated from the grid.
  
OPERATION

GRIDCOMP copies the second input, keeping the output grid as is
(except perhaps for conversion to double precision), and substituting
for the input grid points, the values that passed through the first
grid would yield the inputs.

For points that fall outside the first grid, an extrapolation process
similar to that of tieconv is used.  This should be avoided, but is
provided as a convenience in case the edge effects are not significant.

PERFORMANCE

Guess about a minute for a 500 x 500 grid.

Restrictions
------------

EACH OF THE TWO INPUTS HAVE A PAIR OF GRIDS THAT COULD BE CALLED
"INPUT" AND "OUTPUT".  THESE MUST SATISFY THE REQUIREMENTS FOR
GEOMV.

THE OUTPUT GRIDS MUST BE UNIFORMLY SPACED IN EACH DIRECTION
(MORE RESTRICTIVE THAN LGEOM).  THE SPACING VERTICALLY DOES
NOT HAVE TO EQUAL THE SPACING HORIZONTALLY THOUGH.
ON THE OTHER HAND, THE OUTPUT GRID VALUES CAN BE FRACTIONAL (LGEOM
REQUIRES WHOLE NUMBERS).  THESE REQUIREMENTS ARE EASY TO MEET IF THE
GRID IS GENERATED MATHEMATICALLY OR IF A PROGRAM SUCH AS TIECONV IS
USED.

Original Programmer: A. L. Zobrist, 23 Jun. 2000.
Current Cognizant Programmer: B. A. McGuffie
Revisions:
  2007-12-29 WLB Switched to USES_ANSI_C AND LIB_CARTO; misc cleanup
  2016-01-08 WLB Migrated to MIPL; replaced printf with zifmessage

PARAMETERS: