include	<math/iminterp.h>

# EL_SUBPIX -- Integrates intensity distribution over target pixel. This
# is an experimental version using the IRAF interpolation routines in
# math$iminterp. Experiments with 3rd degree 2-dim polynomials and
# bi-cubic splines showed no improvement over the existing bi-linear
# interpolation. In particular, the bi-cubic spline interpolation is prone
# to introduce larger 3rd and 4th order harmonic amplitudes at radii
# <= 4 pixels.

real procedure el_subpix (pixels, x, y)

pointer	pixels
real	x, y			# fractional position of target pixel + 1

pointer msi
real	sum, datain[4,4]

real	msisqgrl()

begin	
	datain[1,1] = Memr[pixels]
	datain[2,1] = Memr[pixels+1]
	datain[3,1] = Memr[pixels+2]
	datain[4,1] = Memr[pixels+3]
	datain[1,2] = Memr[pixels+4]
	datain[2,2] = Memr[pixels+5]
	datain[3,2] = Memr[pixels+6]
	datain[4,2] = Memr[pixels+7]
	datain[1,3] = Memr[pixels+8]
	datain[2,3] = Memr[pixels+9]
	datain[3,3] = Memr[pixels+10]
	datain[4,3] = Memr[pixels+11]
	datain[1,4] = Memr[pixels+12]
	datain[2,4] = Memr[pixels+13]
	datain[3,4] = Memr[pixels+14]
	datain[4,4] = Memr[pixels+15]

	call msiinit (msi, II_BISPLINE3)
#	call msiinit (msi, II_BILINEAR)
	call msifit (msi, datain, 4, 4, 4)
	sum = msisqgrl (msi, x+.5, x+1.5, y+.5, y+1.5)
	call msifree (msi)
	return (sum)
end