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# AP_YLEVEL -- Set the aperture to intercept the specified y level.
procedure ap_ylevel (imdata, npts, ylevel, peak, bkg, grow, center, low, high)
real imdata[npts] # Image data
int npts # Number of image points
real ylevel # Y value
bool peak # Is y a fraction of peak?
bool bkg # Subtract a background?
real grow # Grow factor
real center # Center of aperture
real low, high # Equal flux points
int i1, i2, j1, j2, k1, k2
real y, y1, y2, a, b, ycut, x
begin
if ((center < 1.) || (center >= npts) || IS_INDEF (ylevel))
return
if (bkg) {
i1 = nint (center)
i2 = max (1, nint (center + low))
for (k1=i1; k1 > i2 && imdata[k1] <= imdata[k1-1]; k1=k1-1)
;
for (; k1 > i2 && imdata[k1] >= imdata[k1-1]; k1=k1-1)
;
i2 = min (npts, nint (center + high))
for (k2=i1; k2 < i2 && imdata[k2] <= imdata[k2+1]; k2=k2+1)
;
for (; k2 < i2 && imdata[k2] >= imdata[k2+1]; k2=k2+1)
;
a = imdata[k1]
b = (imdata[k2] - imdata[k1]) / (k2 - k1)
} else {
k1 = center
a = 0.
b = 0.
}
i1 = center
i2 = i1 + 1
y1 = imdata[i1] - a - b * (i1 - k1)
y2 = imdata[i2] - a - b * (i2 - k1)
y = y1 * (i2 - center) + y2 * (center - i1)
if (peak)
ycut = ylevel * y
else
ycut = ylevel
if (y > ycut) {
for (j1 = i1; j1 >= 1; j1 = j1 - 1) {
y1 = imdata[j1] - a - b * (j1 - k1)
if (y1 <= ycut)
break
}
if (j1 >= 1) {
j2 = j1 + 1
y2 = imdata[j2] - a - b * (j2 - k1)
x = (ycut + y2 * j1 - y1 * j2) / (y2 - y1) - center
low = max (low, (1.+grow)*x)
}
for (j2 = i2; j2 <= npts; j2 = j2 + 1) {
y2 = imdata[j2] - a - b * (j2 - k1)
if (y2 <= ycut)
break
}
if (j2 <= npts) {
j1 = j2 - 1
y1 = imdata[j1] - a - b * (j1 - k1)
x = (ycut + y2*j1 - y1*j2) / (y2 - y1) - center
high = min (high, (1.+grow)*x)
}
} else {
for (j1 = i1; j1 >= 1; j1 = j1 - 1) {
y1 = imdata[j1] - a - b * (j1 - k1)
if (y1 >= ycut)
break
}
if (j1 >= 1) {
j2 = j1 + 1
y2 = imdata[j2] - a - b * (j2 - k1)
x = (ycut + y2 * j1 - y1 * j2) / (y2 - y1) - center
low = max (low, (1.+grow)*x)
}
for (j2 = i2; j2 <= npts; j2 = j2 + 1) {
y2 = imdata[j2] - a - b * (j2 - k1)
if (y2 >= ycut)
break
}
if (j2 <= npts) {
j1 = j2 - 1
y1 = imdata[j1] - a - b * (j1 - k1)
x = (ycut + y2*j1 - y1*j2) / (y2 - y1) - center
high = min (high, (1.+grow)*x)
}
}
end
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