Index: trunk/psphot/src/psphotIsophotal.c
===================================================================
--- trunk/psphot/src/psphotIsophotal.c	(revision 13983)
+++ trunk/psphot/src/psphotIsophotal.c	(revision 15562)
@@ -3,5 +3,57 @@
 bool psphotIsophotal (pmSource *source, psMetadata *recipe, psMaskType maskVal) {
 
-  psLogMsg ("psphot", PS_LOG_INFO, "not implemented\n");
+  assert (source->extpars);
+  assert (source->extpars->profile);
+  assert (source->extpars->profile->radius);
+  assert (source->extpars->profile->flux);
+
+  bool status;
+
+  psVector *radius = source->extpars->profile->radius;
+  psVector *flux = source->extpars->profile->flux;
+
+  // flux at which to measure isophotal parameters
+  // XXX cache this?
+  float ISOPHOT_FLUX = psMetadataLookupF32 (&status, recipe, "ISOPHOTAL_FLUX");
+
+  // find the first bin below the flux level and the last above the level
+  // XXX can this be done faster with disection?
+  // XXX do I need to worry about crazy outliers?  
+  // XXX should i be smoothing or fitting the curve?
+  int firstBelow = -1;
+  int lastAbove = -1;
+  for (int i = 0; i < flux->n; i++) {
+    if (flux->data.F32[i] > ISOPHOT_FLUX) lastAbove = i;
+    if ((firstBelow < 0) && (flux->data.F32[i] < ISOPHOT_FLUX)) firstBelow = i;
+  }
+
+  // need to examine pixels in this vicinity
+  float isophotalFluxFirst = 0;
+  float isophotalFluxLast = 0;
+  for (int i = 0; i <= PS_MAX(firstBelow, lastAbove); i++) {
+    if (i <= firstBelow) {
+      isophotalFluxFirst += flux->data.F32[i];
+    }
+    if (i <= lastAbove) {
+      isophotalFluxLast += flux->data.F32[i];
+    }
+  }
+  float isophotalFlux    = 0.5*(isophotalFluxLast + isophotalFluxFirst);
+  float isophotalFluxErr = 0.5*fabs(isophotalFluxLast - isophotalFluxFirst);
+
+  float isophotalRad     = 0.5*(radius->data.F32[firstBelow] + radius->data.F32[lastAbove]);
+  float isophotalRadErr  = 0.5*fabs(radius->data.F32[firstBelow] - radius->data.F32[lastAbove]);
+
+  if (!source->extpars->isophot) {
+    source->extpars->isophot = pmSourceIsophotalValuesAlloc ();
+  }
+  
+  // these are uncalibrated: instrumental mags and pixel units
+  source->extpars->isophot->mag    = -2.5*log10(isophotalFlux);
+  source->extpars->isophot->magErr = isophotalFluxErr / isophotalFlux;
+
+  source->extpars->isophot->rad    = isophotalRad;
+  source->extpars->isophot->radErr = isophotalRadErr;
+
   return true;