Index: trunk/psphot/src/psphotPetrosian.c
===================================================================
--- trunk/psphot/src/psphotPetrosian.c	(revision 13983)
+++ trunk/psphot/src/psphotPetrosian.c	(revision 15562)
@@ -3,5 +3,79 @@
 bool psphotPetrosian (pmSource *source, psMetadata *recipe, psMaskType maskVal) {
 
-  psLogMsg ("psphot", PS_LOG_INFO, "not implemented\n");
+  bool status;
+
+  assert (source->extpars);
+  assert (source->extpars->profile);
+  assert (source->extpars->profile->radius);
+  assert (source->extpars->profile->flux);
+
+  psVector *radius = source->extpars->profile->radius;
+  psVector *flux = source->extpars->profile->flux;
+
+  // flux at which to measure isophotal parameters
+  // XXX cache this?
+  float PETROSIAN_R0 = psMetadataLookupF32 (&status, recipe, "PETROSIAN_R0");
+  float PETROSIAN_RF = psMetadataLookupF32 (&status, recipe, "PETROSIAN_FLUX_RATIO");
+
+  // first find flux at R0
+  int firstBelow = -1;
+  int lastAbove = -1;
+  for (int i = 0; i < radius->n; i++) {
+    if (radius->data.F32[i] > PETROSIAN_R0) lastAbove = i;
+    if ((firstBelow < 0) && (flux->data.F32[i] < PETROSIAN_R0)) firstBelow = i;
+  }
+
+  // average flux in this range
+  float fluxR0 = 0.0;
+  int fluxRn = 0;
+  for (int i = PS_MIN(firstBelow, lastAbove); i <= PS_MAX(firstBelow, lastAbove); i++) {
+    fluxR0 += flux->data.F32[i];
+    fluxRn ++;
+  }
+  fluxR0 /= (float)(fluxRn);
+  
+  // target flux for petrosian radius
+  float fluxRP = fluxR0 * PETROSIAN_RF;
+
+  // 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?
+  firstBelow = -1;
+  lastAbove = -1;
+  for (int i = 0; i < flux->n; i++) {
+    if (flux->data.F32[i] > fluxRP) lastAbove = i;
+    if ((firstBelow < 0) && (flux->data.F32[i] < fluxRP)) firstBelow = i;
+  }
+
+  // need to examine pixels in this vicinity
+  float fluxFirst = 0;
+  float fluxLast = 0;
+  for (int i = 0; i <= PS_MAX(firstBelow, lastAbove); i++) {
+    if (i <= firstBelow) {
+      fluxFirst += flux->data.F32[i];
+    }
+    if (i <= lastAbove) {
+      fluxLast += flux->data.F32[i];
+    }
+  }
+  float fluxRPSum    = 0.5*(fluxLast + fluxFirst);
+  float fluxRPSumErr = 0.5*fabs(fluxLast - fluxFirst);
+  // XXX need to use the weight appropriately here...
+
+  float rad     = 0.5*(radius->data.F32[firstBelow] + radius->data.F32[lastAbove]);
+  float radErr  = 0.5*fabs(radius->data.F32[firstBelow] - radius->data.F32[lastAbove]);
+
+  if (!source->extpars->petrosian) {
+    source->extpars->petrosian = pmSourcePetrosianValuesAlloc ();
+  }
+  
+  // these are uncalibrated: instrumental mags and pixel units
+  source->extpars->petrosian->mag    = -2.5*log10(fluxRPSum);
+  source->extpars->petrosian->magErr = fluxRPSumErr / fluxRPSum;
+
+  source->extpars->petrosian->rad    = rad;
+  source->extpars->petrosian->radErr = radErr;
+
   return true;