Index: fact/tools/rootmacros/DrsCalibration.C
===================================================================
--- fact/tools/rootmacros/DrsCalibration.C	(revision 12508)
+++ fact/tools/rootmacros/DrsCalibration.C	(revision 12511)
@@ -42,38 +42,101 @@
 	vector<int16_t> StartCellVector
 ){
-	// TODO this should be a parameter...
-//	const int NumberOfPixel = 1440;
+	// In order to minimize mem free and alloc actions
+	// the destination vector is only resized, if it is way too big
+	// or too small
+	size_t DestinationLength = RegionOfInterest-LeaveOutRight-LeaveOutLeft;
+	if ( destination.size() < DestinationLength || destination.size() > 2 * DestinationLength ){
+		destination.resize( DestinationLength );
+	}
 
-//	if ( RegionOfInterest < drs_triggeroffsetmean.size()/NumberOfPixel ){
-//		cerr << "Error in applyDrsCalibration: ROI of data is:" << RegionOfInterest << endl;
-//		cerr << "While ROI of 'TriggerOffset' is:" << drs_triggeroffsetmean.size()/NumberOfPixel << endl;
-//		cerr << "Data ROI must be <= ROI of TriggerOffset. Aborting..." << endl;
-//		return 0
-//	}
+	// the vector drs_triggeroffsetmean is not 1440 * 1024 entries long
+	// but has hopefully the length 1440 * RegionOfInterest (or longer)
+	if ( drs_triggeroffsetmean.size() < 1440*RegionOfInterest ){
+		return 0;
+	}
 
+	//	We do not entirely know how the calibration constants, which are saved in a filename.drs.fits file 
+	// were calculated, so it is not fully clear how they should be applied to the raw data for calibration.
+	// apparently the calibration constants were transformed to the unit mV, which means we have to do the same to
+	// the raw data prior to apply the calibration
+	//
+	// on the FAD board, there is a 12bit ADC, with a 2.0V range, so the factor between ADC units and mV is
+	// ADC2mV = 2000/4096. = 0.48828125 (numerically exact)
+	//
+	// from the schematic of the FAD we learned, that the voltage at the ADC 
+	// should be 1907.35 mV when the calibration DAC is set to 50000. 
+	// 
+	// One would further assume that the calibration constants are calculated like this:
 
-	destination.clear();
+	// The DRS Offset of each bin in each channel is the mean value in this very bin,
+	// obtained from so called DRS pedestal data
+	// Its value is about -1820 ADC units or -910mV
+
+	// In order to obtain the DRS Gain of each bin of each channel
+	// again data is takes, with the calibration DAC set to 50000
+	// This is called DRS calibration data.
+	// We assume the DRS Offset is already subtracted from the DRS calibration data
+	// so the typical value is assumed to be ~3600 ADC units oder ~1800mV 
+	// As mentioned before, the value *should* be 1907.35 mV 
+	// So one might assume that the Gain is a number, which actually converts ~3600 ADC units into 1907.35mV for each bin of each channel.
+	// So that the calibration procedure looks like this
+	// TrueValue = (RawValue - Offset) * Gain
+	// The numerical value of Gain would differ slightly from the theoretical value of 2000/4096.
+	// But this is apparently not the case. 
+	// The Gain, as it is stored in the DRS calibration file of FACT++ has numerical values 
+	// around +1800.
+	// So it seems one should calibrate like this:
+	// TrueValue = (RawValue - Offset) / Gain * 1907.35
+
+	// When these calibrations are done, one ends up with a quite nice calibrated voltage.
+	// But it turns out that, if one returns the first measurement, and calculates the mean voltages 
+	// in each bin of the now *logical* DRS pipeline, the mean voltage is not zero, but slightly varies 
+	// So one can store these systematical deviations from zero in the logical pipeline as well, and subtract them.
+	// The remaining question is, when to subtract them.
+	// I assume, in the process of measuring this third calibration constant, the first two 
+	// calibrations are already applied to the raw data. 
+
+	// So the calculation of the calibrated volatage from some raw voltage works like this:
+	// assume the raw voltage is the s'th sample in channel c. While the Trigger made the DRS stopp in its t'th cell.
+	// note, that the DRS pipeline is always 1024 bins long. This is constant of the DRS4 chip. 
+
+	// TrueValue[c][s] = ( RawValue[c][s] - Offset[c][ (c+t)%1024 ] ) / Gain[c][ (c+t)%1024 ] * 1907.35 - TriggerOffset[c][s] 
+	
+	
+	
+
 	const float dconv = 2000/4096.0;
-	float vraw, vcal;
-	unsigned int pixel_pt;
-	unsigned int slice_pt;
-	unsigned int cal_pt;
-	unsigned int drs_cal_offset;
+	float vraw;
+//	float vcal;
+//	unsigned int pixel_pt;
+//	unsigned int slice_pt;
+//	unsigned int cal_pt;
+//	unsigned int drs_cal_offset;
 
-	pixel_pt = pixel * RegionOfInterest;
+	if ( RegionOfInterest != AllPixelDataVector.size()/1440 ){
+		cout << "RegionOfInterest != AllPixelDataVector.size()/1440 .. this makes no sense ... I guess ... aborting" << endl;
+		return 0;
+	}
 
-//	unsigned int pixel_pt_TriggerOffset = pixel * drs_triggeroffsetmean.size()/NumberOfPixel;
-//	unsigned int slice_pti_TriggerOffset;
+	int Offset_offset = pixel * drs_basemean.size()/1440;
+	int Gain_offset = pixel * drs_gainmean.size()/1440;
+	int TriggerOffset_offset = pixel * drs_triggeroffsetmean.size()/1440;
+	int Data_offset = pixel * RegionOfInterest;
+	
+	for ( unsigned int sl = LeaveOutLeft; sl < RegionOfInterest-LeaveOutRight ; sl++){
+		vraw = AllPixelDataVector[ Data_offset + sl ] * dconv;
+		vraw -= drs_basemean[ Offset_offset + (sl + StartCellVector[pixel])%1024 ];
+		vraw /= drs_gainmean[ Gain_offset + (sl + StartCellVector[pixel])%1024 ];
+		vraw *= 1907.35;
+		vraw -= drs_triggeroffsetmean[ TriggerOffset_offset + sl ];
 
-	for ( unsigned int sl = LeaveOutLeft; sl < RegionOfInterest-LeaveOutRight ; sl++){
-		slice_pt = pixel_pt + sl;
-		//slice_pt_TriggerOffset =  pixel_pt_TriggerOffset + sl;	
-		drs_cal_offset = ( sl + StartCellVector[ pixel ] ) % 1024;
-		cal_pt    = pixel_pt + drs_cal_offset;
+//		slice_pt = pixel_pt + sl;
+//		drs_cal_offset = ( sl + StartCellVector[ pixel ] ) % RegionOfInterest;
+//		cal_pt    = pixel_pt + drs_cal_offset;
+//		vraw = AllPixelDataVector[ slice_pt ] * dconv;
+//		vcal = ( vraw - drs_basemean[ cal_pt ] - drs_triggeroffsetmean[ slice_pt ] ) / drs_gainmean[ cal_pt ]*1907.35;
+//		destination.push_back(vcal);
 
-		vraw = AllPixelDataVector[ slice_pt ] * dconv;
-		vcal = ( vraw - drs_basemean[ cal_pt ] - drs_triggeroffsetmean[ slice_pt ] ) / drs_gainmean[ cal_pt ]*1907.35;
-
-		destination.push_back(vcal);
+		destination[sl-LeaveOutLeft] = vraw;
 	}
 
