pro cntrd, img, x, y, xcen, ycen, fwhm, SILENT= silent, DEBUG=debug, $ EXTENDBOX = extendbox, KeepCenter = KeepCenter ;+ ; NAME: ; CNTRD ; PURPOSE: ; Compute the centroid of a star using a derivative search ; EXPLANATION: ; CNTRD uses an early DAOPHOT "FIND" centroid algorithm by locating the ; position where the X and Y derivatives go to zero. This is usually a ; more "robust" determination than a "center of mass" or fitting a 2d ; Gaussian if the wings in one direction are affected by the presence ; of a neighboring star. ; ; CALLING SEQUENCE: ; CNTRD, img, x, y, xcen, ycen, [ fwhm , /KEEPCENTER, /SILENT, /DEBUG ; EXTENDBOX = ] ; ; INPUTS: ; IMG - Two dimensional image array ; X,Y - Scalar or vector integers giving approximate integer stellar ; center ; ; OPTIONAL INPUT: ; FWHM - floating scalar; Centroid is computed using a box of half ; width equal to 1.5 sigma = 0.637* FWHM. CNTRD will prompt ; for FWHM if not supplied ; ; OUTPUTS: ; XCEN - the computed X centroid position, same number of points as X ; YCEN - computed Y centroid position, same number of points as Y, ; floating point ; ; Values for XCEN and YCEN will not be computed if the computed ; centroid falls outside of the box, or if the computed derivatives ; are non-decreasing. If the centroid cannot be computed, then a ; message is displayed and XCEN and YCEN are set to -1. ; ; OPTIONAL OUTPUT KEYWORDS: ; /SILENT - Normally CNTRD prints an error message if it is unable ; to compute the centroid. Set /SILENT to suppress this. ; /DEBUG - If this keyword is set, then CNTRD will display the subarray ; it is using to compute the centroid. ; EXTENDBOX = {non-negative positive integer}. CNTRD searches a box with ; a half width equal to 1.5 sigma = 0.637* FWHM to find the ; maximum pixel. To search a larger area, set EXTENDBOX to ; the number of pixels to enlarge the half-width of the box. ; Default is 0; prior to June 2004, the default was EXTENDBOX= 3 ; /KeepCenter = By default, CNTRD finds the maximum pixel in a box ; centered on the input X,Y coordinates, and then extracts a new ; box about this maximum pixel. Set the /KeepCenter keyword ; to skip then step of finding the maximum pixel, and instead use ; a box centered on the input X,Y coordinates. ; PROCEDURE: ; Maximum pixel within distance from input pixel X, Y determined ; from FHWM is found and used as the center of a square, within ; which the centroid is computed as the value (XCEN,YCEN) at which ; the derivatives of the partial sums of the input image over (y,x) ; with respect to (x,y) = 0. In order to minimize contamination from ; neighboring stars stars, a weighting factor W is defined as unity in ; center, 0.5 at end, and linear in between ; ; RESTRICTIONS: ; (1) Does not recognize (bad) pixels. Use the procedure GCNTRD.PRO ; in this situation. ; (2) DAOPHOT now uses a newer algorithm (implemented in GCNTRD.PRO) in ; which centroids are determined by fitting 1-d Gaussians to the ; marginal distributions in the X and Y directions. ; (3) The default behavior of CNTRD changed in June 2004 (from EXTENDBOX=3 ; to EXTENDBOX = 0). ; (4) Stone (1989, AJ, 97, 1227) concludes that the derivative search ; algorithm in CNTRD is not as effective (though faster) as a ; Gaussian fit (used in GCNTRD.PRO). ; MODIFICATION HISTORY: ; Written 2/25/86, by J. K. Hill, S.A.S.C., following ; algorithm used by P. Stetson in DAOPHOT. ; Allowed input vectors G. Hennessy April, 1992 ; Fixed to prevent wrong answer if floating pt. X & Y supplied ; W. Landsman March, 1993 ; Convert byte, integer subimages to float W. Landsman May 1995 ; Converted to IDL V5.0 W. Landsman September 1997 ; Better checking of edge of frame David Hogg October 2000 ; Avoid integer wraparound for unsigned arrays W.Landsman January 2001 ; Handle case where more than 1 pixel has maximum value W.L. July 2002 ; Added /KEEPCENTER, EXTENDBOX (with default = 0) keywords WL June 2004 ;- On_error,2 ;Return to caller if N_params() LT 5 then begin print,'Syntax: CNTRD, img, x, y, xcen, ycen, [ fwhm, ' print,' EXTENDBOX= , /KEEPCENTER, /SILENT, /DEBUG ]' PRINT,'img - Input image array' PRINT,'x,y - Input scalars giving approximate X,Y position' PRINT,'xcen,ycen - Output scalars giving centroided X,Y position' return endif else if N_elements(fwhm) NE 1 then $ read,'Enter approximate FWHM of image in pixels: ',fwhm sz_image = size(img) if sz_image[0] NE 2 then message, $ 'ERROR - Image array (first parameter) must be 2 dimensional' xsize = sz_image[1] ysize = sz_image[2] dtype = sz_image[3] ;Datatype ; Compute size of box needed to compute centroid if not keyword_set(extendbox) then extendbox = 0 nhalf = fix(0.637*fwhm) > 2 ; nbox = 2*nhalf+1 ;Width of box to be used to compute centroid nhalfbig = nhalf + extendbox nbig = nbox + extendbox*2 ;Extend box 3 pixels on each side to search for max pixel value npts = N_elements(x) xcen = float(x) & ycen = float(y) ix = round( x ) ;Central X pixel ;Added 3/93 iy = round( y ) ;Central Y pixel for i = 0,npts-1 do begin ;Loop over X,Y vector pos = strtrim(x[i],2) + ' ' + strtrim(y[i],2) if not keyword_set(keepcenter) then begin if ( (ix[i] LT nhalfbig) or ((ix[i] + nhalfbig) GT xsize-1) or $ (iy[i] LT nhalfbig) or ((iy[i] + nhalfbig) GT ysize-1) ) then begin if not keyword_set(SILENT) then message,/INF, $ 'Position '+ pos + ' too near edge of image' xcen[i] = -1 & ycen[i] = -1 goto, DONE endif bigbox = img[ix[i]-nhalfbig : ix[i]+nhalfbig, iy[i]-nhalfbig : iy[i]+nhalfbig] ; Locate maximum pixel in 'NBIG' sized subimage mx = max( bigbox) ;Maximum pixel value in BIGBOX mx_pos = where(bigbox EQ mx, Nmax) ;How many pixels have maximum value? idx = mx_pos mod nbig ;X coordinate of Max pixel idy = mx_pos / nbig ;Y coordinate of Max pixel if NMax GT 1 then begin ;More than 1 pixel at maximum? idx = round(total(idx)/Nmax) idy = round(total(idy)/Nmax) endif else begin idx = idx[0] idy = idy[0] endelse xmax = ix[i] - (nhalf+extendbox) + idx ;X coordinate in original image array ymax = iy[i] - (nhalf+extendbox) + idy ;Y coordinate in original image array endif else begin xmax = ix[i] ymax = iy[i] endelse ; --------------------------------------------------------------------- ; check *new* center location for range ; added by Hogg if ( (xmax LT nhalf) or ((xmax + nhalf) GT xsize-1) or $ (ymax LT nhalf) or ((ymax + nhalf) GT ysize-1) ) then begin if not keyword_set(SILENT) then message,/INF, $ 'Position '+ pos + ' moved too near edge of image' xcen[i] = -1 & ycen[i] = -1 goto, DONE endif ; --------------------------------------------------------------------- ; Extract smaller 'STRBOX' sized subimage centered on maximum pixel strbox = img[xmax-nhalf : xmax+nhalf, ymax-nhalf : ymax+nhalf] if (dtype NE 4) and (dtype NE 5) then strbox = float(strbox) if keyword_set(DEBUG) then begin message,'Subarray used to compute centroid:',/inf print,strbox endif ir = (nhalf-1) > 1 dd = indgen(nbox-1) + 0.5 - nhalf ; Weighting factor W unity in center, 0.5 at end, and linear in between w = 1. - 0.5*(abs(dd)-0.5)/(nhalf-0.5) sumc = total(w) ; Find X centroid deriv = shift(strbox,-1,0) - strbox ;Shift in X & subtract to get derivative deriv = deriv[0:nbox-2,nhalf-ir:nhalf+ir] ;Don't want edges of the array deriv = total( deriv, 2 ) ;Sum X derivatives over Y direction sumd = total( w*deriv ) sumxd = total( w*dd*deriv ) sumxsq = total( w*dd^2 ) if sumxd GT 0 then begin ;Reject if X derivative not decreasing if not keyword_set(SILENT) then message,/INF, $ 'Unable to compute X centroid around position '+ pos xcen[i]=-1 & ycen[i]=-1 goto,DONE endif dx = sumxsq*sumd/(sumc*sumxd) if ( abs(dx) GT nhalf ) then begin ;Reject if centroid outside box if not keyword_set(SILENT) then message,/INF, $ 'Computed X centroid for position '+ pos + ' out of range' xcen[i]=-1 & ycen[i]=-1 goto, DONE endif xcen[i] = xmax - dx ;X centroid in original array ; Find Y Centroid deriv = shift(strbox,0,-1) - strbox deriv = deriv[nhalf-ir:nhalf+ir,0:nbox-2] deriv = total( deriv,1 ) sumd = total( w*deriv ) sumxd = total( w*deriv*dd ) sumxsq = total( w*dd^2 ) if (sumxd GT 0) then begin ;Reject if Y derivative not decreasing if not keyword_set(SILENT) then message,/INF, $ 'Unable to compute Y centroid around position '+ pos xcen[i] = -1 & ycen[i] = -1 goto, DONE endif dy = sumxsq*sumd/(sumc*sumxd) if (abs(dy) GT nhalf) then begin ;Reject if computed Y centroid outside box if not keyword_set(SILENT) then message,/INF, $ 'Computed X centroid for position '+ pos + ' out of range' xcen[i]=-1 & ycen[i]=-1 goto, DONE endif ycen[i] = ymax-dy DONE: endfor return end