[ncl-talk] png with background transparent

[Francesco Trotta]

Dear ncl-support

   I'm generating a contour_map plot for the ocean temperature

   I would like to generate a png with a background transparent beacause I want overlay the map on google map.

   I'm using the following scripts but I got a white background


   How can I get a transparent background?

thanks

Francesco



 ;============== open a file ========================================
   wks = gsn_open_wks(typefile,namefile)

   setvalues NhlGetWorkspaceObjectId()
      "wsMaximumSize" : 300000000
   end setvalues

   ;============== miscellanous =======================================
   gsn_define_colormap(wks,colorsMap)

   ;============== map resources ======================================
   res = True
   ;--- Title attributes -----------------------------------
   res at tiMainFont        = 0
   res at tiMainPosition    = "Center"
   res at tiMainFontHeightF = 0.011
   ;res at tiMainOffsetYF    = 0.01
   ;--- GSN attributes -------------------------------------
   res at gsnDraw         = False
   res at gsnFrame        = False
   res at gsnMaximize     = True
   res at gsnAddCyclic    = False
   res at gsnSpreadColors = True
   ;--- Map attributes (window) ----------------------------
   res at mpLimitMode = "LatLon"    ;Zoom in on the plot area
   res at mpMinLatF   = minLatF
   res at mpMaxLatF   = maxLatF
   res at mpMinLonF   = minLonF
   res at mpMaxLonF   = maxLonF
   ;--- Map attributes (projection) ----------------------------
   res at mpProjection = "mercator"
   ;--- Map attributes (land outlines) ---------------------
   res at mpDataBaseVersion = "HighRes"           ;use GMT coastline
   res at mpOutlineOn       = lmapOutlineOn       ;turn on continental outlines
   res at mpOutlineBoundarySets = "AllBoundaries" ;turn on country boundaries (all the boundaries database in use)
   res at mpGeophysicalLineThicknessF = 2.5       ;thickness of outlines
   res at mpGeophysicalLineColor= "Black"         ;color of cont. outlines
   res at mpOutlineDrawOrder    = "PostDraw"      ;draw continental outline last
   res at mpFillOn              = False      ;turn on gray fill for continental background
   res at pmTickMarkDisplayMode = "Always"
   ;--- Map attributes (lat/lon grid lines) ----------------
   res at mpGridAndLimbOn       = False           ;turn on lat/lon lines
   res at mpGridLineDashPattern = 2               ;... with xxxx lines
   res at mpGridLineColor       = "black"         ;color of the lines
   res at mpGridLatSpacingF     = 10              ;space (in degree) between 2 lines
   res at mpGridLonSpacingF     = 10              ;idem
   res at mpGridLineThicknessF  = 2.0             ;thickness of the lines
   res at mpGridAndLimbDrawOrder= "PostDraw"      ;draw lat/lon lines last
   res at mpLabelFontHeightF    = 0.005           ;label font size
   ;--- contours attributes --------------------------------
   res at cnFillOn              = True            ;areas between levels filled with color
   res at cnFillMode            = "AreaFill"      ;AreaFill
   res at cnLevelSelectionMode  = "ManualLevels"  ;set manual contour levels
   res at cnMinLevelValF        = minLevelVal     ;set max contour level
   res at cnMaxLevelValF        = maxLevelVal     ;set min contour level
   res at cnLevelSpacingF       = levelSpacing
   res at cnLinesOn             = False           ;turn off the contour lines
   res at cnLineLabelsOn        = False           ;turn off the line labels
   res at cnLineLabelFontHeightF= 0.005
   res at cnLineLabelDensityF   = 1               ;label spacing when cnLineLabelPlacementMode != "Constant"
   res at cnLineLabelFontAspectF= 1               ;shape of the line label. > 1 :thinner characters. < 1 :wider
   res at cnLineLabelInterval   = 1
   res at cnInfoLabelOn         = False           ;turn off "CONTOUR FROM X TO X BY X" legend.
   res at cnLabelMasking        = True            ;mask lines where label appear
   res at cnConstFEnableFill    = True            ;allow constant values (1=sig) to fill
   res at cnConstFLabelOn       = False
   ;--- LabelBar attributes --------------------------------
   res at lbLabelBarOn       = llabbarOn
   res at lbLabelsOn         = llabbarLabelsOn
   ;res at lbTitleString      = "("+field at units+")"
   res at lbOrientation      = "vertical"          ; vertical label bar
   ;res at pmLabelBarHeightF = 0.7
   res at lbTitlePosition    = "Bottom"
   res at lbAutoManage       = False         ; Necessary to control sizes
   res at pmLabelBarWidthF      = 0.1               ; default is shorter
   ;res at pmLabelBarHeightF     = 0.1               ; default is taller
   ;res at lbLabelFontHeightF    = .018              ; default is HUGE
   res at lbPerimOn             = False             ; default has box
   res at lbLeftMarginF = 0.0
   res at lbTopMarginF = 0.0 ; eliminate any margin at the top and left of the colorbar
   res at lbBottomMarginF = 0.0 ; eliminate any margin at the top and left of the colorbar
   res at lbLabelOffsetF = .05 ; this specifies the offset between the colorbar and the labels as a fraction of vpWidthF
   res at lbLabelAutoStride = True ; (optional) this ensures that labels do not overlap when the font height is made very large.
   res at lbLabelFontHeightF = 0.015
   res at lbTitleFontHeightF = 0.015
   ;res at lbLabelStride      = 5       ; Label every 5th entry
   if (ismissing(minLevelVal) .or. ismissing(maxLevelVal)) then res at lbLabelStrings = ""
   else res at lbLabelStrings = "" + sprintf(labformatStr,fspan(minLevelVal,maxLevelVal, \
                                  floattoint((maxLevelVal-minLevelVal)/levelSpacing)+1)) end if

   ;-------------- scalars attributes ----------------------
   res at sfXArray = lon2d(0:ny-1,0:nx-1)       ; abscisses
   res at sfYArray = lat2d(0:ny-1,0:nx-1)       ; ordonnees

   ;=== draw contour/contour lines ====================================
   plot = gsn_csm_contour_map(wks,field(:,:),res)

   draw(plot)
   frame(wks)

   system("convert -alpha off -background white -geometry "+typefile at wkWidth+"x"+typefile at wkHeight+ \
          " -density 300 -trim "+namefile+".png "+namefile+".png")

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