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Hints to NCL graphics exercises

Basic graphics exercise hints

  1. You can change the size of the X11 window in one of two ways: inside the script or via the ".hluresfile" file. Doing it inside the ".hluresfile" is recommended if you always want this size for your X11 window.

    The script below shows how to do it inside the script itself, using the resources wkWidth and wkHeight.

    [Answer script] [Image]

  2. To have the script send the graphical output to a PS, PDF, or PNG file, change "x11" to "ps", "pdf", or "png" in the call to gsn_open_wks. The second argument will be the filename (with the appropriate ".ps", ".pdf". or ".png" appended).

    [Answer script] [Image]

  3. To rotate the image for PS or PDF output, you need to use the resource wkOrientation. Read the documentation on gsn_open_wks for information on how to set this resource.

    [Answer script] [Image]

  4. Setting gsnMaximize to True forces the plot to be maximized in the output device you're using. This means if you are drawing to a PS file that will be sent to an 8-1/2 x 11 piece of paper, the plot may be rotated to landscape mode if the plot is wider than it is high.

    Note that you must make sure the resource variable ("res" in the answer script below) is set to True in order for any resources to take effect.

    [Answer script] [Image]

  5. Drawing another plot is simple. Just add another call to gsn_csm_y, and pass it the new set of y points.

    Note that if your output is going to an X11 window, then when you run the script, you will need to click on the X11 window with your left mouse button to advance the frame to the second picture, and then to close the window.

    If your output is going to a PS, PDF, or PNG file, each plot will appear on a separate page.

    [Answer script] [Image 1] [Image 2]

  6. Setting gsnFrame to False causes the frame not to be advanced, and hence if you draw a second plot, it will be drawn on the same frame (or page), as the previous plot.

    To set gsnFrame to False for one plot and not the other, set it to False before the first call, and then back to True for the second call. Or, you can just delete it before you generate the second plot.

    If your output is going to an X11 window, you will only see one X11 window pop up, and the second plot will be drawn to the same window.

    If your output is going to a PS, PDF, or PNG file, both plots will be draw on the same page.

    [Answer script] [Image]

  7. Use vp resources to reposition and resize individual plots.

    To advance the frame manually, call the frame procedure. You must advance the frame after you are done drawing to it, because otherwise the PS, PDF or PNG file will not be closed properly.

    For an X11 window, if you leave out the frame call, you will see a window pop up and then disappear. When you include the frame call, it waits for you to click on it before it disappears.

    Note that when you use "vp" resources to position plot(s) on a page, the gsnMaximize resource will no longer have effect.

    [Answer script] [Image]

    Tip: if your plots are the exact same size, and you want to panel them on a page, then use gsn_panel to do this.

Color table exercise hints

  1. Use the procedure gsn_draw_colormap to draw the color table associated with the current workstation. Each color will have its corresponding index included. Color index 0 represents the background color, and 1 represents the foreground color.

    [Answer script] [Image]

  2. Use the procedure gsn_define_colormap to change the color table (also known as a "color map"). It is recommended that you call this procedure immediately after calling gsn_open_wks. It is generally a good idea to have only one color table defined per frame (page).

    Note: as of NCL V6.1.0 and later, the use of gsn_define_colormap may not be as necessary. This is because you can now set color tables for contours and vectors directly via contour and vector resources; see the "tip" below.

    [Answer script] [Image]

  3. Use the function NhlNewColor to add a new color to a color table. The existing color table must have less than 256 colors, as this is the maximum number of colors you can have. The color will be added to the end of your color table.

    The color gray is represented by RGB triplets that have the same values, like (0.1,0.1,0.1) or (0.85,0.85,0.85). The larger the values, the lighter the shade of gray.

    Note: in NCL versions 6.0.0 and earlier, one of the main purposes of NhlNewColor was to add "named" colors to a color map, so you could reference them later. For example, in order to use "red" as a color, you had to make sure the RGB triplet (1.,0.,0.) was in your workstation color map. This is no longer required in NCL V6.1.0 and later.

    [Answer script] [Image]

    Tip: to replace a color in an existing color table, you can use NhlSetColor.

  4. Use the procedure gsn_define_colormap to create a new color table using RGB triplets.

    [Answer script] [Image]

    There are other ways to create your own color table. For more information, see the short write up on creating your own color table.

  5. Use the procedure gsn_define_colormap to create a new color table using named colors.

    To see all the color names and their corresponding colors, see the named colors gallery. You can use the gsn_draw_named_colors procedure to quickly draw a set of named colors to see how they look on your screen.

    [Answer script] [Image]

Tip: with NCL version 6.1.0, you can assign a color table directly to a contour, vector, or streamline plot without having to "define" it first. Use the resources cnFillPalette, vcLevelPalette, or stLevelPalette. If you only want to use part of a color table for one of these resources, you can use read_colormap_file to read in the color map as an array of RGB triples, and then subscript it as necessary.

Title exercise hints

  1. Resources for changing attributes of the main title start with "tiMain" and are listed alphabetically in the title resource page.

    [Answer script] [Image]

  2. Resources for changing attributes of the X or Y axis titles start with "tiXAxis" and "tiYAxis". The resources for the main title, X axis title, and Y axis title have the same structure, i.e. "tiMainFont", "tiXAxisFont", and "tiYAxisFont". This makes it easier to remember resource names.

    [Answer script] [Image]

  3. If you change the orientation of text, this may also change the direction of the text. This means you need to set two resources in order to change the orientation of the Y axis title. You unfortunately can't have a title on both Y axes using the "ti" resources. You would need to use gsn_add_text if you wanted to add an additional title to one of the axes.

    [Answer script] [Image]

XY plot exercise hints (set 1)

  1. Resources for changing attributes of an XY line start with "xyLine" and are listed alphabetically in the XY resource page.

    [Answer script] [Image]

  2. All line thickness resources have names like xyLineThicknessF, xyLineThicknesses, cnLineThicknessF, cnLineThicknesses, gsLineThicknessF, etc. The default is usually 1.0, so a value of 2.0 makes the line twice as thick. Sometimes you need to make lines thicker so they'll look better in a PS, PDF, or PNG file.

    [Answer script] [Image]

  3. Changing an XY plot so that markers are used instead of lines involves changing a "Mode" resource. To then change the look of a marker, there are a series of "xyMarker" resources you can set. You can see a list of predefined markers in the marker table.

    [Answer script] [Image]

    Try playing with these and other xyMarker resources to change the marker, color, size, etc.

  4. If you don't like any of the predefined markers, you can define your own using NhlNewMarker. This function allows you to use any of the characters in any of the font tables as a marker. In the script below, the character "p" is used from font table 35.

    If you don't want to plot every value, you can use the "::" syntax to subscript your data array. Using "::2" will grab every other point, while, "::3" will grab every third point.

    [Answer script] [Image]

    Try playing with these and other xyMarker resources to change the marker, color, size, etc.

XY plot exercise hints (set 2)

  1. Use fspan to generate the X array. In order to use it in a call to gsn_csm_xy, it must be the same length as its corresponding Y array.

    [Answer script] [Image]

  2. You can reorder the dimensions by naming both dimensions of "y", and then using NCL's reorder syntax to transpose the dimensions.

    [Answer script] [Image]

  3. Whenever you have a single and plural version of a resource, like "xyLineColor" and "xyLineColors", there's usually a corresponding "Mono" resource ("xyMonoLineColor") that takes a logical True/False value. This allows you to specify whether to use the singular or plural version of the resource to apply to that specific plot feature.

    You need to know whether the corresponding Mono resource's default is True or False to know whether you need to change it.

    [Answer script] [Image]

  4. Changing the area that the data is displayed in involves a transformation of the data to a new "data space", so you need to use Transformation resources to change the axes limits.

    [Answer script] [Image]

  5. Adding a legend involves using a PlotManager resource called pmLegendDisplayMode. The PlotManager is something that manages additional annotations of a plot, like labelbars, titles, and legends.

    [Answer script] [Image]

  6. Legends are somewhat confusing in NCL. To change various aspects of a legend, you may need to use xy, pm, and lg resources. For this particular exercise, no "lg" resources are involved.

    [Answer script] [Image]

Contour plot exercise hints

  1. There are three ways you can set contour levels: 1) let NCL do it automatically, 2) set a minimum level, maximum level, and a level spacing, or 3) set an array of contour levels.

    The default behavior is #1. To do #2 or #3, you need to set the cnLevelSelectionMode resource to the "mode" you want, and then set the other resources depending on the mode you choose. The #3 method should be used if you need to use irregularly spaced contour levels.

    [Answer script] [Image]

  2. The default is for each contour line to be the same color. To change this, see the cnMonoLineColor resource. Other contour line resources start with "cnLine".

    Note: as of NCL V6.1.0 and later, it is better to use named colors like "blue", "red", "forestgreen" for color, rather than index values, because then you don't need to worry about using the correct color table.

    [Answer script] [Image]

    See "contour_ex02_namedcolors ncl" for a way of doing this using named colors.

  3. When you turn contour fill on, the current colormap in use will be spanned.

    Note: if you only want a particular part of your color map spanned, then you set the resources gsnSpreadColorStart and gsnSpreadColorEnd to the desired start and end color indexes. See a later example below for another way to do this using cnFillPalette.

    [Answer script] [Image]

  4. A labelbar is added automatically when contour fill is set. If there are lots of contour levels, then your labelbar labels will automatically be strided (this wasn't the case in versions 6.0.0 or earlier.) For a vertical labelbar, look at the labelbar resources, which start with "lb". Look for the word "orientation".

    [Answer script] [Image]

  5. Tickmark resources can be set for XY, vector, streamline, and contour plots, and hence they have their own set of resources that start with the letters "tm".

    There are three ways you can change the labels on a tickmark axis: 1) let NCL do it automatically, 2) set a minimum and maximum tickmark value and a tickmark spacing, or 3) set arrays of tickmark values and labels you want at those values. For this exercise, you will use #2 for the Y axis, and #3 for the X axis.

    Each of the four axes has its own set of tickmark resources. By default, however, the right/left axes will use the same resources values, as will the top/bottom axes (unless instructed otherwise). To set explicit labels for the bottom X axis, look for the "tmXB" resources, and for the left Y axis, look for "tmYL".

    [Answer script] [Image]

  6. Use the cnFillPalette resource to set a color map for color contours. This resource was added in NCL V6.1.0.

    [Answer script] [Image]

  7. Use the read_colormap_file function to read in a predefined color table as an array of RGBA quadruplets (ncolors x 4). You can then subscript the array as desired to only use part of it.

    [Answer script] [Image]

  8. The "A" part of the RGBA array is an alpha value that you can set to make colors more transparent (they are fully opaque by default). Use values between 0 and 1 (inclusive) to control the transparency level.

    [Answer script] [Image]

Contours over maps exercises (set 1)

  1. If you add "printVarSummary(t)" to the script, you will see that "t" already has latitude and longitude coordinate information attached to it. If you further do a "printVarSummary(t&lat)" you will see that the latitude coordinate array has units of "degrees_north" which is also required for overlaying data on a map. (Same for the "lon" array).

    Thus, all you need to do to draw your contours over a map is to call gsn_csm_contour_map instead of gsn_csm_contour. This function defaults to a cylindrical equidistant map projection.

    [Answer script] [Image]

  2. If you have global data, then it may or may not have a longitude "cyclic" point. The gsn_csm_xxxx scripts try to determine if your data is cyclic or not, but sometimes you have to help by setting the gsnAddCyclic resource to True or False, to indicate whether the cyclic point should be added for you. gsnAddCyclic is set to True for global data, so if your data is already cyclic, you need to set this to False. [Note: gsnAddCyclic is set to False if you have two-dimensional latitude/longitude arrays, which is not covered here.]

    [Answer script] [Image]

  3. Most of the resources used for this example were discussed in the contour plot exercises section above.

    [Answer script] [Image]

  4. Most of the resources used for this example were discussed in the contour plot exercises section above. The main title was discussed in the title exercises.

    [Answer script] [Image]

Contours over maps exercises (set 2)

  1. To attach coordinate arrays to a variable, you need to name each variable dimension, assign each one a coordinate array, and, for lat/lon coordinate arrays, make sure the "units" attributes are set to "degrees_north" and "degrees_east" respectively.

    [Answer script] [Image]

  2. There are several ways to zoom in on a map. See the documentation for mpLimitMode for more information. For gsn_csm_contour_map, if the default projection (cylindrical equidistant) is used, the default limit mode is "LatLon".

    [Answer script] [Image]

  3. Most of the resources used for this example were discussed in the contour plot exercises section above. Note that in this case, we are not setting a minimum or maximum contour level; we are only setting the spacing. This is possible when you use a cnLevelSelectionMode setting of "ManualLevels".

    [Answer script] [Image]

  4. Use the mpProjection resource to change the map projection. Once you do this, the limit mode is no longer "LatLon" as described for the previous example, so you'll need to set this resource. Also, the map projection may not look right, so you need to change the center latitude and longitude values as well.

    [Answer script] [Image]

  5. The gsnXXXString resources allow you to add smaller subtitles to the top of your plot, which will appear under the main title (tiMainString). To turn on lat/lon grid lines, visit the "mp" resource page, and look for the "mpGrid" resources.

    [Answer script] [Image]

  6. This example introduces the use of the DrawOrder resources in order to control what order aspects of the plot get drawn in. For example, you can change the draw order of contour lines, filled contours, map grid lines, and map outlines. For changing the lat/lon grid, see the "mp" resource page and look for the "mpGrid" resources.

    [Answer script] [Image]

Vector plot exercise hints

  1. Making the vectors multi-colored involves the use of one of the "vcMono" resources.

    [Answer script] [Image]

  2. Changing the reference magnitude is probably one of the most common things you'll want to do to a vector plot. Check out the vcRefMagnitudeF resource as a start.

    [Answer script] [Image]

  3. Use the vcLevelPalette resource to set a color map for the vectors. See the contour exercises above to see how to span only certain sections of a color map.

    [Answer script] [Image]

  4. Moving the reference annotation box involves the use of a vector orthogonal resource to move the box up and down, and a parallel resource to move the box left and right. Generally you have to use trial and error to get the box where you want it.

    To reorient the labelbar, visit the labelbar resources and look for the word "orientation".

    [Answer script] [Image]

  5. The default "mode" for setting vector levels is "AutomaticLevels", where NCL tries to pick "nice" levels based on the magnitude of your data. In order to set the levels yourself, see the vcLevelSelectionMode resource.

    Use tiMainString to change the title. See the related function code resource for changing the function code from a ":" to a "~".

    [Answer script] [Image]

  6. The default vector style is straight line vectors. To change to curly vectors or wind barbs, see the vcGlyphStyle resource.

    [Answer script] [Image]

  7. There are 64 timesteps on the file. Instead of going to an "x11" window, then, it might be better to go to an NCGM file and use idt to animate the NCGM file.

    Use the ismissing function to test for missing data.

    [Answer script] [Animation]

Primitives exercise hints (set 1)

  1. You need to set gsnFrame to False so that you can draw the marker before the frame is advanced. Use the gsn_polymarker procedure to add the marker using plot coordinates.

    [Answer script] [Image]

  2. To set some marker resources, use the "gs" (GraphicStyle) resources that start with "gsMarker".

    [Answer script] [Image]

  3. You can use the same resource list ("mkres") to change the markers for each set that you draw. To draw more than one marker of the same style, size, and color, pass an array of X and Y values to gsn_polymarker.

    [Answer script] [Image]

  4. This exercise produces the exact same plot as the previous exercise, only using a slightly different method. When you use any of the "gsn_add_polyxxx" functions, you must use a unique variable on the lefthand side for each call.

    You should also turn off the initial drawing of the plot and draw the plot manually after you add the markers.

    [Answer script] [Image]

Primitives exercise hints (set 2)

  1. Use the gsn_add_polyline function to add the lines using lat/lon coordinates. You need to set gsnFrame to False so that you can draw the lines before the frame is advanced. To set some lines resources, use the "gs" (GraphicStyle) resources that start with "gsLine".

    [Answer script] [Image]

  2. Use the gsn_add_polygon function to draw the polygon. You can use the same lat/lon coordinates that you used for the box lines. To set polygon resources, use the "gs" (GraphicStyle) resources that start with "gsFill".

    [Answer script] [Image]

  3. Use the gsFillOpacityF resource, which was added in NCL version 6.1.0.

    [Answer script] [Image]

  4. As with the above exercise, you can use "gsFill" resources to change the fill pattern and color of your polygon. To see a table of the available fill patterns, go to http://www.ncl.ucar.edu/Document/Graphics/Images/fillpatterns.png.

    [Answer script] [Image]

  5. Use the gsn_add_polymarker function to add the polymarkers. Note that when you use the "gsn_add" functions, the variables on the lefthand side must be unique, and they must not be deleted.

    [Answer script] [Image]

  6. Use the gsn_add_text function to add the text. The text resources start with "tx" (TextItem). Again, the variable on the lefthand side must be unique.

    [Answer script] [Image]

Primitives exercise hints (set 3)

  1. Use one "tmXB" resource to accomplish this. You can remove the other tmXB resources being set.

    [Answer script] [Image]

  2. All NCL plots are drawn in a unit square, using NDC coordinates. The drawNDCGrid procedure is in the shea_util.ncl script, so before you can use this procedure, you need to load this script with: 
    load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/shea_util.ncl"
    This procedure should be called before you draw anything else, so it doesn't cover up your plot.

    [Answer script] [Image]

  3. Use the gsn_polyline_ndc procedure to draw the box. Note that there is no "function" version of this procedure, like with gsn_add_polyline. Use one of the "gsLine*" resources to change the line color.

    You will need to set gsnFrame to False so that you can draw the lines before the frame is advanced. Of course, you could draw the lines before the plot was drawn, but then your plot would obscure the lines. It's usually better to draw the plot first, and then any primitives that you want.

    [Answer script] [Image]

  4. Instead of xpos and ypos being scalars, turn them into arrays of eight elements each, and then loop across each element to draw each box in a different color.

    [Answer script] [Image]

  5. Use the gsn_text_ndc procedure to draw the text strings. Use txFontHeightF to change the font height.

    The NCL default is to position the text based on the dead center of the box that contains the text string. Since we want the text to be left-justified to the right of each box, use the txJust resource to change the justification from "CenterCenter", to "CenterLeft".

    [Answer script] [Image]

  6. Use the gsn_polygon_ndc procedure to draw a filled box, and gsFillColor to set the fill color.

    [Answer script] [Image]

  7. Use four vp resources to reposition and resize the bar plot. All plots are positioned using the upper left corner of the plot box (in this case, this position is currently at (0.2,0.8)). A good position to start with is (0.17,0.9).

    [Answer script] [Image]

Paneling exercise hints

  1. Turn the "plot" variable into an array of 4 elements of type "graphic". To make sure each dataset is unique, you can use a different seed in the generate_2d_array function.

    [Answer script] [Image] (only first image shown)

  2. Use the gsn_panel to panel the individual plots.

    [Answer script] [Image] (only last image shown)

  3. The gsnDraw and gsnFrame resources control the drawing of the individual plots.

    [Answer script] [Image]

  4. See the resource cnLevelSelectionMode.

    [Answer script] [Image]

  5. This example also uses the cnLevelSelectionMode resource, but sets it to a different value.

    [Answer script] [Image]

  6. To turn off individual labelbars, see lbLabelBarOn. To turn on a common labelbar in a panel plot, see gsnPanelLabelBar. A few other resources are added in this script to turn off the individual informational labels, and to set the label auto stride for the labelbar in the panel plot.

    [Answer script] [Image]

  7. Normally, you would use the title resource tiMainString to add a title, but for paneled plots which don't recognize the "ti" resources, you can use txString instead. gsn_panel is set up to recognize txString as a special resource.

    Note that in this script, the main title font is changed to font 22 (helvetica bold) using the "F" text function code.

    [Answer script] [Image]

  8. See the resources gsnPanelFigureStrings and gsnPanelYWhiteSpacePercent. Note there is a similar resource for white space in the X direction.

    [Answer script] [Image]

  9. To turn off tickmarks, you need to set tickmark resources for the individual plots. See tmXTOn and tmYROn.

    [Answer script] [Image]