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New color capabilities (V6.1.0 or later)

NCL V6.1.0 introduced a new color model in which color is specified in a natural and flexible manner. Colors may have degrees of transparency, and there is no longer a limit of 256 colors in a plot. Backwards compatibility with the previous color model is also retained. This new color model is limited to PS, PDF, PNG, and X11 output.

In versions of NCL prior to v6.1.0, colors are specified relative to a colortable that is associated with the workstation. Most commonly, a color-resource is assigned the index into the colortable of the desired color. Alternatively, a named-color or an RGB-triplet may be given to designate a specific color; the color is added to the lookup table if there is room, or the most similar color in the table is used. The workstation colortable has a limit of 256 entries, and those are the only colors that may appear in a plot.

In the new model, color may -- and should -- be used independently of a workstation colormap. Users choose colors directly by names or RGB-triplets, without appeal to a workstation colormap. Colors might even be computed dynamically based upon a data source. There is no practical limit on the number of colors that may be used in a plot.

Colors may also be partially transparent, by specifying an opacity value (sometimes referred to as alpha in computer graphics parlance). Opacity is given as a floating-point in the range [0.,1.], where 0. means completely transparent, and 1. means fully opaque. Opacity values may be given for individual colors by specifying a 4-tuple, in which the components are red, green, blue, and opacity values. Alternatively, newly introduced resources can set the opacity for entire classes of graphical primitives (described below).

Finally, to maintain backwards compatibility, the one exception to the foregoing discussion is that if color is ever specified as an integer index, it is interpreted to be a color relative to the workstation's colormap. Workstations still have an associated colormap that may be changed and manipulated as before, however this usage is discouraged for new scripts.

Specifying Opacity
Again, opacity values range from 0. to 1. The opacity of individual colors can be specified by giving a 4-tuple of red, green, blue, opacity values. For example:

  res@gsLineColor = (/ 1., 0., 0., .5 /)
specifies a partially transparent red color.

New resources are available to specify the opacity of classes of graphical elements:

cnFillOpacityF sets the opacity of all colors in a contour plot
vcGlyphOpacityF sets the opacity of all vector glyphs in a vector plot
stLineOpacityF sets the opacity of all streamlines in a streamline plot
gsFillOpacityF sets opacity for color-filled polygons
gsLineOpacityF sets opacity for boundaries of polygons, polylines, etc.
gsMarkerOpacityF sets opacity for marker-based graphics
txFontOpacityF sets opacity for textual graphics

Import note: in NCL V6.3.0 and earlier, there's a bug in which the labelbar does not reflect the same opacities as filled contours or vectors that have cnFillOpacityF or vcGlyphOpacityF set. This bug was fixed in NCL V6.4.0. If this fixed behavior is not desired, set lbOverrideFillOpacity to True.

A new perspective on colormaps
While the use of a workstation colormap is no longer necessary, color lookup tables are a useful means to map color onto data, such as in a contour plot or vector plot. The following new resources can be used to define a colormap for particular plot:
cnFillPalettecolormap to used for a filled contour plot
vcLevelPalettecolormap to use in a filled vector plot
stLevelPalettecolormap to use in a filled streamlines plot

You can also use existing color resources to set colors via an RGB or RGBA array:

cnFillColorsRGB(A) values used in a contour plot
vcLevelColorsRGB(A) colors used in a vector plot
stLevelColorsRGB(A) colors used in a streamlines plot
mpFillColorsRGB(A) colors used for filling map areas

Colormaps used in this fashion are still limited to 256 colors, but each instance of these plot types may have its own colormap. Thus for example, multiple contour plots appearing in a panel plot may employ differing colormaps. Or several plots using distinct colormaps incorporating partial transparency may be overlain to depict multiple data aspects, as in the example) below.

A new function, read_colormap_file is available to make it easy to load existing NCL system colormaps, or user-created colormaps. Note that this function always returns 4-component colors, comprised of red, green, blue, opacity values; the opacity defaults to 1 (fully opaque) where ever it is not explicity given. User created colormaps may freely intermix 3-component and 4-component tuples.

newcolor_1.ncl: This example simply illustrates that with V6.1.0 of NCL, you no longer need to add named colors to your color map in order to use them. This example uses a 256-color map (BlueYellowRed) that contains no gray in it.

The first frame shows what the graphic would look like if you ran this script with NCL version 6.0.0 or earlier. You get a bluish color for the land, because this was the closest match to gray that NCL found in the workstation color map.

The second frame shows the graphic as generated by NCL versions 6.1.0 or later, in which you should see gray-filled land.

Internally, the land areas are being filled with "LightGray".

newcolor_2.ncl: A simple example showing possibilities with text opacity resources.

txFontOpacityF is set to 0.10 to produce a highly-transparent text string.

newcolor_3.ncl: Re-creates the original opaque markers example that showed how to achieve transparency effects with previous versions of NCL, in combination with external tools. Here, its simply a matter of using the new gsMarkerOpacityF resource to achieve the desired effect.
newcolor_4.ncl: Adapted from an example of scatter plots. The original plot is re-created using the new color model idioms, and a second version is drawn using partially transparent colors. Notice how markers that are obscured in the first version are visible in the second plot.
newcolor_5.ncl: This example shows how to use two large color maps in the same contour plot, both containing 254 colors (BlueRed and GreenYellow). The three frames show how to make the GreenYellow contours increasingly more transparent.

To do this, you first need to set the new resource cnFillPalette to the desired colormap (NCL will automatically span it). To control then opacity, set the new cnFillOpacityF resource to 1.0 for a fully opaque plot, and 0.4 for a mostly transparent plot. A value of 0.0 is fully transparent.

Important note: in NCL V6.3.0 and earlier, there's a bug in which the colors in the labelbar do not correctly reflect the opacity applied to the filled contours. This bug has been fixed in NCL V6.4.0. See the next example for more information.

newcolor_5.ncl: This example is identical to the previous one, except it shows what the labelbar will look like in NCL V6.4.0, now that the colors reflect the same opacity values reflected in the filled contours.

If you do not want the labelbar to show the same fill opacities as the color contours, set lbOverrideFillOpacity to True.

newcolor_6.ncl - Shows how to use transparency to de-emphasize a particular area in a plot.

In the first frame, cnFillOpacityF is used to first draw the full plot with a transparency of 0.5, and then the second subsetted plot is drawn with no transparency.

In the second frame, gsFillOpacityF is used to draw a partially transparent filled box over an area to "hide" it.

Important note: in NCL V6.3.0 and earlier, there's a bug in which the colors in the labelbar do not correctly reflect the opacity applied to the filled contours. This bug has been fixed in NCL V6.4.0. See example newcolor_5.ncl for more information.

newcolor_7.ncl: Adapted from an example on overlay techniques. Here the original plot is created using multiple and independent colormaps for the contours, vectors, and streamlines. Then two additional versions of the plot are generated, varying the levels of opacity of the contours and streamlines. Notice how opacity can be used to (de)emphasize or declutter overlain graphics.

Other concepts illustrated are the use of new resources cnFillPalette and vcLevelPalette to load desired colormaps, and the direct specification of color for the map backgrounds, rather than by giving colormap indices.

In the script, code using constructs of the previous color model has been commented out with special annotations, and is followed immediately by equivalent idioms in the new model, to contrast the different usages.

Important note: in NCL V6.3.0 and earlier, there's a bug in which the colors in the labelbar do not correctly reflect the opacity applied to the filled contours. This bug has been fixed in NCL V6.4.0. See example newcolor_5.ncl for more information.

newcolor_8.ncl: This example shows how to draw partially transparent filled vectors over filled contours. The vectors are drawn fully opaque in the first frame. In the second frame, vcGlyphOpacityF is set to 0.3.

This example also shows another method for subscripting a color palette, if you don't want to use the whole thing. It first uses read_colormap_file to get an RGBA array, and then passes a subset of it cnFillPalette.

newcolor_9.ncl: This example makes use of overlays and opacity to plot full color imagery. The red, green, and blue channels of a source image are plotted separately as "contour maps". The red channel is plotted with full opacity, while the green and blue channels are plotted as completely transparent. When the green and blue channels are overlain on top of the red image, the colors combine to recreate the colors of the image, but upper layers do not obscure lower ones due to their transparency.

Notice that the colormaps for the red, green, blue contour maps are computed as a ramp-function, from 0. to 1.

The open source tool gdal_translate was used to convert an original image (in .png, .jpg, .gif, etc.), into a NetCDF file with the color-channels pre-separated:

  gdal_translate -ot Int16 -of netCDF fuji_orig.jpg fuji.nc

This example only works for "x11" or "png" output, and not with "ps" and "pdf" output.

newcolor_10.ncl: This example shows how you can use the 256-color OceanLakeLandSnow color table to draw filled terrain from a WRF output file, and then use the 103-color WhViBlGrYeOrRe color table to overlay filled contours showing reflectivity.

The cnFillPalette resource is used to set the color palette.

The first color for reflectivity is set to transparent by setting the "A" component of the RGBA color array to 0.0.

overlay_12.ncl: This example is very similar to the previous newcolor_10.ncl example, except the labelbar for the terrain plot is also drawn.

Important note: In NCL V6.3.0 and earlier the labelbar does not reflect the same opacities as the filled contours; this bug was fixed in NCL V6.4.0. A new resource called lbOverrideFillOpacity was introduced in NCL V6.4.0 which allows you to keep the labelbar colors fully opaque independent of the opacity of the filled contours.

The first frame shows the partially opaque labelbar, and the second frame shows a fully opaque labelbar created by setting lbOverrideFillOpacity to True.

newcolor_11.ncl: This example is similar to example #9. It recreates a JPEG image using overlays and opacity. It then attaches lat/lon information to the jpeg image, allowing us to change the projection to "satellite", and overlay map outlines and contour lines.

As with example #9, the open source tool gdal_translate was used to convert the jpeg file to a NetCDF file:

  gdal_translate -ot Int16 -of netCDF EarthMap_2500x1250.jpg EarthMap_2500x1250.nc

This example only works for "x11" or "png" output, and not with "ps" and "pdf" output.

newcolor_12.ncl: This example shows how to draw partially transparent filled polygons using gsFillOpacityF.

The point of this example is to show how various boxes look when they overlaid in a different order. The middle column was drawn starting with the red box starting first. The right column was drawn with the yellow box starting first.

newcolor_13.ncl: This example shows how to draw four panelled contour plots, each with a different color map. With older versions of NCL, you had to draw each plot before you changed the color map, or you had to merge all color maps into one single color map that was fewer than 256 colors.

With NCL V6.1.0 and later, you can use the cnFillPalette resource to define a color palette for each filled contour plot.

newcolor_14.ncl: This example is similar to example 13, in that it shows you how to draw three sets of panelled contour plots, each with a different color map. This example is identical to panel example #26 except it shows you the easier way to do this using cnFillPalette.
newcolor_15.ncl: The first frame of this example uses the cnFillPalette resource to set the "BlueYellowRed" color map for the contours. In the second frame, it sets cnSpanPalette to False to tell it not to span the full colormap. In the third frame, it then uses read_colormap_file to subset just the red/yellow portion of the color map.
newcolor_16.ncl: This example overlays three filled XY plots. In the second frame, it uses gsnXYFillOpacities (a new resource that was added after V6.1.2 was released) to specify an opacity for each of the filled areas.

In order to use this new resource with NCL V6.1.0, 6.1.1, or 6.1.2, you must download the fill_opacities_fix.ncl file and load it in your script after "gsn_csm.ncl" is loaded:

load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl"
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_csm.ncl"
load "./fill_opacities_fix.ncl"
newcolor_17.ncl: This example uses transparency in order to show where positive and negative contours in two different plots overlap.

cnLevels is set to 0.0 so that we only have two contour levels (< 0 and > 0). Then, cnFillColors is set to RGBA values so we can set the opacity for each contour fill color for each plot:

base plot:
res@cnFillColors = (/(/0.60,0.60,0.60,1./),(/1.,1.,1.,1./)/)

overlay plot:
res@cnFillColors = (/(/0.60,0.60,0.60,1./),(/1.,1.,1.,0./)/)

By setting the white contours in the overlay plot to fully transparent, this means that you only see white in locations where both datasets are positive.

The contour lines are drawn in different colors just so you can see where the contours actually are in the overlay.

newcolor_18.ncl: The purpose of this example is to show a work-around for a bug in NCL V6.3.0 and earlier, in which the labelbar colors do not reflect the same transparency as the filled contours when cnFillOpacityF is used to set the opacity.

This bug has been fixed in NCL V6.4.0. If you do not want the labelbar to reflect the same opacities, you can set the lbOverrideFillOpacity resource to True.

The first frame of this example draws a fully opaque contour plot, using cnFillPalette to specify the colors to use.

The second frame draws a partially opaque contour plot by setting cnFillOpacityF to 0.5. If you have NCL V6.3.0 or earlier, you will note that the labelbar colors do not reflect the same opacities.

The third frame shows a work-around. It draws the same partially opaque contour plot, except by 1) calling span_color_indexes to generate a set of integer indexes that fully span the "GMT_hot" color table, 2) using these indexes in an RGBA array (254 x 4) generated by calling read_colormap_file, and 3) setting the "A" component of this RGBA array to 0.5. This has the same effect as setting cnFillOpacityF to 0.5, except now the labelbar colors also appear partially opaque.