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NMPlot User's Guide, Chapter 10: Color Gradient Plots

Color gradient plots display a grid using smoothly varying colors. Such plots are visually striking, and can bring out subtle details in your data that are difficult to visualize using contours.

The Color Gradient Table

To create a color gradient plot, you must define a color gradient table, which maps grid data values to color/opacity pairs. For example:

Data Level Color Opacity (%)
10   Black   0
20   Dark Green   60
30   Light Green   100

NMPlot draws a color gradient plot by computing grid data values at a large number of locations, then, at each location, interpolating in the color gradient table to find the corresponding color to draw, and the opacity with which to draw it. For example, based upon the color gradient table above, a point on a plot where the grid data value is 25 would be colored an 80% opaque medium green.

Opacities may range from 0% (totally transparent) to 100% (totally opaque). An intermediate opacity will tint an area with a translucent color that allows a background map to show through.

Creating a Color Gradient Plot

To create a color gradient plot, follow these steps.

  1. Go to the Color Gradient page of the Plot Options dialog box.

  2. Check the box labeled Show Grid Using Color Gradient.

  3. Resolution: Type the resolution of the color gradient plot. This should be an integer between 2 and 10,000. Higher values produce smoother gradients, but also use more memory, and take longer to display.

    It is recommended that you initially try a resolution between 500 and 1000. If your color gradient plot appears blocky, increase the resolution.

    If your computer takes an excessively long time to display a color gradient plot, decrease the resolution. When NMPlot is creating a color gradient plot, it displays the message "Building color gradient image" on the status bar.

  4. Color Interpolation Space: Select the color space used to interpolate colors in the color gradient table.

    NMPlot supports three color spaces for use in interpolating colors: RGB (red green blue), HLS (hue luminance saturation), and CIE-LAB (International Commission on Illumination L*a*b*, 2° observer, D65 illuminant).

    The CIE-LAB color space is more perceptually uniform than the other color spaces, and therefore will often produce the most pleasing results. However, it is computationally intensive, and therefore somewhat slower to use.

  5. Select the method used to specify the color gradient table. You have two choices.

    • Automatically compute mapping of grid levels to colors - Choose the colors and opacities associated with the lowest and highest data values in your grid. NMPlot will automatically construct the color gradient table.

    • Manually specify mapping of grid levels to colors - Enter two or more levels, and the color and opacity associated with each. Press the Add Row button to add another row to the table. Press the Remove Row button to remove a row.

      You can specify the rows in any order. NMPlot will sort them if necessary.

See Color Control for information on using the drop-down color control to specify colors.

Tips and Suggestions

Tip:

Set your plot's background color to the color associated with the lowest level in your color gradient table.

Tip:

Avoid color gradient plots where the colors vary wildly across the spectrum. Numerous studies have shown that the eye does not perceive a natural visual ordering to colors, with the exception of reddish hues, which are interpreted as representing higher levels than other colors. Consider color gradients that vary by lightness or opacity instead of hue --- for example, dark to light gray, with reddish hues to highlight extreme levels.

Tip:

For a powerful plot, consider adding bold contours to a color gradient plot.


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