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

Color gradient plots display a grid using smoothly varying colors. They 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 colors. For example:

Data Level		Color
10		Black
20		Dark Green
30		Light Green

NMPlot draws a color gradient plot by computing grid data values at a large number of locations, then interpolating in the color gradient table to find the corresponding colors to draw. For example, based upon the color gradient table above, a point on a plot where the grid data value is 25 would be colored medium green.

Creating a Color Gradient Plot

To create a color gradient plot, follow these steps.

Go to the Color Gradient page of the Plot Options dialog box.
Check the box labeled Show Grid Using Color Gradient.
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.
Color Interpolation Space: Select the method used to interpolate colors in the color gradient table.

There are two common ways to represent colors mathematically. One method uses the relative intensities of the three primary colors: red, green, and blue. The other method use the color's hue, luminance, and saturation. You can select which method is used to represent colors when interpolating in the color gradient table.

It is not necessary to understand the details of the interpolation methods. Simply be aware that the two methods will produce different results. You should try both, and select the one that produces the best results for your particular grid and color table.
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 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 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 across the spectrum. Numerous studies have shown that the eye does not naturally give a visual ordering to colors, with the exception of red, which is interpreted as representing higher levels than other colors. Favor color gradients that vary by lightness and not by hue. For example, have your color gradient vary from dark green to light green. Alternatively, vary your colors from dark to light gray, with red to highlight extreme levels.

Tip:

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