
<section id='NMPlot_ColorGradientPlots_id'
webfilename='colorgradientplots'>
    <title>
        Color Gradient Plots
    </title>
    <para>
        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.
    </para>
    <exhibit>
        <img src='NMPlot_Figure_SampleColorGradientPlot.jpg'/>
    </exhibit>
    <section id='NMPlot_ColorGradientTable_id'>
        <title>
            The Color Gradient Table
        </title>
        <para>
            To create a color gradient plot, you must define a <termtext>color
            gradient table</termtext>, which maps grid data values to color/opacity
            pairs. For example:
        </para>
        <exhibit>
            <table>
                <tableheader>
                    <tablecolumnheader horzjust='center'>
                        Data Level
                    </tablecolumnheader>
                    <tablecolumnheader horzjust='left'>
                        Color
                    </tablecolumnheader>
                    <tablecolumnheader horzjust='left'>
                        Opacity (%)
                    </tablecolumnheader>
                </tableheader>
                <tablerow>
                    <tablecell>
                        10
                    </tablecell>
                    <tablecell>
                        Black
                    </tablecell>
                    <tablecell>
                        0
                    </tablecell>
                </tablerow>
                <tablerow>
                    <tablecell>
                        20
                    </tablecell>
                    <tablecell>
                        Dark Green
                    </tablecell>
                    <tablecell>
                        60
                    </tablecell>
                </tablerow>
                <tablerow>
                    <tablecell>
                        30
                    </tablecell>
                    <tablecell>
                        Light Green
                    </tablecell>
                    <tablecell>
                        100
                    </tablecell>
                </tablerow>
            </table>
        </exhibit>
        <para>
            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.
        </para>
        <para>
            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.
        </para>
    </section>
    <section id='NMPlot_CreatingColorGradientPlot_id'>
        <title>
            Creating a Color Gradient Plot
        </title>
        <para>
            To create a color gradient plot, follow these steps.
        </para>
        <orderedlist>
            <listitem>
                <para>
                    Go to the Color Gradient page of the <link
                    target='#NMPlot_PlotOptionsDialogBox_id'>Plot Options dialog box</link>.
                </para>
                <exhibit>
                    <img src='NMPlot_ScreenCap_PlotOptions_ColorGradient.png'/>
                </exhibit>
            </listitem>
            <listitem>
                <para>
                    Check the box labeled <italic>Show Grid Using Color Gradient</italic>.
                </para>
            </listitem>
            <listitem>
                <para>
                    <italic>Resolution:</italic> 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.
                </para>
                <para>
                    It is recommended that you initially try a resolution between
                    500 and 1000. If your color gradient plot appears blocky, increase
                    the resolution.
                </para>
                <exhibit>
                    <img src='NMPlot_Figure_ColorGradientResolution.png'
                    standard_colors='true'/>
                </exhibit>
                <para>
                    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 <link
                    target='#WCF_ApplicationMainStatusBar_id'>status bar</link>.
                </para>
                <exhibit>
                    <img
                    src='NMPlot_ScreenCap_StatusBar_BuildingColorGradientImage.png'/>
                </exhibit>
            </listitem>
            <listitem>
                <para>
                    <italic>Color Interpolation Space:</italic> Select the color space
                    used to interpolate colors in the color gradient table.
                </para>
                <para>
                    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&deg; observer,
                    D65 illuminant).
                </para>
                <para>
                    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.
                </para>
            </listitem>
            <listitem>
                <para>
                    Select the method used to specify the color gradient table. You
                    have two choices.
                </para>
                <unorderedlist>
                    <listitem>
                        <para>
                            <italic>Automatically compute mapping of grid levels to colors</italic>
                            - Choose the colors and opacities associated with the lowest and
                            highest data values in your grid. NMPlot will automatically construct
                            the color gradient table.
                        </para>
                    </listitem>
                    <listitem>
                        <para>
                            <italic>Manually specify mapping of grid levels to colors</italic>
                            - 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.
                        </para>
                        <para>
                            You can specify the rows in any order. NMPlot will sort them if
                            necessary.
                        </para>
                    </listitem>
                </unorderedlist>
            </listitem>
        </orderedlist>
        <para>
            See <iref target='WCF_ColorControl_id'/> for information on using
            the drop-down color control to specify colors.
        </para>
    </section>
    <section id='NMPlot_ColorGradientPlotTips_id'>
        <title>
            Tips and Suggestions
        </title>
        <note type='tip'>
            <para>
                Set your plot's <link target='#WCGIS_MapBackground_Color_id'>background
                color</link> to the color associated with the lowest level in
                your color gradient table.
            </para>
        </note>
        <note type='tip'>
            <para>
                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.
            </para>
            <exhibit>
                <img src='NMPlot_Figure_RecommendedColorGradients.png'
                standard_colors='true'/>
            </exhibit>
        </note>
        <note type='tip'>
            <para>
                For a powerful plot, consider adding bold contours to a color
                gradient plot.
            </para>
            <exhibit>
                <img src='NMPlot_Figure_ColorGradientWithContours.png'/>
            </exhibit>
        </note>
    </section>
</section>
