WPF 3D - mapping gradient brush on complex geometry

Question

I wanted to ask if anyone knows how to map gradient brush on complex objects in WPF 3D. The result should look similar to 3D images in matlab (3D function for example). Lets say you have some 3-dimensional data you wish to visualize and you want to diferentiate certain levels of values by color.

Answer 1

Given a GradientBrush defined something like this:

        <LinearGradientBrush x:Name="RedYellowGradient">
            <GradientStop Color="Blue" Offset="0.01" />
            <GradientStop Color="Purple" Offset="0.25"/>
            <GradientStop Color="Red" Offset="0.5"/>
            <GradientStop Color="Orange" Offset="0.75"/>
            <GradientStop Color="Yellow" Offset="1.0"/>
        </LinearGradientBrush>

Fundamentally, you assign the GradientBrush to the DiffuseMaterial of your MeshGeometry3D. When defining the brush, set its ViewportUnits property to "Absolute".

Something like this would work directly in the code-behind of a XAML form (otherwise, create the brush in code (in your ViewModel) using the corresponding properties, or call it from your resource dictionary):

MyMaterial = New DiffuseMaterial(RedYellowGradient) With {.ViewportUnits = BrushMappingMode.Absolute}

Then, for each Point3D in your geometry, assign a value between 0.0 and 1.0 to the corresponding texture coordinate. Generically, for a pre-sized Point array it could look like this:

    Parallel.For(0, positions.Count - 3, Sub(i)
                                             Dim p = positions(i)
                                             Dim plotValue = GetYourValue(p.X, p.Y, p.Z)
                                             Dim t = (plotValue - minPlot) / (maxPlot - minPlot)
                                             If t < 0 Then t = 0
                                             If t > 1 Then t = 1                                                
                                             arr(i) = New Point(t, 0.5)
                                         End Sub)

If your facets are very long or the values between vertices very far apart, your plotting will look odd. But given the strictures of WPF 3D it's probably the best you can do without a lot of UV Mapping.

(If you need C#, the Roslyn CTP has a VS Add-on which will convert VB code on the clipboard to C#...)