assigning values to shader parameters in the XNA content pipeline

Question

I have tried creating a simple content processor that assigns the custom effect I created to models instead of the default BasicEffect.

[ContentProcessor(DisplayName = "Shadow Mapping Model")]
public class ShadowMappingModelProcessor : ModelProcessor
{
    protected override MaterialContent ConvertMaterial(MaterialContent material, ContentProcessorContext context)
    {
        EffectMaterialContent shadowMappingMaterial = new EffectMaterialContent();
        shadowMappingMaterial.Effect = new ExternalReference<EffectContent>("Effects/MultipassShadowMapping.fx");
        return context.Convert<MaterialContent, MaterialContent>(shadowMappingMaterial, typeof(MaterialProcessor).Name);
    }
}

This works, but when I go to draw a model in a game, the effect has no material properties assigned. How would I go about assigning, say, my DiffuseColor or SpecularColor shader parameter to white or (better) can I assign it to some value specified by the artist in the model? (I think this may have something to do with the OpaqueDataDictionary but I am confused on how to use it--the content pipeline has always been a black box to me.)

Answer 1

Kikaimaru gave me some helpful tips, so here is an answer (a code example) to my own question based on what I've learned about custom shaders and building materials in the content pipeline:

[ContentProcessor(DisplayName = "Shadow Mapping Model")]
public class ShadowMappingModelProcessor : ModelProcessor
{
    protected override MaterialContent ConvertMaterial(MaterialContent material, ContentProcessorContext context)
    {
        // We want to use a custom shader we wrote ourselves, so we build our .fx file, putting shader parameter 
        // values into opaque data and textures using our shader variable names as the keys.

        // You can see these default "key (value)" entries on "material" by using the content pipeline debug project here:
        // (http://badcorporatelogo.wordpress.com/2010/10/31/xna-content-pipeline-debugging-4-0/)

        // default "material" opaque data:
        // ===============================
        // DiffuseColor (vector3)
        // EmissiveColor (vector3)
        // SpecularColor (vector3)
        // Alpha (float)
        // SpecularPower (float)
        // VertexColorEnabled (bool)
        //
        // default "material" textures:
        // ============================
        // Texture (external texture reference)

        // use our custom effect
        EffectMaterialContent shadowMappingMaterial = new EffectMaterialContent();
        shadowMappingMaterial.Effect = new ExternalReference<EffectContent>("Effects/MultipassShadowMapping.fx");

        // transfer opaque data
        shadowMappingMaterial.OpaqueData.Add("DiffuseColor", new Vector4((Vector3)material.OpaqueData["DiffuseColor"], (float)material.OpaqueData["Alpha"]));
        shadowMappingMaterial.OpaqueData.Add("SpecularColor", material.OpaqueData["SpecularColor"]);
        shadowMappingMaterial.OpaqueData.Add("SpecularPower", material.OpaqueData["SpecularPower"]);

        // transfer texture
        if (material.Textures.ContainsKey("Texture"))
        {
            shadowMappingMaterial.OpaqueData.Add("DiffuseTextureEnabled", true);
            shadowMappingMaterial.Textures.Add("DiffuseTexture", material.Textures["Texture"]);
        }

        return context.Convert<MaterialContent, MaterialContent>(shadowMappingMaterial, typeof(MaterialProcessor).Name);
    }
}