In the image3d context, when geometry is specified (i.e. metaball geometry or particles), you can iterate over the metaballs which affect a point in space.

forpoints ( position [, distance] ) {

}

…where the position is a vector representing a point in space. The statement will be executed once for each metaball/particle at the position passed in.

If you specify the distance, all metaballs/particles within the distance of the point specified will be iterated through. The distance parameter is optional and may result in slower execution of the shader.

Inside the loop, you can call the mdensity and mattrib functions to query the contribution of the current point instead of getting a “blended” value.

For example, the following code will take the point color of the metaball which contributes the maximum weight to the point in space:

float d = 0, max = 0;
vector clr = 0;
vector blended_color;

forpoints ( P ) {
    d = mdensity(P);
    if (d > max) {
        clr = mattrib("Cd", P);
        max = d;
    }
    blended_color = d * clr;
}

Note that when you call mattrib inside a forpoints loop, the attribute is not pre-blended by the density of the metaball.