We begin by using our bounding box's top face as a grid to be rayed. We convert the grid to points and use the reverse of the grid's N value to choose the direction to shoot the rays. Think of rays as rays of light. They illuminate the first surface that they come into contact with. In the case of the ray SOP, it shoots rays from one geometry to another and records the first surface that the ray hits. We can see below how raying onto a squab will only record the first point of contact.
We now have an idea of what the top surface of a geometry looks like. However, we are still unsure as to the types of angles that we're dealing with. For example, there are very steep angles on the squab geometry that wouldn't gather snow very well. To find these areas, we can find all the points that point directly up. A face that points directly up will have a N.y value of 1.
If we know the faces that are flat (point straight up), we can use that to find a gradient. We can use that value to determine a multiplier for the density of our rayed points. The multiplier can be put in a variable called 'mult'.
Once we have our 'mult' value, we can use it to drive the density of point and the scale of those points to allow steeper areas to not have as much snow. Below you can see the effect of this.
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