A solver is essentially a reduce function. It’s passed, as its first argument, the result of the previous loop’s evaluation. A reduce function in Javascript: const array1 = [1, 2, 3, 4]; function reducer(accumulator, currentValue) { return accumulator + currentValue } ; result = array1.reduce(reducer) //=> 10 Inside a solver: As an example, we create a network with a single attribute @num = 0 and pass it into the solver.

The idea: Use a solver to accumulate boxes from each frame of a “simulation”. E.g. we’re creating a reduce function that looks like this: func accumulate(boxes []box, currBox box) { boxes = append(boxes, currBox) return boxes } To get started, we create one point on a grid on each frame. We then copy a randomly colored box to each of those points, on each frame e.g. the box is given a different color on each frame, and then copied to the new point on that frame.

Here we have a bunch of curves that change color over time and transfers their @Cd to the grid when they come into contact with it. When a frame cooks, Houdini evaluates the contact only on that frame, it has no “memory” as to whether they’ve touched before. So how do we persist the color? We do the attribute transfer inside a solver node, which will allow us to “accumulate” the paint just as we did with boxes.