The "Function Graph: f(x,y)" window shows the graph of z(x,y) = -0.5x - 0.5y + 1.

The stepX and stepY variables determine the position of the two sweeping cross sections and are controlled by "tape deck" controllers.

There are two ways to perform the double integration: You can either sweep out along the x-axis, and then let the resulting area sweep along the y-axis (this is called first summing or integrating over x, and then over y). The other possibility is to first integrate over y and then integrate over x. This demo shows that the order of the summation/integration does not change the actual value of the integral. No matter which variable you choose to sum over first, you will always get the same volume under the function graph.