As a child, I became the proud owner of a wonderful toy "Spirograph": plastic ruler with two large holes with jagged edges and a few jagged circles smaller with several holes-holes at various distances from the center. That's all.
But it was enough to put in a pen in one of the holes a small circle and gently ride along the great circle on the paper started showing incredible figure for my child's perception of beauty. Admired in the first place, the smooth lines being drawn and inexhaustible number of options received by the figures. At this session was spent not one hour, what do not regret :)
Moreover, I am now "ride circles" in the program written by me, I know that the resulting figure is called "gipotrohoida" I know how it differs from the "epitrochoid" and "hypocycloids" .... and excited about the process of drawing it all the same :)
Management tried to make the most intuitive:
In the "Settings" (the left-most button) touch the desired circle and move to the right and to the left (this is recalculated and displayed on the screen fading outline future gipotrohoidy)
Button «Play» (the rightmost button) starts the process of drawing the figure of the button to press to pause the process of drawing.
The next button hides / shows the service elements in drawing (circle «R», «r» and the segment «d»)
At any time, you can take a screen shot. The resulting image is saved in the root directory of the SD-card in the file «spirograph.png».