How to play
Sandbox: set where the impact happens (along the wall, how high, how far out), how fast the drops fly and whether gravity is on, and see the pattern it leaves. With Measure for me on, every stain is measured exactly so you can watch the method work; turn it off to measure them yourself. Find the source hides the source: tap a stain, slide the calipers to its edges, save it, and once three or more are saved the lab works out where they came from. Check your answer to see the real source and how far off you were. Within 30 cm counts as solved.
The science
Shape gives the angle. A drop that hits a surface straight on leaves a round stain. The shallower it comes in, the longer the stain: the width over the length is the sine of the impact angle, so a stain twice as long as it is wide hit at 30°. Analysts measure the ellipse and leave out the tail, which sits on the side the drop was travelling towards.
Lines give the place. Draw a line through each stain's long axis, back the way the drop came. On the wall those lines cross near one spot, the area of convergence: the point on the wall straight in front of the source.
Angles give the distance. Each stain's distance from the convergence point, times the tangent of its impact angle, says how far out from the wall its drop started. That is the area of origin. At a real scene this is done with strings taped to the wall at each stain's angle, or with software that does the same sums.
Why it's an area, not a point. Real drops fall as they fly. A slow drop curves down, so it meets the wall at a steeper angle than a straight line from the source would, and the strings put the source further out than it was. Turn gravity off in the sandbox and the strings meet exactly; turn it on with slow drops and watch them miss. That is why reports give an area and why analysts pick small, well-formed stains from fast drops.
How accurate is it?
The angle formula, the convergence and the tangent method are the ones taught in bloodstain pattern analysis courses. Drops fly in a vacuum here (no air drag), the stains are perfect ellipses and the wall is smooth paper, so real patterns are messier. Stains are drawn ten times life size on the wall so you can tap them; the close-up shows them at their real size in millimetres.
Is anything saved? Only how many sources you have found and your best result, on this device.