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Technical
Hoop Movement

By Samir Patel

Ok, I've been inspired to spend longer than anyone should staring at pictures taken from behind Reg Bamford of him running hoop 1 at a sharp angle firmly, but at ground level, during the Final of the World Championship earlier this year (2012 Adelaide).

Reg Bamford starting to run hoop

Photograph before hoop run

Reg Bamford after running hoop

... after hoop run

The two photographs below are two pixel-matched crops from larger photos. The first frame is from just before the red ball hits the hoop, and the second from just after it has run. The two frames were taken from a tripod with a cable release so are as steady as you can reasonably get - and a pixel comparison of the location of Reg's feet, hoop 5, another ball and what I think must be a distant piece of fencing bear this out. What there isn't there is the ideal intermediate frame with the ball against the far wire - which is why high-speed video would be much more useful. I've got a timestamp of 02:27 UK time, which was just under 2 hours after the match started, so I'm guessing this is the start of game 2 - the hoops were checked and firmed up between games. The courts were former bowling greens, and I'm assuming that the hoops were in new holes for the final and set to 1/32".

Hop position before shot

Hoop positon before shot

Hoop position after shot

Hoop positon after shot

Difference

Difference image

Unfortunately, I didn't envisage this sort of thing at the time and I thought I should keep the camera off the court, which is a long way of saying that the focus isn't quite perfect. This means that although I can tell you that Reg didn't shave his legs that morning, the hoop isn't quite in perfect focus - so there's a bit of guesswork as to exactly where the hoop starts and stops. The best scaling measurement that I can get from the hoop uprights is 24 pixels. Assuming that the hoops were exactly to spec, that gives us a "convenient" measure of a pixel as 5/192".

Enough pre-amble, down to measuring hoop movement. The crown of the hoop has moved 3 pixels up - that's 5/64".

The whole hoop has moved to the right. The angle makes it difficult to tell how much of this movement is to the right or backwards - indeed, movement directly away from the camera is effectively hidden. Assuming a minimum movement, just to the right, the movement is 2 pixels, or 5/96", or in round numbers 1/20". Given that the hoops will have been set (I assume) to 1/32", this is a lot of movement!

Difference detail showing pixels

Difference image showing pixels

At the level of the crown, the hoop as moved a whopping 8 pixels - that's 5/24" (1/5" or 5mm)

Now, a few caveats. A combination of resolution and focus means that there is a margin for error in these measurements. However, I've been as careful as I can in comparing pixel colours and contrasts. If you want to have a go yourself, good luck! Secondly, the "after" image is a short time after the ball left the hoop. The hoop could have moved further at its most extreme (which I would expect to be when the striker's ball was in the middle of the hoop) - Andrew's estimate of 15mm is certainly plausible. Oh, and this is all subject to me being able to count and add up accurately. Any reliance you place upon these figures is entirely at your own risk.

So what? Well, this all shows is that the whole hoop has effectively moved to the right (i.e. away from the stroke) and rotated up out of the ground while pivoting on the far wire - but by quite a lot. This is intuitively what I'd expected. However, most of all, the fact that the hoop , at ground level, remained 1/20" further to the right than before the stroke. This reinforces the idea that a hoop will move to accommodate where the ball wants to go.

 

Author: Samir Patel
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Updated 28.i.16
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