Software Tool for Measuring Area in Digital Pictures

General Discussion
1

The Tool ImageJ.

A use: Approximating ‘Projected’ Bottom area… to start.

The ImageJ tool will allow you to approximate flat regions on a digital image. Though surfboards are rarely flat, this is still a pretty handy tool for approximating area from a digital picture. In fact, if you know just one reference measurement, you can pretty much get them all from the picture - well, to a point, as mentioned, surfboards are rarely flat.

To make it work you will have to at least know one actual measurement, for example, the deck or bottom length of the board. Fairly clear instructions are provided.

Why would you care…

If you don’t really care about surface area, at least its precise measurement, then my guess is you wouldn’t care. However, if you do -i.e. you’re a closet geek and have your own reasons for wanting a quick way to generate this kind of information, whether it be for boards or fins or whatever, then it may be worth exploring.

Sadly, unless you know the curvature (or rocker, for surfboards), you can not determine the exact surface area. However, it looks like it will be possible to add a plugin which will take incremental rocker measurements… wait for it, or do it yourself.

The value of knowing the area…

A big part of planing as applied to surfing is about wetted-area. A big part of design is about how a rider can bring wetted area on and off-line.

An application…

Aside from my interest in bottom area, I’m also interested in fin area. I’m slowly puting together a database of various fins and their respective surface areas. Why? That’s my problem. But ImageJ has very helpful. I’m also doing the same for templates, again as to why being my problem.

… as always

Geshalt works in surfboard design, so does the ‘Hey, how about this’ approach. In fact it has gotten design to where it is today. Journeymen and garage builders ‘don’t need no stinking physics.’ Like I mentioned, this is just for the closet geeks.

kc

2

you forgot about fins

3

Here’s an example using a fin,

When you set it [software] up you can also get it to give the centroid coordinates - the XM and YM in the diagram - which are nice for modeling. They also might come in handy when you’re trying to figure out why your ‘rad’ fin design, which you’ve glassed on, keeps cracking at the tail end of it’s base, or why your fin was ripped off on your first big power bottom turn -i.e. it can be used to give a quick and dirty ‘torque’ analysis. That’s assuming you had a picture of the fin prior to loosing it at sea.

kc

4

Lucky you…once again, my World is flat, so here’s another quicky illustration, this time with a cutaway.

… Cut what away?

Obviously as to exactly what was cutaway on the Walden 7.5 cutaway in the diagram is debatable. I’m not to sure I like my assumption as to what actually was cut away, my curve is a little funky, but its just an example.

Here, if you buy my crude sketch of the full fin the area is 32.581 sq. in., let’s call it 32.6 sq. in. . The cutaway is 28.631 sq. in., say it’s 28.6 sq. in. . That’s a difference of 4 sq. in., or about 12% of the original fin. Also, check out the change in the centroid coordinates.

Of course, what part of the fin that is being cut away matters, but here you have an actual number. Admittedly, this may or may not mean all that much to most. Also, I am not suggesting that fin design is merely about acreage – it’s just an example. But gee… 4 sq. in. or 12%? Curious, that does seem to be a fairly important 12%, …but I guess that’s another thread.

kc

5

I think that is the most important element of this software,since the program can’t account for rocker.

6

the free BOARDCAD program gives you surface area measurments. but this program is great for fins, cheers mr kasey

7

Thanks.

Like the thread title states, it’s a ‘Software Tool for Measuring Area in Digital Pictures’… wouldn’t think of, nor propose a replacement for any of the existing CAD packages. It’s just another tool.

When mastered, which takes a few minutes, it’s a quick and dirty way to get some numbers off of digital pictures. In fact, on occasion, you can use it to get, though admittedly still approximate, wave heights. It’s tricky and I’d be hesitant to go public with any findings, but it’s sort of neat nevertheless.

In my opinion CAD, or whatever CAD-like packages emerge, are likely to own the future.

kc

8

Here’s another quick and dirty kind of analysis.

In the figure I’ve used a semi-popular longboard fin (which shall remain nameless to protect the innocent?) Here the fin has a height of 8 inches.

What I’ve done is just sketch in some variations of the basic fin to get some idea as to what might be happening. That is, I kind of know what a fin that has a shape indicated by 4 sort of feels like – mucho stiff for starters. But what happens when I start to peel away ‘fin’ as I’ve done in curves 3, 2 and 1 (1 is the original fin.) It just so happens that I sort of know how the shape indicated by 1 behaves too, so I guess that this a is less than an ‘objective’ example.

Obviously the surface area is changing fairly dramatically, and it’s likely the fin will be ‘loosening’ up with each modification, when moving from 4 back to 1. But with decreasing surface area, the geometric center, or centroid of the fin is also changing. Tamely at first, almost traveling alone a straight line. But somewhere between 2 and 1 things change dramatically.

The labels prefixed by the c are the centroids for the respective shapes. Centroids are handy as they are where you could model all the force developed by the fin to be centered. (Assuming the fin didn’t change its shape when subject to a force, which admittedly is not likely here.) Nevertheless, by the time you get to the original fin, 1 with centroid c1, things have changed significantly.

To appreciate this, think about loosing a nut with a wrench. The longer the handle, the easier it is to loosen the nut. That is, the less force you have to apply to get the nut turning.

Here, the centroid is moving forward as you approach the original fin, but then somewhere between 2 and 1 takes a sort of vertical turn. The impact is likely to be sensed in the roll response of the board, as opposed to the yaw or ‘which way am I headed’ kind of response. So you might, as a sort of rough assessment conclude some increase stability, say for ‘walking’ around –i.e. walking the board, but also that you might want to watch out on those big bottom turns - especially if the fin is capable of storing a lot of elastic energy – I did say it would be less than an objective example.

This is pretty quick and dirty, but fun.

kc