## The CompGeoJS Project

I’ve received a few emails from readers of this (now closed) blog regarding my current development direction. I’m actually taking some time off after working on a very long Flex gig to get involved in some new stuff. As I look back over the span of time from 2004 (when I started this blog) to nearly 2014, my two favorite open-source projects were Degrafa and Singularity. The Singularity project provided an Actionscript library for commonly used tools from the field of computational geometry. I wrote a series of free TechNotes to explain the mathematics behind the techniques. Unfortunately, the TechNotes did not follow any reasonable pedagogical order, and I kind of regret the approach.

I’m very interested in reviving this development for the new wave of interactive developers who are concentrating their efforts in HTML 5 and JS. The goal of the CompGeoJS project is to provide interactive developers with a comprehensive, open-source library for addressing problems arising from the fields of analytic and computational geometry. Unlike Singularity, the library will be developed first, along with a robust collection of demos and online documentation. The process will be much more organized this time around and will eventually enable developers to tackle a wide variety of applications such as architecture, floor planning, advanced programmatic animation, and gaming.

Once the library reaches a solid ‘version one’ state, I will write an eBook to accompany the online documentation that discusses the mathematics behind all the tools. I hope this will be of use to developers who wish to modify the library for very customized uses as well as instructors of numerical analysis and computational geometry.

It’s at a VERY early stage of development, but you can check out what is available right now and where the library is going at the CompGeoJS project home page.

Thanks and enjoy!

## Time To Say Goodbye

When I started this blog, I wasn’t sure how much time I would devote to blogging, so I took the free WordPress route. I’ve been incredibly busy over the last year and a half and the lack of posts reflects that situation. I’m now in a position to devote a small amount of time to blogging and I decided to take the leap into the 21st century and create a WordPress site at algorithmist.net (my business site).

Since I now have a blog at my site, I will no longer maintain this one. I will leave all the posts online since many of them have code samples from which readers may realize some benefit.

I will continue blogging about math and programming, just in a different place. I hope you enjoyed the content of this blog and will keep up with the new one.

thanks!

– jim armstrong

## Trig Plus Algebra Plus Geometry Plus Vectors Equals Fun

Math teachers hear the most often-uttered student phrase in history a lot more than I do, but my ears have twitched to the infamous, “I’ll never use that” or its many variants over the years. Funny how people know right now what will and will not be needed or used for the rest of their lives 🙂

Now, taken in strict isolation, a single trig or algebra formula might appear to have limited or no use in the myriad of life situations in someone’s future. It is, however, quite interesting how often problems can be solved by strategic use of a single formula from multiple areas of study. The problem discussed in this post is one such example.

In the prior post, I illustrated how some basic trig concepts could be used to solve a rotation and bounding-box problem without any presumptions or special considerations of the programming environment. This example is similar in that it illustrates how to rotate a box around an arbitrary point in its interior. The box is represented by a sequence of four coordinate pairs. It is not a Flash symbol or anything other than a sequence of coordinates. The drawing environment is simple; it can move the pen, draw lines, and draw filled circles. The programming environment contains the typical set of math functions.

Our math background includes a semester of trig, analytic geometry, and algebra. We know the very basics of vectors, but have not been introduced to matrices or matrix/coordinate transforms. As it happens, all we need to know to solve the stated problem is

1 – Polar coordinates, i.e. x = r cos(a) , y = r sin(a)

2 – Parametric equation of a straight line, i.e. P = (1-t)P0 + tP1

3 – How to add and subtract vectors

4 – How to compute the distance between two points

Refer to the diagram below.

The four points of the box are A (x1,y1), B (x2,y2), C (x3,y3), and D (x4,y4). The rotation point is R (rx,ry). The example code was written in Flex and the coordinate system in the Flash player is y-down. We are not given the coordinates of R. In fact, the box may be at an angle to the horizontal as shown in the lower, right part of the diagram. The only thing we know about R is that the component parallel to each side of the box is a fraction of that side’s length. This is where vector math can be useful.

The coordinates of R can be computed by adding the vectors **V1** and **V2**. The percentage inputs can be converted into numbers t1 and t2 in [0,1]. The exact coordinates of the terminal points of these vectors are obtained from the parametric equation of a line. Let the non-bolded notation V1 and V2 refer to vectors from the origin representing these terminal points. Then, V1 = (1-t1)A + t1B and V2 = (1-t2)A + t2D. Since the initial point of both **V1** and **V2** is A, the vector constituents (Δx and Δy) are immediately available. Add the vectors and add the result to A to obtain the coordinates for R. So, we can adjust sliders that change the percentage along each side and quickly compute R regardless of any prior rotation since we should always recompute the correct coordinates for A, B, C, and D.

When R is set, imagine lines drawn from R to each of the vectors A, B, C, and D. Let the distances be r1, r2, r3, and r4, respectively. Also compute the angle each line segment makes with the horizontal using the atan2() function. Since this returns a value in [-Π, Π], a small adjustment is made to always record the angles in [0, 2Π]. This makes studying the angle computations a bit easier if you want to trace out the results as part of deconstructing the code.

Also record the initial rotation angle when R is assigned. Subsequent changes to this value rotate the box around R by some delta. If the initial angle of the box is θ, and the delta is δ, then the four points are rotated around R by an angle θ+δ. Compute the coordinates of each new vector, A, B, C, and D by using the formula for polar coordinates and add the center, R.

These concepts are illustrated in an online demo.

The percentage along each side used to compute R is adjusted by sliders as well as the rotation angle. The red dot visually identifies the rotation point, R. You may optionally display the circle traced by each of the four corner points as the box is rotated. There is a *_clear* (Boolean) variable in the code that you may adjust to either clear the drawing each rotation update or choose to draw over each update of the box. If the value is false, you can produce some spirograph-style drawings.

The UI was created in Flex, but the actual code is very straightforward and does not rely on anything unique to Actionscript. It could be ported to many other environment quite easily. There is also some internal documentation on how to use trig identities to reduce the number of trig computations at each rotation update.

Even more importantly, there is nothing in the code or the algorithm that relies on a box being rotated other than computing R. The actual rotation is applied to a sequence of points using the simplest of concepts from a few math disciplines. I hope this provides some sense of appreciation of problems that can be solved using only the most fundamental techniques from a few branches of applied math.

## Typescript and Jangaroo

I think I’m about to set a new personal record for lowest number of posts in a given year. Is it really October already?

Well, at least I’m working and on a pretty interesting project from both a mathematical and programming perspective, so I should be thankful for that.

I am, however, still interested in doing something new and interesting this year and I think it will be along the lines of a computational geometry library and/or version of the Freehand Drawing Library in Javascript. The CG library is likely to be released through a series of posts in an open-source context similar to the TechNotes series I did in Actionscript for Flash/Flex programmers. The JS FDL is intended for clients doing mobile development in an HTML/JS environment. Okay, it’s mostly for me to have fun – I confess! However, an extra license or two never hurt anyone 🙂

To this end, I’m currently looking into both Typescript and Jangaroo. Coffeescript is on the list as well, but I’m really looking to get back into Visual Studio and C++/C# development, so Typescript has some natural appeal. Perhaps sufficient time will arise to deep-dive into all three?

I guess we’ll have to wait and see. Sorry again for the waiting part. This has been a strange (but fortunately very profitable) year.

## Privatizing Freehand Drawing Library

I’m into an extension of my prior gig, so time is still very limited. In addition to family issues, what little time remains is dedicated to supporting existing beta users of the Freehand Drawing Library. And, these users just scored big time. I’ve decided to keep the library private and license it only to customers, providing requested customization at an hourly rate.

Existing beta users will continue to receive free updates of the core library and the current beta period is now permanently closed. I will blog about development of the library as a segue into discussing some of the math behind the programming. I also hope to start a series on computational geometry in Javascript some time this fall.

Thanks for bearing with the extreme lack of posts this year 🙂

## An Extension

Well, this is what every contractor likes to hear – “you’ve been extended.” Although relatively short, it means that between the gig, taking care of family, and supporting users of the Freehand Drawing Library, my lack of blogging will also be extended for a while.

On a personal note, I hope to get back into tennis after being out of practice since April due to a foot injury. Looking forward to hitting this afternoon instead of just watching the US Open 🙂

## QA and Change Requests

Okay, well, I hope that tells you where my summer is at this point and helps explain the lack of posts. With some luck, I’ll be back to work on the Freehand Drawing Library later this month.

Thanks!