## Saturday, January 23, 2016

### Creating scalable images on the cheap

I just finished working on a banner for my univeristy. The design involves three logos for the three entities involved in this activity: the school mascot, the student organization and the house of representatives. I preferred the mascot vs the seal since I believe it's more natural for students to identify with. And to keep things light and informal I also preferred an easily recognizable figure rather than the official seal. There is indeed a nice such figure on their website here. Unfortunately, the image is small and the quality makes it even harder to use.

I don't really know much about graphics design or printing; only enough to know one needs high quality scalable images for something as large as a banner. To get a feel for the complexity of the issue, you may take a look at this webpage.

Since I really wanted to use this house figure for the banner, I decided to go through creating a scalable version. Here's what I had to do:

0. For all image editing I used the free Pinta Image Editor. Cropping out the text got me started with the circle I ended up spending long hours staring at.

1. I found a decent web service for tracing bitmap images into SVG files: vectormagic.com. They offer a couple free conversions after you sign up using your email. I tried so many inputs and made a single download once I was happy with it. Of course I was hoping I will just upload my cute circle and get back a scalable version, but here's what I got:

2. The logo has 50 little dots for each of the states around the circle containing the house figure. As the image comes in such low resolution, these dots are far from perfectly circular and the tracing software lumped each into a unique weird shape. So, it seemed I'll need to erase those from my input and figure out a way to put them back once I get the house.

3. It turns out the house itself isn't exactly symmetric, which means tracing ends up with different curves for corresponding features on the left and right. Not good. The only way I could thing of was to fire up my trusty Pinta and make sure the image is symmetric by copying the left half into a new layer, flipping horizontally and merging down.

This made things better, but for some reason, even with identical pixels on both sides, tracing didn't always produce the same result. So, I had to edit few pixels here and there to make it do what I wanted. The result was actually decent, at least for my purposes. Now I have my the main component in SVG format.

4. I should add that the circular boundary wasn't perfect either. Erasing the dots around it might have contributed to that. I spent sometime trying to fix it then I just gave up. Later on, I figured a fix, but this will have to wait for now.

5. Creating a separate SVG file with the 50 dots was the easiest part. It suffices to observe that there's a dot right at 12 o'clock, starting from there one can take steps of $\frac{2\pi}{50}$ to place each next dot. I wrote a script to generate the 50 dots of radius $r$ around a circle of radius $R$ with a controllable offset and scaling. This will come in handy as I had to tune the radii and offset to get something close to the original. This step provides a second SVG.

6. To merge the two SVGs, I found a free online SVG editor: vectorpaint.yaks.co.nz. I didn't know what to expect but they had a particularly nice feature that allows it to distinguish different parts of the SVG and manipulate each separately. I was happy to learn I could use this to remove the uneven boundary. This editor can draw some shapes. To get a perfect circle I found a donut shape that seemed to work for my purposes. However, I figured I'd better edit the script to include a background circle along with the 50 dots so I have fewer things to align.

Let me mention the editor couldn't load two SVGs so I did the merge manually, by copying the markup from one into the other, so I can load a single file. And that's it. Here's the final result, which I hope communicates what is intended:

Luckily, the university provides scalable trademark images including the mascot. As for the logo of student organization, which is a simple 3 letter design, convertio.co was able to do it easily.

I can't wait to see the 2'x12' banner!

## Sunday, November 29, 2015

### Shortest path touching a circle

This came up a week or so ago in a discussion with a lab mate. The governing principle is well-known from physics, but I was too lazy to find a derivation online. Later on, I had an idea and wanted to give it a try, so here goes.

We're interested in the shortest path starting at point $a$ to point $b$ with the requirement that it intersects a circle of radius $r$ centered at point $c$, and does so before stopping at $b$. When the circle intersects the line segment $\overline{ab}$, the segment coincides with the shortest path. Hence, we are interested in paths that reflect off the circle, just like a ray of light does when it hits a shiny surface. I'll be referring to the figure to the right.

Eventually, we're interested in a statement involving the angle of incidence $\theta$. However, I found it easier to work instead with the central angle $\theta'$. We might as well parameterize the path by the point of incidence $x$ and write the length as:

$f(x) = |\overline{ax}| + |\overline{xb}|.$

Now, what do we know about each of $\overline{ax}$ and $\overline{xb}$? Assume $d_a$ and $d_b$ are the distances from $c$ to $a$ and $b$, respectively. Let's look at $\triangle{cxa}$. By the law of cosines, we have that:

$|\overline{ax}|^2 = d_a^2 + r^2 - 2 d_a r \cos{\theta'}.$
Likewise for $\triangle{cxb}$, where $\phi' = \angle{acb}$, we get:
$|\overline{xb}|^2 = d_b^2 + r^2 - 2 d_b r \cos{(\phi' - \theta')}.$

It's quite disappointing that we don't obtain nice expressions for $|\overline{ax}|$ and $|\overline{xb}|$ to plug into $f(x)$. But, since we're only interested in minimizing $f$, we might hope that finding the derivative will get us around that. Let's start by rewriting $f$ as:

$f(x) = \sqrt{|\overline{ax}|^2} + \sqrt{|\overline{xb}|^2}.$

We can then write the derivative as:

$\frac{\partial f(x)}{\partial \theta'} = \frac{1}{2\sqrt{|\overline{ax}|^2} }\frac{\partial |\overline{ax}|^2}{\partial \theta'} + \frac{1}{2\sqrt{|\overline{xb}|^2}.}\frac{\partial |\overline{xb}|^2}{\partial \theta'}.$

Plugging in $\frac{\partial |\overline{ax}|^2}{\partial \theta'} = 2d_ar\sin{\theta'}$ and $\frac{\partial |\overline{xb}|^2}{\partial \theta'} = -2d_br\sin{(\phi'-\theta')}$ we get:

$\frac{\partial f(x)}{\partial \theta'} = \frac{d_ar\sin{\theta'}}{|\overline{ax}|} - \frac{d_br\sin{(\phi' - \theta')}}{|\overline{xb}|}.$

Setting $\frac{\partial f(x)}{\partial \theta'} = 0$, cancelling $r$ and rearranging, we find that:

$\frac{d_a\sin{\theta'}}{|\overline{ax}|} = \frac{d_b\sin{(\phi' - \theta')}}{|\overline{xb}|}.$

It would really help to realize that $d_a\sin{\theta'} = |\overline{aa'}|$ and $d_b\sin{(\phi'-\theta')} = |\overline{bb'}|$, where $a'$ and $b'$ are the projections of $a$ and $b$, respectively, on $\overrightarrow{cx}$. Making this substitution we get:

$\frac{|\overline{aa'}|}{|\overline{ax}|} = \frac{|\overline{bb'}|}{|\overline{xb}|}.$

Letting $\phi = \angle axb$, this is the same as:

$\sin{\theta} = \sin{(\phi - \theta)}.$

Or,

$\theta = \phi - \theta.$

Which says that the angle of incidence should be equal to the angle of reflection, for the path to be optimal. Which is already known.

Update 01/14/16: There remains the issue of determining the point $x$ that minimizes the shortest path defined by $f$. I found it easier to work with angles $\alpha = \angle{cax}$ and $\beta = \angle{cbx}$. Applying the law of sines in $\triangle{cxa}$ and $\triangle{cxb}$ we find that:

$\sin{(\pi - \theta)} = \sin{\theta} = \frac{d_a}{r}\sin{\alpha}, \quad \sin{(\phi - \theta)} = \frac{d_b}{r}\sin{\beta}.$

Since the point $x$ we are interested in makes $\theta = \phi - \theta$, we get:

$d_a\sin{\alpha} = d_b\sin{\beta}.$

In a way, this equation only describes the angle of incidence from $a$ or $b$ to points on the circle. We still need a second equation to ensure these two angles correspond to a single point of incidence. This can be achieved by examining $\triangle{abx}$. Letting $\alpha' = \angle{cab}$ and $\beta' = \angle{cba}$ we can write:

$(\alpha' - \alpha) + (\beta' - \beta) + \phi = \pi.$

Rewriting $\phi = 2 \theta$ and rearranging we get:

$\beta = (\alpha' + \beta' - \pi) - \alpha + 2\theta.$

Noting that $\theta = \arcsin{(\frac{d_a}{r}\sin{\alpha})}$, we can now eliminate $\beta$ using the first equation to get an equation for $\alpha$ only, and the same can be written for $\beta$:

$\alpha = (\alpha' + \beta' - \pi) + 2 \arcsin{(\frac{d_a}{r}\sin{\alpha})} - \arcsin{(\frac{d_a}{d_b}\sin{\alpha)}}.$

Other than a numerical approach, it is plausible to assume $\alpha$ is small especially when $d_a \gg r$. This might justify linearizing both the sines and arcsines to get something like:

$\alpha \approx \frac{\alpha' + \beta' - \pi}{1 + \frac{d_a}{d_b} - 2 \frac{d_a}{r}}.$

## Sunday, March 23, 2014

### PS to PDF: convert and crop

I've done this task once before and forgot how. It took me a while to do it again, so I'm posting this here in case somebody else needs it. It might not be the best way to do it so if you can make it better, I'd love to hear from you.

I'm using pstill_dist and pdfcrop with two subdirectories for all input and output files.
#!/bin/bash
for FILE in $(find ./input -name '*.ps'); do FNAME="${FILE##*/}"
FNAME="${FNAME%.*}" ./pstill "./input/${FNAME}.ps" -o "./output/${FNAME}.pdf" pdfcrop --clip "./output/${FNAME}.pdf" "./output/\${FNAME}.pdf"
done


## Sunday, May 8, 2011

### Sorting Facebook feeds in chronological order

College students spend lots of time on Facebook and virtually use it to share and organize their shcedules and course materials. Obviously, things get out of control and you end up with a very long and entangled feed that you might find yourself in a real need to explore or even worse, scan for something important.

So, one day a student of mine posted (!), wondering if there were any group settings that would arrange posts in reverse chronological order, presumably to save some time. That post caught my attention and eventually, I decided to prepare a bookmarklet for this exact purpose. It was pretty much straightforward but unfortunately, it would only work for groups. Someone may find it worthwhile to adapt it to the home/profile feeds, though.

Update: It seems the student got interested in this sort of JS magic and she actually took it a step further to work for the home feed as well.
// This snippet is licensed under a Creative Commons Attribution 3.0 License.
// Authors: Ahmed Abdelkader and Nancy Iskander
javascript:{
var flist = document.getElementById('home_stream');
if (flist == null)
flist = document.getElementById('pagelet_group_mall').children[0].children[0];
var far = new Array();
var dates = {};
while (flist.children.length) {
var abbrs = flist.children[0].getElementsByTagName('abbr');
if (abbrs.length) {
dates[flist.children[0].id] = abbrs[0].getAttribute('data-utime');
far.push(flist.children[0]);
}
flist.removeChild(flist.children[0]);
}
far.sort(function(a, b){return dates[b.id] - dates[a.id];});
for (var i = 0; far.length - i; ++i)
flist.appendChild(far[i]);
void(0);
}
To create the bookmarklet, we just need to compress that and put it in a hyperlink. Just drag this link: Feed-Repairo!, into your browser's Bookmarks Toolbar to add the bookmark and you can easily rename it if you please. That's it! Whenever you need to sort out your group feeds, all you need to do is click the bookmark. Let me know if it works for you or you encoutner any bugs. Enjoy!

Update 8/23/2013: Drag this link if you want to sort ascendingly Repairo-Feed!
Update 3/23/2014: flist is not at the root container anymore; so we added another .children[0] and all is fine. (Many thanks to Jeff E)

## Wednesday, January 26, 2011

### A lightweight anonymous visitor for Java collections

I got myself into a situation where I really wished that Java collections accepted a visitor interface of some kind so I could define one on the fly and get my job done. Alas, I decided to implement the utility myself and here's the result.

public interface IVisitor<T> { void visit(T item);}public class CollectionUtils { public static<T> void applyVisitor(Collection<T> collection, IVisitor<T> visitor) {  for (T item : collection)   visitor.visit(item); }}
A typical usage example would be:
public static void main(String[] args) { CollectionUtils.applyVisitor(Arrays.asList(1, 2, 3, 4, 5), new IVisitor<Integer>() {  public void visit(Integer x) {   System.out.println(x);  }    });}

## Sunday, October 24, 2010

### Efficient enumeration of all integers with a given pop count

Population count is the number of '1' bits in the binary representation of an integer.

The basic trick we employ is to get the next higher number with the same number of 1-bits, which we borrow from the Hacker's Delight (whole book). Below is a Java implementation of all 4 version of the snoob function.

public static int snoob1(int x) {    int r = x + (x & -x);    return r | ((x ^ r) >>> (2 + Integer.numberOfTrailingZeros(x)));}public static int snoob2(int x) {    int s = x & -x, r = x + s;    return r | ((x ^ r) >>> (33 - Integer.numberOfLeadingZeros(s)));}public static int snoob3(int x) {    int r = x + (x & -x);    return r | ((1 << (Integer.bitCount(x ^ r) - 2)) - 1);}public static int snoob4(int x) {    int s = x & -x, r = x + s;    return r | (((x ^ r) >> 2) / s);}
We utlize this formula to implement the method below, where the width parameter is added for convenience.
public static void printAllIntegersOfWidthAndPop(int w, int n) {    if (n <= 0 || n > w || w > 32) return;    int x = (1 << n) - 1, g = x << (w - n), c = 0;    while (true) {        String s = Integer.toBinaryString(x);        while(s.length() < w) s = "0" + s;        System.out.println(++c + "\t" + s);        if (x == g) break;        else x = snoob*(x);    }}
It is easy to figure out there will be Choose(w, n) such numbers. We know where the sequence starts and since we also know where it stops, we don't need to compute this number.

If you'd like to learn how to find the number of leading/trailing zeros or the pop count or learn about more cool bit twiddling hacks you can consult the book above (which the java.lang.Integer implementation of these methods is based on) or this magnificent web page.

## Saturday, July 17, 2010

### Generating combinations in lexicographical order using Java

Below is a Java port of the algorithm by James McCaffrey as presented in the MSDN article Generating the mth Lexicographical Element of a Mathematical Combination.

Please do take into consideration that this implementation only uses int as Java does not allow array indexing with long. This means that the valid input range is more restricted. Watch out for overflows!

/** * Based on the Combinadic Algorithm explained by James McCaffrey * in the MSDN article titled: "Generating the mth Lexicographical * Element of a Mathematical Combination" * <http://msdn.microsoft.com/en-us/library/aa289166(VS.71).aspx> *  * @author Ahmed Abdelkader * Licensed under Creative Commons Attribution 3.0 * <http://creativecommons.org/licenses/by/3.0/us/> */public class Combinations { /** returns the mth lexicographic element of combination C(n,k) **/ public static int[] element(int n, int k, int m) {  int[] ans = new int[k];  int a = n;  int b = k;  int x = (choose(n, k) - 1) - m;  // x is the "dual" of m  for (int i = 0; i < k; ++i) {   a = largestV(a, b, x);          // largest value v, where v < a and vCb < x   x = x - choose(a, b);   b = b - 1;   ans[i] = (n - 1) - a;  }  return ans; } /** returns the largest value v where v < a and Choose(v,b) <= x **/ public static int largestV(int a, int b, int x) {  int v = a - 1;  while (choose(v, b) > x)   --v;  return v; } /** returns nCk - watch out for overflows **/ public static int choose(int n, int k) {  if (n < 0 || k < 0)   return -1;  if (n < k)   return 0;  if (n == k || k == 0)   return 1;  int delta, iMax;  if (k < n - k) {   delta = n - k;   iMax = k;  } else {   delta = k;   iMax = n - k;  }  int ans = delta + 1;  for (int i = 2; i <= iMax; ++i) {   ans = (ans * (delta + i)) / i;  }  return ans; }}
The code below produced the output that follows:
public static void main(String[] args) { int n = 5, k = 3; int total = choose(n, k); for(int i = 0; i < total; i ++) {  for(int x : element(n, k, i))   System.out.print(x + " ");  System.out.println(); }}0 1 2 0 1 3 0 1 4 0 2 3 0 2 4 0 3 4 1 2 3 1 2 4 1 3 4 2 3 4