Skip to main content

Quadrilateral Centroid Algorithm

Oh men... This time google wasn't helping much...

I was looking for an algorithm to find the centroid from a quadrilateral polygon. It wasn't necessary to be a general, any polygon, centroid algorithm... just one which give me the centroid of a regular convex quadrilateral. It was hard to find something that simple. But finally, here is.
The following code is a C example on how to compute the area and the centroid from a regular convex quadrilateral polygon:

#include<stdio.h>

int main(void){
float verticesX[5];
float verticesY[5];
float centroidX = 0;
float centroidY = 0;

verticesX[0] = 3.58; verticesY[0] = 1.90;
verticesX[1] = 4.48; verticesY[1] = 1.88;
verticesX[2] = 4.56; verticesY[2] = 2.71;
verticesX[3] = 3.64; verticesY[3] = 2.74;
verticesX[4] = 3.58; verticesY[4] = 1.90; // Repeat the first vertex


int i, k;
float area = 0.0f;
float tmp = 0.0f;

for (i = 0; i <= 4; i++){
k = (i + 1) % (4 + 1);
tmp = verticesX[i] * verticesY[k] -
verticesX[k] * verticesY[i];
area += tmp;
centroidX += (verticesX[i] + verticesX[k]) * tmp;
centroidY += (verticesY[i] + verticesY[k]) * tmp;
}
area *= 0.5f;
centroidX *= 1.0f / (6.0f * area);
centroidY *= 1.0f / (6.0f * area);
printf("Centroid = (%1.2f, %1.2f), area = %1.2f\n", centroidX, centroidY, area);

return 0;
}

Comments

Popular posts from this blog

More trickery with gnuplot dumb terminal

In my post "Plotting memory usage on console" the chart doesn't pan the data.
Now, using a named pipe, the effect got a little bit nicer.
First, we have to run the memUsage.sh script to get a file filled with memory usage info:
./memUsage.sh > memUsage.dat &
Then we have to create a named pipe:
mkfifo pipe
Now we have to run another process to tail only the last 64 lines from the memUsage.dat
while [ 1 ]; do tail -64 memUsage.dat> pipe; done &
And now we just have to plot the data from the pipe:
watch -n 1 'gnuplot -e "set terminal dumb;p \"pipe\" with lines"'
And that is it!

uSleep on windows (win32)

I am facing a terrible issue regarding timing on windows.

Googling arround, I've found those infos:
Using QueryPerformanceCounter and QueryPerformanceFrequency APIs in Dev-C++
(http://yeohhs.blogspot.com/2005/08/using-queryperformancecounter-and_13.html)
QueryPerformanceCounter() vs. GetTickCount()
http://www.delphifaq.com/faq/delphi_windows_API/f345.shtml
How to time a block of code
http://www.cryer.co.uk/brian/delphi/howto_time_code.htm
And Results of some quick research on timing in Win32 http://www.geisswerks.com/ryan/FAQS/timing.html
With that I'm trying to write something like a uSleep function for windows:


#include<windows.h>

voiduSleep(int waitTime){
__int64 time1 = 0, time2 = 0, sysFreq = 0;

QueryPerformanceCounter((LARGE_INTEGER *)&time1);
QueryPerformanceFrequency((LARGE_INTEGER *)&freq);
do{
QueryPerformanceCounter((LARGE_INTEGER *)&time2);

// }while((((time2-time1)*1.0)/sysFreq)<waitTime);
}while( (time2-time1) <waitTime);
}

There is also already a nanosleep…

Checking auth.log for ssh brute force attacks

As I am letting my personal computer always on, as a homelinux server, I decided to check if someone is trying to breaking in with SSH brute force attacks.

First I did a grep for fail at the /var/log/auth.log. (grep -i /var/log/auth.log)

And I got lots of lines with the string "fail". With [grep -i /var/log/auth.log | wc -l] I figured out that were 1164 fail entries at auth.log

With an [grep -i fail auth.log | cut -d " " -f 6 | sort | uniq] I checked that were two kind of failed attempts:
Failed
pam_unix(sshd:auth):

So I wrote the following line to check with which users they were attempting to log:
grep Failed auth.log | cut -d " " -f 11 | sort | uniq | while read line ; do echo -n $line" "; grep $line auth.log | wc -l; done | sort -n -k 2

Here, the field position (the number 11 at the above command lines [-f 11]) may change in some systems. At my desktop at work, the username came at the position 9.

Here are the "top ten":
root 2922
user 2884