Senduran's Blog

Wednesday, November 1, 2023

How to Search and Replace the First Letter to Upper Case in GEdit

Following is the Find and Replace with syntax, make sure the Regular expression is selected

Find: (\w)(\w+)

Replace with: \u\1\2

Tuesday, March 1, 2022

Setting up Jupyter Notebook for python virtual environment

Create a virtual environment (my-env)

$ python3 -m venv my-env

$ source my-env/bin/activate

(my-env) $

Install ipykernel module

$ pip install ipykernel

Install iPython Kernel for virtual environment

$ python -m ipykernel install --user --name=my-env-kernel

Start jupyter and select my-env-kernel from kernel

Sunday, December 19, 2021

Reducing image size with GIMP in batch mode

First we define a function which contains all the needed procedures. and save the script in

~/.gimp-2.8/scripts

with .scm extension.

In 2.10, the script directory is ~/.config/GIMP/2.10/scripts

(define (batch-reduce-img-size pattern reduceby)
  (let* ((filelist (cadr (file-glob pattern 1))))
    (while (not (null? filelist))
           (let* ((filename (car filelist))
                  (image (car (gimp-file-load RUN-NONINTERACTIVE filename filename)))
                  (imageWidth (car (gimp-image-width image)))
		   (imageHeight (car (gimp-image-height image)))
		   (newWidth (* imageWidth reduceby))
		   (newHeight (* imageHeight reduceby))
                  (drawable (car (gimp-image-get-active-layer image))))
				   (gimp-image-scale image newWidth newHeight)
                                  (gimp-file-save 1 image drawable filename filename)
                                  
(gimp-image-delete image))
           (set! filelist (cdr filelist)))))

run the script as

gimp -i -b '(batch-reduce-img-size "*.JPG" 0.5)' -b '(gimp-quit 0)

IMPORTANT: The images will get over written

Sunday, May 17, 2020

JavaScript Arrow Function, Spread & Rest Operators And Destructuring Syntax

With ECMAScript6 (ES6) JavaScript got new syntax. Followings are some of the new syntax.

Arrow Functions

New syntax for defining a function

Traditional Function	ES6 Arrow Function
function myFunction() { ... }	const myFunction = () => { ... }
function myFunction(name) { ... }	const myFunction = (name) => { ... } also without parenthesis if only one argument const myFunction = name => { ... }
function add(n, m) { return n+m; }	const add = (n, m) => n+m; if a single line return then the return keyword and the curly brackets can be omitted

Spread Operator

Used to split up array elements OR object properties

const newArray = [...oldArray, 1, 2];


const newObject = {...oldObject, newProerty:newValue}

newArray is a one dimensional array contains all the elements in oldArray and additionally 1, 2

newObject has all the properties of oldObject in addition to the it has newProperty

(if the oldObject already has newProperty it will be overwritten)

Spread operator is useful when copying an array or an object.

Rest Operator

Used to merge a list of function arguments into an array

function sortArgs(...args) {
       return args.sort();
}

Destructuring

Easily extract array elements or object properties and store them in variables

Array Destructuring

[a, b] =  ["Hello", "World"]
console.log(a) // Hello
console.log(b) // World

Object Destructuring

{name} = {"name": "Aang", "age" : 112}
console.log(name) // Aang
console.log(age) // undefined

Sunday, May 3, 2020

How to draw a quarter circle in HTML Canvas

A circle can be drawn in the Canvas element using the arc function.
For example

<html>
    <body>
        <canvas id="drawArea" width="1024" height="768"></canvas>
    </body>

    <script>
        var canvas = document.getElementById("drawArea");
        var canvasContext = canvas.getContext("2d");

        canvasContext.fillStyle = "black";
        canvasContext.beginPath();
        canvasContext.arc(100, 100, 50, 0, Math.PI*2, true);
        canvasContext.fill();
    </script>
</html>

Lets explore the arc function's parameters

arc(x-coordinate, y-coordinate, radius, start-angle, end-angle, [counter-clockwise])

x-coordinate : position x from the top left corner of the canvas area

y-coordinate : position y from the top left corner of the canvas area

radius : radius of the arc

start-angle : the angle(in Radian) where the arc should begin. The canvas angle is always measured in the clockwise starting from 0.

i.e in a clock, 3 represent 0

6 --> 𝝅/2 (90 degree )

9 --> 𝝅 (180 degree)

12 --> 3𝝅/2 (270 degree)

end-angle: the angle where the arc should end

counter-clockwise : optional parameter. To draw the arc from the start-angle end- angle in the counter clockwise, default value is false (i. by default it is clock wise)

drawing a circle is straight forward, start angle is 0 and end angle is 2𝝅 or (360 degree). but when drawing a half circle we have two option whether to draw the upper half or lower half.

Drawing an upper half circle

canvasContext.arc(100, 100, 50, 0, Math.PI, true)

here the begin angle is set to zero and end to 180 degree( we can imagine from 3 to 9 in a clock) these two can be connected either via 6 or 12, since the counter-clockwise parameter is set to true an arc is drawn from 3 to 9 via 12 so it is a upper half circle.

Drawing a lower half circle

Simply changing the counter-clockwise parameter to false, the arc will be drawn from 3 to 9 via 6 (clockwise)

Let's see how to draw a quarter circle

If we try to draw a quarter circle similar to the above approach by giving a 90 degrees, i.e

canvasContext.arc(100, 100, 50, 0, Math.PI/2, false)

This will draw a segment of circle rather than quarter sector. This is because the arc function will connect the points specified by the angle begin and angle end parameter.

That is from the start angle (3'O clock) to end angle (6'O clock)an arc is drawn (end to start because of the counter clockwise is true)

So to draw a quarter circle, we have to draw two lines, those are connecting the start angle and end angle position towards the center of the circle

First the position is moved to the center and a line is drawn to the starting angle position then the arc is drawn and from the ending position another line is drawn towards the center.

canvasContext.fillStyle="black";

canvasContext.beginPath();
canvasContext.moveTo(100,100);
canvasContext.lineTo(150,100);
canvasContext.arc(100, 100, 50, 0, Math.PI/2, false);
canvasContext.lineTo(100, 100);
canvasContext.fill();

Drawing without filling will be like this

canvasContext.storeStyle="black";
canvasContext.beginPath();
canvasContext.moveTo(100,100);
canvasContext.lineTo(150,100);
canvasContext.arc(100, 100, 50, 0, Math.PI/2, false);
canvasContext.lineTo(100, 100);
canvasContext.stroke();

Wednesday, April 8, 2020

Invoking a C function from Java using JNI

This is our C function which we need to invoke from Java

hello.c

#include <stdio.h>

void sayHello() {
        printf("Hello world, from C\n");
}

Let's write the Java class which contains the native method to calls the C function

HelloWorld.java

public class HelloWorld {
    
    private native void callCHello();

    static {
        System.loadLibrary("chello");
    }

    public static void main(String[] args) {
        new HelloWorld().callCHello();
    }
    
}

Here we are loading the chello C library (we will be creating in the later part) which contains the implementation for the native method callCHello()

Compile the HelloWorld.java

javac HelloWorld.java

Create the native method header file

javah HelloWorld

This will generate HelloWorld.h file

Let's write the C code including the HelloWorld.h, the function is named with the following convention.
Java_<Java class name>_<java native method name>

so here it will be:
Java_HelloWorld_callCHello

chello.c will call our original C function (sayHello()) which we needed to call from Java
chello.c

#include <jni.h>
#include <stdio.h>
#include "HelloWorld.h"#include "hello.c"
JNIEXPORT void JNICALL Java_HelloWorld_callCHello(JNIEnv *env, jobject obj)  {
    sayHello();
    return;
}

Les's create the C library file to which we loaded in the Java code earlier with the following command

gcc chello.c -I $JAVA_HOME/include/ -I $JAVA_HOME/include/linux/ -shared -o libhello.so -fPIC

Please note that the so file name is libhello i.e with the "lib" prefix to the name we loaded in Java. Also I am including the linux related header files because I am working on linux. If it is windows it has to be changed as win32

let's run the HelloWorld java. We have to set the java.library.path to where the so file is located (in this case the current directory)

java HelloWorld

The output will be "Hello World, from C"

Sunday, January 28, 2018

WSO2 ESB - Sample Class Mediator

In WSO2 ESB / EI when we want to write our own logic of mediation, class mediators are very helpful.

A class mediator can be written by extending AbstractMediator

Following is a sample class mediator which simply logs a sting message (Doesn't actually do any mediation)

This source can be used as a template to start a class mediator quickly

public class SampleClassMediator extends AbstractMediator {

    private static final Log log = LogFactory.getLog(SampleClassMediator.class);

    public boolean mediate(MessageContext messageContext) {
        log.info("This is a sample class mediator");
        return true;
    }
}

Full project can be found in https://github.com/bsenduran/sample-class-mediator/tree/v1.0