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CS101 - Introduction to Computing - Lecture Handout 41

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Images & Animation

(Web Development Lecture 14)

During the last lecture we discussed String Manipulation

  • We became familiar with methods used for manipulating strings
  • We became able to solve simple problems involving strings

String Manipulation in JavaScript

  • In addition to the concatenation operator (+) JavaScript supports several advanced string operations as well
  • Notationaly, these functions are accessed by referring to various methods of the String object
  • Moreover, this object also contains the ‘length’ property

String Methods

FORMAT

string.methodName( )
EXAMPLE:
name = “Bhola” ;
document.write( name.toUpperCase( ) ) ;
document.write( name.bold( ) ) ;

BHOLABhola

Two Types of String Methods

  1. HTML Shortcuts
  2. All Others

String Methods: HTML Shortcuts

String Methods

String Methods: All Others

String Methods  All Others

Automatic Conversion to Strings

  • Whenever a non-string is used where JavaScript is expecting a string, it converts that non-string into a string
  • Example:
    • The document.write( ) method expects a string (or several strings, separated by commas) as its argument
    • When a number or a Boolean is passed as an argument to this method, JavaScript automatically converts it into a string before writing it onto the document

The ‘+’ Operator

  • When ‘+’ is used with numeric operands, it adds them
  • When it is used with string operands, it concatenates them
  • When one operand is a string, and the other is not, the non-string will first be converted to a string and then the two strings will be concatenated

Strings In Mathematical Expressions

When a string is used in a mathematical context, if appropriate, JavaScript first converts it into a number. Otherwise, a “NaN” is the result

Strings In Mathematical Expressions

document.write( "2" * Math.PI ) ;
document.write( "Yes" ^ 43 ) ;

The ‘toString’ Method

Explicit conversion to a string

EXAMPLE:

Convert 100.553478 into a currency format
a = 100.553478 ;
b = a.toString( ) ;
decimalPos = b.indexOf( ".", 0 ) ;
c = b.substring( 0, decimalPos + 3 ) ;
document.write( c ) ;
Conversion

Explicit conversion to a string

parseInt( ) and parseFloat( ) methods

Today’s Goal

(Images & Animation)

  • To become able to add and manipulate images and simple animations to a Web page

Images in HTML

  • It is quite straight forward to include gif and jpg images in an html Web page using the <IMG> tag
    Format: <IMG src=URL, alt=text height=pixels width=pixels
    align="bottom|middle|top">

Plea: Don’t use images just for the sake of it!

Image Demo

<HTML><HEAD>
<TITLE>Image Demo</TITLE>
</HEAD><BODY>
<H1>Image Demo</H1>
Here is an image <IMG src="die5.gif">
<IMG src="die5.gif" height="63" width="126"> <P>
Here is another <IMG align="middle" src=
"http://www.vu.edu.pk/images/logo/logotop.jpg">
</BODY></HTML>

Images in JavaScript

  • Images in JavaScript can be manipulated in many ways using the built-in object Image
  • Properties: name, border, complete, height, width, hspace, vspace, lowsrc, src
  • Methods: None
  • Event handlers: onAbort, onError, onLoad, etc.

Image Preloading

  • The primary use for an Image object is to download an image into the cache before it is actually needed for display
  • This technique can be used to create smooth animations or to display one of several images based on the requirement

The Image Pre-Loading Process

  1. An instance of the Image object is created using the new keyword
  2. The src property of this instance is set equal to the filename of the image to be preloaded
  3. That step starts the down-loading of the image into the cache without actually displaying it
  4. When a pre-loaded image is required to be displayed, the src property of the displayed image is set to the src property of the pre-fetched image

Let us revisit an example that we first saw in lecture 35

revisit an example

revisit an example 1

<HTML>
<HEAD>
<TITLE>Roll the Die</TITLE>
<SCRIPT>
JavaScript Code
</SCRIPT>
</HEAD>
<BODY >
HTML Code
</BODY>
</HTML>
<IMG name="die" src="die6.gif">
<FORM>
<INPUT type="button" value="Roll the Die"
onClick="rollDie( )">
</FORM>

dieImg = new Array( 7 ) ;
for( k = 1; k < 7; k = k + 1 ) { //Preload images
dieImg[ k ] = new Image( ) ;
dieImg[ k ].src = "die" + k + ".gif" ;
}
function rollDie( ) {
dieN = Math.ceil( 6 * Math.random( ) ) ;
document.die.src = dieImg[ dieN ].src ;
}

Another Example

  • Develop a Web page that displays six thumbnail images and a main image
  • The main image should change to a larger version of the thumbnail as soon as the mouse moves over on a thumbnail image

Another Example

<HTML>
<HEAD>
<TITLE>Image Selector</TITLE>
<SCRIPT>
JavaScript Code
</SCRIPT>
</HEAD>
<BODY >
HTML Code
</BODY>
</HTML>
dieImg = new Array( 7 ) ;
for( k = 1; k < 7; k = k + 1 ) { // Preload images
dieImg[ k ] = new Image( ) ;
dieImg[ k ].src = "die" + k + ".gif" ;
}

<IMG name="big" src="die6.gif" width="252" height="252"><P>
<IMG src="die1.gif" width="63" height="63"
onMouseOver=
"document.big.src=dieImg[ 1 ].src">

<IMG src="die6.gif" width="63" height="63"
onMouseOver=
"document.big.src=dieImg[ 6 ].src">

Where Else Can We Use This?

  • Automobile Web site
  • ???

Animation Example 1

  • Take 16 images and cycle through them to create an animation effect

Take 16 images and cycle through them to create an animation effect

<HTML>
<HEAD>
<TITLE>Animation 1</TITLE>
<SCRIPT>
JavaScript Code
</SCRIPT>
</HEAD>
<BODY >
HTML Code
</BODY>
</HTML>

<CENTER>
<IMG name="circle" src="circle1.gif" onLoad="setTimeout( 'circulate( )', gap
)">
</CENTER>

setTimeout( ) executes circulate( )

gap = 100 ;
imageN = 1 ;
circImg = new Array( 17 ) ;
for( k = 1; k < 17; k = k + 1 ) { // Preload images
circImg[ k ] = new Image( ) ;
circImg[ k ].src = "circle" + k + ".gif" ;
}

Animation Example 2

function circulate( ) {
document.circle.src =
circImg[ imageN ].src ;
imageN = imageN + 1 ;
if( imageN > 16 )
imageN = 1 ;
}

Animated Gifs

  • We could have saved the 16 gif images of the previous example in a single file in the form of an animated gif, and then used it in a regular <IMG> tag to display a moving image
  • However, JavaScript provides better control over the sequencing and the gap between the individual images

Animation Example 2

  • Take 16 images and cycle through them to create an animation effect
  • Provide buttons to slow down or speed up the animation

<HTML>
<HEAD>
<TITLE>Animation 2</TITLE>
<SCRIPT>
JavaScript Code
</SCRIPT>
</HEAD>
<BODY >
HTML Code
</BODY>
</HTML>

Animation Example 3

<CENTER>
<IMG name="circle" src="circle1.gif" onLoad="setTimeout( 'circulate( )',
gap )">
</CENTER>
<FORM>
<INPUT type="button" value="Slow Down"
onClick="slowDown( )">
<INPUT type="button" value="Speed Up"
onClick="speedUp( )">
</FORM>

gap = 100 ;
imageN = 1 ;
circImg = new Array( 17 ) ;
for( k = 1; k < 17; k = k + 1 ) { // Preload images
circImg[ k ] = new Image( ) ;No change
circImg[ k ].src = "circle" + k + ".gif" ;
}
function circulate( ) {
document.circle.src =
circImg[ imageN ].src ;
imageN = imageN + 1 ;
if( imageN > 16 )
imageN = 1 ;No change
}
function slowDown( ) {
gap = gap + 20 ;
if( gap > 4000 )
gap = 4000 ;
}Two new functions
function speedUp( ) {
gap = gap - 20 ;
if( gap < 0 )
gap = 0 ;

Animation Demo

Flash Animation

  • Designed for 2-D animations, but can be used for storing static vector-images as well
  • A special program (called a plug-in) is required to view Flash files in a Web browser
  • Can be used to design complete, animated Web sites with hardly any HTML in it
  • Binary-file storage

Structured Vector Graphics

  • New format; may become more popular than Flash
  • Plug-in required
  • Text-file storage; search engine friendly

During Today’s Lecture …

  • We became able to add and manipulate images and simple animations to a Web page

Our 15th & Final Web Dev Lecture:

(Programming Methodology)

  • To understand effective programming practices that result in the development of correct programs with minimum effort
  • To become familiar with simple debugging techniques