Bits & Bytes

Posts Tagged ‘css’

Clipping in JavaScript with Absolute Positioning

If you program games in JavaScript, you will often find it necessary to clip the region of an image that lies outside of the viewport. The solution is easy, but not obvious. The elements in a game are generally positioned using the “absolute” positioning designation: For example, the line qrMadonna.style.position = “absolute”; in the program below sets the positioning that the image of the Madonna, qrMadonna, refers to to absolute. (For an explanation of absolute positioning, see our post on positioning elements.)

Below, we have the code for an HTML file and a JavaScript file. The HTML file is essentially blank; it is simply used to call the JavaScript file, “ClipImage.js” and execute the code.

The file “ClipImage.js” contains four variables that refer to four nested HTML elements. The outermost element is the body; this element was created in the HTML file and is retrieved via a call to the getElementsByTagName() function, along with the array operator. After this, we create a div called qrOuterDiv to hold everything; this outer div is created to allow the code inside to flow normally, since it does not have absolute positioning (Otherwise, it is not needed.) The next element is qrInnerDiv, and it contains the image element that we are clipping; it is necessary that this element have its position as “absolute” and its overflow as “hidden”. Finally, the image element qrImage is 200×149 and is positioned at (125, 100) inside of the div, which is 300×200. So, the image hangs outside of the div by 25 and 49 pixels, respectively.

To illustrate this, we have an resulting image of what this clipping looks like below. The faint region that lies outside the dark gray rectangle is actually clipped. We show this region so that you can see what has been clipped. We also have the original image of the “Madonna and Child with Cherubs” that we used in the example, at the top of the article.

HTML File: “ClipImage.html”


<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
    <meta http-equiv="Content-Type" content="text/html;charset=utf-8" />
    <title>XoaX.net's Javascript Clipping Example</title>
</head>
<body>
    <script type="text/javascript" src="ClipImage.js"></script>
</body>
</html>

JavaScript File: “ClipImage.js”


var qrBody;
var qrOuterDiv;
var qrInnerDiv;
var qrImage;

function PageLoaded() {
    // Get the main body element, first/only element of array
    qrBody = document.getElementsByTagName("body")[0];

    qrOuterDiv = document.createElement("div");
    qrBody.appendChild(qrOuterDiv);

    qrInnerDiv = document.createElement("div");
    qrInnerDiv.style.backgroundColor = "#444444";
    qrInnerDiv.style.width = "300px";
    qrInnerDiv.style.height = "200px";
    qrInnerDiv.style.position = "absolute";
    qrInnerDiv.style.overflow = "hidden";
    qrOuterDiv.appendChild(qrInnerDiv);

    qrImage = document.createElement("img");
    qrImage.src = "MadonnaAndChildWithCherubs.jpg";
    qrImage.style.position = "absolute";
    qrImage.style.left = "125px";
    qrImage.style.top = "100px";
    qrInnerDiv.appendChild(qrImage);
}

window.onload = PageLoaded;

Three Ways to Apply CSS Styles: Inline, Embedded, and External

There are three ways to apply a style to an HTML element: Inline, Embedded, and External. Below, I demonstrate each of these methods by applying a single style to the same element three times. The necessary code is shown in bold.

(Proverbs 6:16-19) There are six things the Lord hates, seven that are detestable to him: haughty eyes, a lying tongue, hands that shed innocent blood, a heart that devises wicked schemes, feet that are quick to rush into evil, a false witness who pours out lies and a person who stirs up conflict in the community.

Inline

The inline method is most suitable for setting the style on a single element. This is because the inline method requires us to repeat the entire specification for each element on which we want to apply it. Also, if we want to change the style, we must change it on each element. If you are setting the style for multiples elements, you should consider using either the embedded or external style sheet method.

To use the inline method, simply add style=”<< style specification >>” to the element that you want to style, where << style specification >> is the CSS style that you want to apply. For example, the element above would be written as:

<div style=”width:420px; height:120px; padding:20px; margin:30px; background-color:#ffddcc; color:#885522;“>(Proverbs 6:16-19) There are six things the Lord hates, seven that are detestable to him: haughty eyes, a lying tongue, hands that shed innocent blood, a heart that devises wicked schemes, feet that are quick to rush into evil, a false witness who pours out lies and a person who stirs up conflict in the community.</div>

Embedded

The embedded style sheet method is most suitable for setting the style for a many elements in the same document. With this method, we define a style specification as a class inside the head element and give it a name. Then we can apply the style repeatedly in the document by using the defined class inside each of the desired elements.

To use an embedded style sheet, define all styles in a style tag inside the head, as shown below. In this case, we have defined a class called “prog.” The style associate with this class can be used by adding class=”prov” to a tag, as shown in the example code below. This class can be applied to any number of elements in the document, and the usage of the class allows us to subsequently change the style for all of the elements just by simply changing the class style definition.

<!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN”
“http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd”>
<html xmlns=”http://www.w3.org/1999/xhtml” xml:lang=”en” lang=”en”>
<head>
    <title>CSS Applying a Style</title>

    <style>
        .prov {
            width:420px;
            height:120px;
            padding:20px;
            margin:30px;
            background-color:#ffddcc;
            color:#885522;
        }
    </style>

</head>

<body>
<div class=”prov”>(Proverbs 6:16-19) There are six things the Lord hates, seven that are detestable to him: haughty eyes, a lying tongue, hands that shed innocent blood, a heart that devises wicked schemes, feet that are quick to rush into evil, a false witness who pours out lies and a person who stirs up conflict in the community.</div>
</body>
</html>

External

The external style sheet is most suitable for setting a style that is used over multiple documents. Like the embedded style sheet, the external style sheet allows us to change all of the defined styles by making a single change to the style definition. However, the external style sheet uses a separate file to define the style so that it can be used in several files at once.

To use an external style sheet, create a file with a .css extension and put whatever style definitions you would like inside it. Below, I show an example with a file named “MyStyles.css” with the external style specification in it. In the HTML document, we need to add the line <link rel=”stylesheet” type=”text/css” href=”MyStyles.css” /> inside the head tag. This link tag can be used inside as many HTML documents as you would like. Then we only need to add class=”prov” to any element that we want to style, just as we did with the embedded style sheet. Note that in this example, we have the HTML document and the .css file in the same directory. However, if we want to put the style sheet file (the .css file) in a separate directory, we need to add the path to the href.

<!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN”
“http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd”>
<html xmlns=”http://www.w3.org/1999/xhtml” xml:lang=”en” lang=”en”>
<head>
    <title>CSS Applying a Style</title>
    <link rel=”stylesheet” type=”text/css” href=”MyStyles.css” />
</head>

<body>
<div class=”prov”>(Proverbs 6:16-19) There are six things the Lord hates, seven that are detestable to him: haughty eyes, a lying tongue, hands that shed innocent blood, a heart that devises wicked schemes, feet that are quick to rush into evil, a false witness who pours out lies and a person who stirs up conflict in the community.</div>
</body>
</html>

MyStyles.css

.prov {
    width:420px;
    height:120px;
    padding:20px;
    margin:30px;
    background-color:#ffddcc;
    color:#885522;
}

Creating a Walkable 2D Game Terrain in JavaScript

The Demonstration

This post demonstrates how to program the basic elements of a 2D tiled game with terrain images. This is an update to my prior post with terrain images added, along with an image for our game character. You can try out the demo, by left-clicking this link and then using the arrow keys to move around.

The Code

Most of this code was explained already in my prior post; so, I will explain the minor changes that have been made, beginning with the HTML file. In the HTML file, I have changed the 25 div elements to img elements so that they can hold images. I have also added an additional img element to hold the character image at the end of the background div.

Inside the JavaScript File, I have added variables to hold the terrain images and adapted the functions to use these images. At the top of the file, there are four new variables declared to hold the images for the terrain types: grass, forest, water, and bushes. In the Initialize() function, we allocate and set these terrain image variables. The function GetRandomTerrain() has been changed from the prior GetRandomColor() function to return an image object rather than a color. The FillBoard() function has been changed from the prior ColorBoard() function so that it now returns a null image for squares outside of the map; this has the effect of leaving these squares medium gray, as defined by the background element. The rest of the changes consist of calling these to functions instead of the prior functions. So, the changes are simple and straightforward.

Again, the HTML code can be put into any file with a .html extension, but the JavaScript file should be named “XoaXGameTerrain2D.js” in order to work with the code as is. Also, you will need 100 pixel by 100 pixel terrain images and a character image, which should be stored in the same folder with the HTML and JavaScript files.

HTML File


<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">

<head>
<title>XoaX.net's Javascript 2D Game Board Example</title>

<script type="text/javascript" src="XoaXGameTerrain2D.js"></script>
</head>

<body>

<div id="gameboard" style="position:absolute; width:500px; height:500px; background:#aaaaaa; margin:30px;">
<img id="A11" style="left:0px; top:0px; position:absolute; width:100px; height:100px;" />
<img id="A12" style="left:100px; top:0px; position:absolute; width:100px; height:100px;" />
<img id="A13" style="left:200px; top:0px; position:absolute; width:100px; height:100px;" />
<img id="A14" style="left:300px; top:0px; position:absolute; width:100px; height:100px;" />
<img id="A15" style="left:400px; top:0px; position:absolute; width:100px; height:100px;" />

<img id="A21" style="left:0px; top:100px; position:absolute; width:100px; height:100px;" />
<img id="A22" style="left:100px; top:100px; position:absolute; width:100px; height:100px;" />
<img id="A23" style="left:200px; top:100px; position:absolute; width:100px; height:100px;" />
<img id="A24" style="left:300px; top:100px; position:absolute; width:100px; height:100px;" />
<img id="A25" style="left:400px; top:100px; position:absolute; width:100px; height:100px;" />

<img id="A31" style="left:0px; top:200px; position:absolute; width:100px; height:100px;" />
<img id="A32" style="left:100px; top:200px; position:absolute; width:100px; height:100px;" />
<img id="A33" style="left:200px; top:200px; position:absolute; width:100px; height:100px;" />
<img id="A34" style="left:300px; top:200px; position:absolute; width:100px; height:100px;" />
<img id="A35" style="left:400px; top:200px; position:absolute; width:100px; height:100px;" />

<img id="A41" style="left:0px; top:300px; position:absolute; width:100px; height:100px;" />
<img id="A42" style="left:100px; top:300px; position:absolute; width:100px; height:100px;" />
<img id="A43" style="left:200px; top:300px; position:absolute; width:100px; height:100px;" />
<img id="A44" style="left:300px; top:300px; position:absolute; width:100px; height:100px;" />
<img id="A45" style="left:400px; top:300px; position:absolute; width:100px; height:100px;" />

<img id="A51" style="left:0px; top:400px; position:absolute; width:100px; height:100px;" />
<img id="A52" style="left:100px; top:400px; position:absolute; width:100px; height:100px;" />
<img id="A53" style="left:200px; top:400px; position:absolute; width:100px; height:100px;" />
<img id="A54" style="left:300px; top:400px; position:absolute; width:100px; height:100px;" />
<img id="A55" style="left:400px; top:400px; position:absolute; width:100px; height:100px;" />

<img src="Man.png" style="left:200px; top:200px; position:absolute; width:100px; height:100px;" />
</div>

</body>
</html>

JavaScript File


var qpBkdg = null;
var qppBoard = null;
var qppMap = null;
var iLocX = 10;
var iLocY = 10;
var qpGrass = null;
var qpForest = null;
var qpWater = null;
var qpBushes = null;

function KeyHandler(qKeyEvent) {
    var iKeyDown = 0;
    var iLeftArrow = 37;
    var iUpArrow = 38;
    var iRightArrow = 39;
    var iDownArrow = 40;

    if (qKeyEvent) {
        iKeyDown = qKeyEvent.which;
    } else {
        iKeyDown = window.event.keyCode;
    }

    if (iKeyDown === iLeftArrow) {
        if (iLocX > 0) {
            iLocX = iLocX - 1;
        }
    } else if (iKeyDown === iRightArrow) {
        if (iLocX < 19) {
            iLocX = iLocX + 1;
        }
    } else if (iKeyDown === iUpArrow) {
        if (iLocY > 0) {
            iLocY = iLocY - 1;
        }
    } else if (iKeyDown === iDownArrow) {
        if (iLocY < 19) {
            iLocY = iLocY + 1;
        }
    }
    FillBoard();
    return false;
}

function GetRandomTerrain() {
    var iRnd = Math.floor(Math.random()*10);
    switch(iRnd) {
        case 0:
        {
            return qpWater;
        }
        case 1:
        {
            return qpForest;
        }
        case 2:
        {
            return qpBushes;
        }
        default:
        {
            return qpGrass;
        }
    }
}

function FillBoard() {
    for (var iY = iLocY - 2; iY < iLocY + 3; iY++) {
        for (var iX = iLocX - 2; iX < iLocX + 3; iX++) {
            if (iY < 0 || iX < 0 || iY > 19 || iX > 19) {
                qppBoard[iY - iLocY + 2][iX - iLocX + 2].src = null;
            } else {
                qppBoard[iY - iLocY + 2][iX - iLocX + 2].src = qppMap[iY][iX].src;
            }
        }
    }
}

function Initialize() {
    qpGrass = new Image();
    qpGrass.src = "Grass.png"
    qpForest = new Image();
    qpForest.src = "Forest.png"
    qpWater = new Image();
    qpWater.src = "Water.png"
    qpBushes = new Image();
    qpBushes.src = "Bushes.png"

    // Generate map
    qppMap = new Array(20);
    for (var iY = 0; iY < 20; iY++) {
        qppMap[iY] = new Array(20);
        for (var iX = 0; iX < 20; iX++) {
            qppMap[iY][iX] = GetRandomTerrain();
        }
    }

    // Allocate the visible board
    qppBoard = new Array(5);
    for (var iY = 0; iY < 5; iY++) {
        qppBoard[iY] = new Array(5);
        for (var iX = 0; iX < 5; iX++) {
            qppBoard[iY][iX] = document.getElementById('A'+((iY+1)*10+(iX+1)));
        }
    }
    FillBoard();

    document.onkeydown = KeyHandler;
    qpBkdg = document.getElementById('gameboard');
}

window.onload = Initialize;