Bits & Bytes

Posts Tagged ‘color’

Responding to Keyboard Events in JavaScript

Key Events

In this post, we demonstrate how to catch and handle a keyboard event. There are three basic key events: key down, key up, and key press. The “key press” event is a bit different, since it describes a combination of a key up and a key down event and generates a character code. So, we will ignore the key press and concentrate on the key up and key down events, which use key codes rather than character codes.

The key down and key up events are triggered when a key is pushed in and when a key is released. During the event, a key code is generated that determines which key on the keyboard is pushed or released.

Key Codes Versus Character Codes

A key code is different than a character code because the same key can generate multiple different characters. For example, one key generates both of the characters ‘1’ and ‘!’ on the keyboard. So, ‘1’ and ‘!’ have different character codes, but they are generated by the same key and correspond to the same key code. For a full list of the JavaScript key codes, consult our table of key codes.

The JavaScript Code

Below, we have a code sample that can be copied, pasted, and saved into a simple text file that should be saved with a .html extension. Then you can double-click the file, say “KeyEvent.html”, to open it with your default browser.

In the code, the Initialize() function sets the function KeyHandler() as the event handler for key down events, and it also initializes the color of the background rectangle in the div element, myrect, to medium gray. Once the KeyHandler() function is set as the handler for key down events, it is called whenever a key is pushed while the document has focus; focus is given to whatever is currently selected to receive messages, usually the window or the control that was last clicked.

Inside of the KeyHandler() function, we have iKeyDown, which will be used to hold the key code of the key that was pushed, and we have the key codes for the left arrow and right arrow keys stored in iLeftArrow and iRightArrow, respectively. The first “if” statement is used to store the key code; the method varies depending on the browser (the else branch is for IE). The second “if” statement sets the background color to red if the left arrow is pressed, blue if the right arrow is pressed, and green if any other key is pressed.

The Code in Action

To see the code running, left-click the red rectangle above to open a new window with the code running in it; press the arrow keys to see the color change. If the color does not change, then it is because the document does not have focus. I have added a border (not used in the code below) to the rectangle that is displayed when it receives focus. To see the focus change, left-click the address in the address bar of your browser and then left-click the rectangle.



<!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 Keyboard Event Example</title>

<script type="text/javascript">
/*<![CDATA[*/
var qpRect = null;

function KeyHandler(qKeyEvent) {
    var iKeyDown = 0;
    var iLeftArrow = 37;
    var iRightArrow = 39;

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

    if (iKeyDown === iLeftArrow) {
        qpRect.style.backgroundColor = '#ff0000';
    } else if (iKeyDown === iRightArrow) {
        qpRect.style.backgroundColor = '#0000ff';
    } else {
        qpRect.style.backgroundColor = '#00ff00';
    }
    return false;
}

function Initialize() {
    document.onkeydown = KeyHandler;
    qpRect = document.getElementById('myrect');
    qpRect.style.backgroundColor='#888888';
}

window.onload = Initialize;
/*]]>*/
</script>
</head>

<body>

<div id="myrect" style="width: 300px; height: 200px; margin: 50px;">
</div>

</body>
</html>


Setting Colors in HTML with CSS

I have discussed how to specify a color with CSS in a prior post. Here, I will explain how the color specifications are used. To simplify the matter, I will use the hexadecimal color format for all of my color designations.

The basic block element has several distinct regions that can be specified, as I discussed in a another post on the CSS box model. Of these regions, three different colors may be selected for the border, background, and text, as shown below.

(Leviticus 10:9-11) You shall not drink wine nor any thing that may make drunk, thou nor thy sons, when you enter into the tabernacle of the testimony, lest you die: because it is an everlasting precept through your generations: And that you may have knowledge to discern between holy and unholy, between unclean and clean: And may teach the children of Israel all my ordinances which the Lord hath spoken to them by the hand of Moses.

The border is the outermost region that is colored bluish. When the border color is set, we need to specify the width and the pattern as well. In the example above, we set the border with this designation:
border:10px solid #88aacc;
To break this down, the border has a width of 10 pixels, a solid pattern, and is colored #88aacc.

Inside the border we have the background and the text. The background pinkish color is specified as
background-color:#ffddcc;
Against this background, we have a darker reddish, brown color for the text, which is specified as
color:#aa5522;
This is how the text color is set for any element.

The full specification for the div above, with all of the color and size styling, is given by this code below
<div style="width:420px; height:160px; padding:20px; border:10px solid #88aacc; background-color:#ffddcc; color:#aa5522; margin:20px;">
(Leviticus 10:9-11) You shall not drink wine nor any thing that may make drunk, thou nor thy sons, when you enter into the tabernacle of the testimony, lest you die: because it is an everlasting precept through your generations: And that you may have knowledge to discern between holy and unholy, between unclean and clean: And may teach the children of Israel all my ordinances which the Lord hath spoken to them by the hand of Moses.
</div>

Specifying Colors in HTML with CSS

Colors in a computer are made up of three color channels: Red, Green, and Blue. These three channels are combined to form all colors. For example, when the three channels are at their maximum value, we get the color white. On the other hand, if all of the channels are set to zero, then we get the color black. We have several methods for setting a color in CSS.

RGB Values

background-color: rgb(255, 240, 200);

For our first method of setting a color, we use the rgb() functional notation. Inside the parentheses, the three values are given as comma separated decimal integers that range between 0 and 255. Above, we have a div that we set with the values red = 255, green = 240, and blue = 200. Since all of the channels are close to their maximum value of 255, the color is close to white. However, is has an orange tint because the red and green channels are the largest.

RGB Percentage Values

background-color: rgb(65%, 95%, 75%);

Instead of the values 0 to 255, we can use percentages. The notation is similar to the prior RGB Values except that the values are given as percentages instead of integers: rgb(65%, 95%, 75%). So, the value 0% is is the same as 0, and the value 100% is the same as 255, in RGB Values. Percentages can be given as fractional numbers, since they will be mapped to the range of values 0 to 255. For example, the color above, rgb(65%, 95%, 75%), is the same as rgb(255*.65, 255*.95, 255*.75) = rgb(165.75, 242.25, 191.25). Of course, since the color values can only be integers, these values are rounded to integers before they can be displayed. The color above has a larger green component in it, so looks green.

Hexadecimal RGB

background-color: #f4cfe5;

If you are wondering why the color values run from 0 to 255, then you might not be familiar with hexadecimal. However, you should learn it because it is a more convenient format for representing how computers store numbers. If you are not familiar with it I suggest watching our short videos on bits and bytes and binary, octal, and hexadecimal. These videos will go a long way toward clearing up this extremely important topic, which is likely to trouble you until you get it sorted out.

The hexadecimal RGB format uses a # sign to signal the hexadecimal format, which is then followed by three sets of two hexadecimal digits to represent the red, green, and blue channels:

   #f4cfe5.

These values convert to f4 = 244, cf = 207, and e5 = 229 in decimal. It takes some time to get used to hexadecimal. Once you do, however, it is more convenient to work with since the values from 0 to 255 can be represented with exactly two digits in hexadecimal. As you can see, the color above is a light purple because the red and blue channels are the largest.

Hexadecimal RGB Abbreviated

background-color: #adf;

The abbreviated hexadecimal format is similar to the two digit hexadecimal format, except that it only uses one digit to represent two digits. It does this by repeating the single hexadecimal digit. So, the abbreviated hexadecimal value #adf is the same as #aaddff:

   #adf

The color above is greenish-blue because the blue channel is the largest followed by the green.

Color Names

background-color: orange;

In addition to the formats above, it is possible to specify a color by name. There are only 17 names that are standardized. First, there is orange, which is shown above and is equal to the color #ffa500 in hexadecimal. The other 16 names are shown below, along with their hexadecimal equivalents.

black
#000000
gray
#808080
silver
#c0c0c0
white
#ffffff
maroon
#800000
red
#ff0000
purple
#800080
fuchsia
#ff00ff
green
#008000
lime
#00ff00
olive
#808000
yellow
#ffff00
navy
#000080
blue
#0000ff
teal
#008080
aqua
#0000ff

Other color designations are available in HTML 5. However, these formats are not supported in all browsers at this time.