Thursday 6 August 2009

Converting Decimal RGB Values Into a 6 Digit Hexadecimal

Recent posts have looked at the Actionscript required to convert decimal numbers into hexadecimal strings. And if we don't look any deeper everything looks hunkydory for mixing any colour we like using RGB values, and then convert to a hexadecimal number you might use to define a colour in HTML.

But we still have a problem to solve.

Low Numbers v. High Numbers

HTML and Flash can both use hexadecimal values to define colours, but the hexadecimal values must be defined as a 6 digit number - 3 pairs representing the 3 colour channels Red, Green and Blue. For example:

White: Decimal Red 255 + Green 255 + Blue 255 = Hexadecimal FFFFFF

In most cases this won't present a problem, after all decimal 255 equates to hexadecimal FF. The problem comes when the decimal value drops below 16. Lets take a look:

Decimal 255 = hex FF
Decimal 16 = hex 10
Decimal 15 = hex F
Decimal 1 = hex 1
Decimal 0 = hex 0

Notice that as soon as the decimal value drops below 16 the hexadecimal value drops into single digits. Lets see what this means in practice:

Black: Decimal Red 0 + Green 0 + Blue 0 = Hexadecimal 000

This is a problem because to convey colour values to HTML or Flash consistently we need a 6 digit hexadecimal number, but what we have here is only a 3 digit number.

To solve the problem we need to remember how hex counts. As an example let's count to thirty in hex:

0 1 2 3 4 5 6 7 8 9 A B C D E F 10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E

In this example values 16 and above all have 2 digits in hex. We need to find a way of making the lower numbers all have 2 digits too. This is actually very easy, in fact we do this all the time, most commonly when we write the date. Take for example 1st January 2009 written numerically - 01/01/09. We can turn a single digit number into a 2 digit number, without changing its value, very easily by placing a '0' (zero) in front (this is called prepending), like so:

1 becomes 01
2 becomes 02

All we have to do then is prepend single digit hex values with a '0' (zero), like so:

00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F

If we do this we can display black as a 6 digit hex number after all:

Black: Decimal Red 0 + Green 0 + Blue 0 = Hexadecimal 000000

But how do we do this in Flash with Actionscript?

Prepending Single Digit Hexadecimal Numbers With '0' Using Actionscript

I am sure there are many and varied ways to do this, but I prefer the following method. First I will give you the code, then I will explain how it works.

/* R is defined as a value between 0-255 in a text input field by the user */

_root.calc_but.onPress = function() {
var decimalred = new Number(R);
hexred = decimalred.toString(16);
if (decimalred<=15) {
hexredfinal = "0"+hexred;
} else {
hexredfinal = hexred;


Here's how it works:

1. First we take the user inputted value (a number between 0 and 255) and pass it to 'decimalred' specifically defining it as a number.

2. Then we convert the 'decimalred' number into a hexadecimal string and pass the result to 'hexred'.

3. Then we test to see if the original value inputted by the user was 15 or less. We do this because we already know that any decimal between 0 and 15 will convert to a single digit hex number.

4. If the original value inputted by the user was 15 or less, then we use Actionscript to prepend a '0' to the converted 'hexred' value like so:

hexredfinal = "0"+hexred;

Then we pass the result to 'hexredfinal'.

5. If the original value inputted by the user was not 15 or less, then we don't prepend anything, we just pass the 'hexred' value straight to 'hexredfinal' untouched.

In this way we can make sure that in the conversion from decimal to hexadecimal, all values between 0 and 255 are shown as 2 digit hex numbers. This allows us to produce the 6 digit numbers required in HTML and by Flash.

What fun.

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