Convert a RGB colour value to Decimal

How do I convert a RGB colour value to just plain decimal?

So I have: RGB(255,255,255) is white
Its decimal equivalent is: 16777215

I have tried thinking it might just be:

var dec = r*g*b; // but this doesn't work

Although that doesn't work.

Anyone know of the algorithm/equation to convert from RGB (or rgba) to decimal?

Answers:

Answer

RGB integers are typically treated as three distinct bytes where the left-most (highest-order) byte is red, the middle byte is green and the right-most (lowest-order) byte is blue. You can retrieve the values of these individual bytes like this:

var c = 0xff03c0; // 16712640
var components = {
    r: (c & 0xff0000) >> 16, 
    g: (c & 0x00ff00) >> 8, 
    b: (c & 0x0000ff)
};

You can re-create a color from its components by shifting the bytes back in:

c = (components.r << 16) + (components.g << 8) + (components.b);

In your case, simply substitute components.r (etc) with your actual variables.

Answer

A much better answer (in terms of clarity) is this:

'Convert RGB to LONG:
 LONG = B * 65536 + G * 256 + R

'Convert LONG to RGB:
 B = LONG \ 65536
 G = (LONG - B * 65536) \ 256
 R = LONG - B * 65536 - G * 256

LONG is your long integer (decimal) that you want to make. Easy huh? Sure, bitshift

Answer
var dec = (b & 0xff) << 16 + (g & 0xff) << 8 + (r & 0xff);

(I think that's the correct order for the r,g,b values)

Update

I just checked for browser applications and I got the order wrong, so the correct formula for browsers (read HTML+CSS+javascript) is:

var dec = r << 16 + g << 16 + b;

Assuming r,g,b values <= 255

Other API's may expect a different order for the r,g,b values. I seem to remember at least one that has the order reversed (per my original answer), but I think which one it is at the moment.

Answer
if (color.substr(0, 1) === '#') {
    return color;
}
var digits = /(.*?)rgb\((\d+), (\d+), (\d+)\)/.exec(color);
var red = parseInt(digits[2]);
var green = parseInt(digits[3]);
var blue = parseInt(digits[4]);
var rgb = blue | (green << 8) | (red << 16);
return rgb.toString(10);
Answer

You need to left-shift the R by 16, the G left by 8 and or both in to the B. Look at the values as bits and all will become clear:

R 255 = 11111111B G 255 = 11111111B B 255 = 11111111B

Shift R 16: 111111110000000000000000 Shift G 8: 000000001111111100000000

or into B: 111111111111111111111111

Convert to decimal: 16777215

Answer

I agree that bitshift is clearer and probably better performance.

That said, if you want a math answer due to language support (such as doing this in a spreadsheet) modulus % and trunc() or floor() make it simple:

Assuming 8 bit values for red green and blue of course (0-255):

var rgbTotal = red * 65536 + green * 256 + blue;

var R = Math.trunc( rgbTotal / 65536 );
var G = Math.trunc( ( rgbTotal % 65536 ) / 256 );
var B = rgbTotal % 256;

Discussion: Pointing to sam's answer, RGB values are nearly always big endian in terms of order, certainly on webpages, jpeg, and png. Personally I think it's best to multiply red by 65536 instead of blue, unless you're working with a library that requires it otherwise.

To return to separate values:

  • For R, just divide by 65536 and truncate the remainder.
  • For G, we discard R via mod 65536 then dividing by 256, truncating the remainder (remainder is blue).
  • For B, we take mod 256, which disposes of the two higher bytes.

For R & G the number needs to be truncated to discard the remainder, language specific, mainly we want the integer. In javascript Math.trunc() is the easy way, and Math.floor() also works. It's not needed for B as that IS the remainder from the modulus — however this also assumes that we don't need error checking such as from user input, i.e. the value for B is always an integer 0-255.

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