Hex transparency in colors [duplicate]

Answer 1

I built this small helper method for an android app, may come of use:

 /**
 * @param originalColor color, without alpha
 * @param alpha         from 0.0 to 1.0
 * @return
 */
public static String addAlpha(String originalColor, double alpha) {
    long alphaFixed = Math.round(alpha * 255);
    String alphaHex = Long.toHexString(alphaFixed);
    if (alphaHex.length() == 1) {
        alphaHex = "0" + alphaHex;
    }
    originalColor = originalColor.replace("#", "#" + alphaHex);


    return originalColor;
}

Answer 2

Color hexadecimal notation is like following: #AARRGGBB

• A : alpha
• R : red
• G : green
• B : blue

You should first look at how hexadecimal works. You can write at most FF.

Answer 3

This might be very late answer. But this chart kills it.

All percentage values are mapped to the hexadecimal values.

http://online.sfsu.edu/chrism/hexval.html

Answer 4

Short answer

You can see the full table of percentages to hex values and run the code in this playground in https://play.golang.org/p/l1JaPYFzDkI .

Short explanation in pseudocode

Percentage to hex values

1. decimal = percentage * 255 / 100 . ex : decimal = 50*255/100 = 127.5
2. convert decimal to hexadecimal value . ex: 127.5 in decimal = 7*16ˆ1 + 15 = 7F in hexadecimal

Hex values to percentage

1. convert the hexaxdecimal value to decimal. ex: D6 = 13*16ˆ1 + 6 = 214
2. percentage = (value in decimal ) * 100 / 255. ex : 214 *100/255 = 84%

More infos for the conversion decimal <=> hexadecimal

Long answer: how to calculate in your head

The problem can be solved generically by a cross multiplication.

We have a percentage (ranging from 0 to 100 ) and another number (ranging from 0 to 255) then converted to hexadecimal.

• 100 <==> 255 (FF in hexadecimal)
• 0 <==> 0 (00 in hexadecimal)

For 1%

• 1 * 255 / 100 = 2,5
• 2,5 in hexa is 2 if you round it down.

For 2%

• 2 * 255 / 100 = 5
• 5 in hexa is 5 .

The table in the best answer gives the percentage by step of 5%.

How to calculate the numbers between in your head ? Due to the 2.5 increment, add 2 to the first and 3 to the next

• 95% — F2 // start
• 96% — F4 // add 2 to F2
• 97% — F7 // add 3 . Or F2 + 5 = F7
• 98% — F9 // add 2
• 99% — FC // add 3. 9 + 3 = 12 in hexa : C
• 100% — FF // add 2

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I prefer to teach how to find the solution rather than showing an answer table you don't know where the results come from.

Give a man a fish and you feed him for a day; teach a man to fish and you feed him for a lifetime