Hex transparency in colors

Question

Hex transparency in colors

Question

I'm working on implementing a widget transparency option for my app widget although I'm having some trouble getting the hex color values right. Being completely new to hex color transparency I searched around a bit although I couldn't find a specific answer to my question.

I want to set transparency by hex color so let's say my hex color id "#33b5e5" and I want it to be 50% transparent. Then I'll use "#8033b5e5" because 80 is 50%.

I found a useful chart here: http://www.dtp-aus.com/hexadeci.htm . With this data I managed to come up with this:

0% = #00
10% = #16
20% = #32
30% = #48
40% = #64
50% = #80
60% = #96
70% = #112
80% = #128
90% = #144

Now the issues start appearing when I get higher than 100 in hex. Hex color codes can only be 8 symbols long right? For example #11233b5e5 (80%) crashes.

What can I do to enable me to use the higher numbers aswell?

SweSnow

2018/08/10

Read less...

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

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

Hex values to percentage

convert the hexaxdecimal value to decimal. ex: D6 = 13*16ˆ1 + 6 = 214
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

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

Raymond Chenon

2020/05/17

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.

erkangur

2013/04/06

That chart is not showing percents. "#90" is not "90%". That chart shows the hexadecimal to decimal conversion. The hex number 90 (typically represented as 0x90) is equivalent to the decimal number 144.

Hexadecimal numbers are base-16, so each digit is a value between 0 and F. The maximum value for a two byte hex value (such as the transparency of a color) is 0xFF, or 255 in decimal. Thus 100% is 0xFF.

Bryan Herbst

2014/09/11

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;
}

dwbrito

2014/06/24

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