How to make a background 20% transparent on Android
android, textview, transparency
Solution
Make the color have 80% in the alpha channel. For example, for red use `#CCFF0000`:
<TextView
...
android:background="#CCFF0000" />
In the example, `CC` is the hexadecimal number for `255 * 0.8 = 204`. Note that the first two hexadecimal digits are for the alpha channel. The format is `#AARRGGBB`, where `AA` is the alpha channel, `RR` is the red channel, `GG` is the green channel and `BB` is the blue channel.
I'm assuming that 20% transparent means 80% opaque. If you meant the other way, instead of `CC` use `33` which is the hexadecimal for `255 * 0.2 = 51`.
In order to calculate the proper value for an alpha transparency value you can follow this procedure:
- Given a transparency percentage, for example 20%, you know the opaque percentage value is 80% (this is `100-20=80`)
- The range for the alpha channel is 8 bits (`2^8=256`), meaning the range goes from 0 to 255.
- Project the opaque percentage into the alpha range, that is, multiply the range (255) by the percentage. In this example `255 * 0.8 = 204`. Round to the nearest integer if needed.
- Convert the value obtained in 3., which is in base 10, to hexadecimal (base 16). You can use Google for this or any calculator. Using Google, type "204 to hexa" and it will give you the hexadecimal value. In this case it is `0xCC`.
- Prepend the value obtained in 4. to the desired color. For example, for red, which is `FF0000`, you will have `CCFF0000`.
You can take a look at the Android documentation for colors.
Problem
How do I make the background of a `Textview` about 20% transparent (not fully transparent), where there is a color in the background (i.e. white)?