How set alpha/opacity value to color on xml drawable?

Question

I need to set a value of opacity to a color in xml drawable. But when i try to add the two values of opacity (#20C0C0C0) for example, don't work. The color appeares totaly transparent.

Here is my code...

  <?xml version="1.0" encoding="utf-8"?>
  <shape xmlns:android="http://schemas.android.com/apk/res/android" >
     <corners
        android:radius="2dp" />
     <solid
        android:color="#CDCDCD" />
     <stroke
        android:width="2dp"
        android:color="@android:color/darker_gray" />
     <size 
        android:height="60dp"
        android:width="80dp"/>
   </shape>

Anyone have an idea? Thanks for your help.

Answer 1

The color "appears" totally transparent b/c it is almost totally transparent. Hex colors are normally 6 digits #RRGGBB but if you would like to set the opacity you pass in 2 digits in hexadecimal scale (base-16) at the start, so in your case #20C0C0C0 your opacity is 20 (in base-16).

Here are some typical decimal alpha values mapped to hexadecimal ones

• 0 -> 0 (this is completely transparent)
• 32 -> 20 (this is your opacity)
• 255 -> FF (this is completely opaque)

So your opacity is only ~12%.

If you would like it to be more opaque (less transparent) use a higher number (7F will give you about 50% opacity)

So:

<solid android:color="#7FC0C0C0" />

Answer 2

Giving an updated answer for when people see this, post 2018. As much as I've seen, when developing for API versions > Lollipop, one does not need to use hexadecimal values for alpha. I checked this yesterday and was surprised by this as well.

In the android color schemes, we use Hex values as in #FFFFFF for the color white. This represents RGB channels in said color. In otherwords, as mentioned by @Cumulo Nimbus above, #RRGGBB.

For opacity/transparency, you need to include the alpha channel as well which would give #AARRGGBB. In decimal/denary (base 10), R,G,B Channels each range from 0-255 whilst the alpha channel ranges from 0-100 so if I wanted to use the color sample you provided with 50% opacity, All I need to do is something like:

<solid android:color="#50C0C0C0"/>

where 50 is the percentage of opacity you require. For a fully opaque option i.e. 100% opacity, you do not need to use #FFC0C0C0 or #100C0C0C0(Note: this would show nothing). Just leave it as #C0C0C0 instead.

Answer 3

i may be a little late, but if someone else comes to this post and is looking for some alpha values. Jared Rummler did the work and provids us with every posibile value https://stackoverflow.com/a/27813407/5973229

So he uses this Method to calculate every opacity value in Hex code:

for (double i = 1; i >= 0; i -= 0.01) {
i = Math.round(i * 100) / 100.0d;
int alpha = (int) Math.round(i * 255);
String hex = Integer.toHexString(alpha).toUpperCase();
if (hex.length() == 1) hex = "0" + hex;
int percent = (int) (i * 100);
System.out.println(String.format("%d%% — %s", percent, hex));

And then this is the Result:

100% — FF
99% — FC
98% — FA
97% — F7
96% — F5
95% — F2
94% — F0
93% — ED
92% — EB
91% — E8
90% — E6
89% — E3
88% — E0
87% — DE
86% — DB
85% — D9
84% — D6
83% — D4
82% — D1
81% — CF
80% — CC
79% — C9
78% — C7
77% — C4
76% — C2
75% — BF
74% — BD
73% — BA
72% — B8
71% — B5
70% — B3
69% — B0
68% — AD
67% — AB
66% — A8
65% — A6
64% — A3
63% — A1
62% — 9E
61% — 9C
60% — 99
59% — 96
58% — 94
57% — 91
56% — 8F
55% — 8C
54% — 8A
53% — 87
52% — 85
51% — 82
50% — 80
49% — 7D
48% — 7A
47% — 78
46% — 75
45% — 73
44% — 70
43% — 6E
42% — 6B
41% — 69
40% — 66
39% — 63
38% — 61
37% — 5E
36% — 5C
35% — 59
34% — 57
33% — 54
32% — 52
31% — 4F
30% — 4D
29% — 4A
28% — 47
27% — 45
26% — 42
25% — 40
24% — 3D
23% — 3B
22% — 38
21% — 36
20% — 33
19% — 30
18% — 2E
17% — 2B
16% — 29
15% — 26
14% — 24
13% — 21
12% — 1F
11% — 1C
10% — 1A
9% — 17
8% — 14
7% — 12
6% — 0F
5% — 0D
4% — 0A
3% — 08
2% — 05
1% — 03
0% — 00