聊聊Color中的alpha值

序

本文主要介紹下java Color對象中的alpha值。

alpha

java/awt/Color.java

/**
     * Creates an opaque sRGB color with the specified red, green,
     * and blue values in the range (0 - 255).
     * The actual color used in rendering depends
     * on finding the best match given the color space
     * available for a given output device.
     * Alpha is defaulted to 255.
     *
     * @throws IllegalArgumentException if <code>r</code>, <code>g</code>
     *        or <code>b</code> are outside of the range
     *        0 to 255, inclusive
     * @param r the red component
     * @param g the green component
     * @param b the blue component
     * @see #getRed
     * @see #getGreen
     * @see #getBlue
     * @see #getRGB
     */
    public Color(int r, int g, int b) {
        this(r, g, b, 255);
    }

    /**
     * Creates an sRGB color with the specified red, green, blue, and alpha
     * values in the range (0 - 255).
     *
     * @throws IllegalArgumentException if <code>r</code>, <code>g</code>,
     *        <code>b</code> or <code>a</code> are outside of the range
     *        0 to 255, inclusive
     * @param r the red component
     * @param g the green component
     * @param b the blue component
     * @param a the alpha component
     * @see #getRed
     * @see #getGreen
     * @see #getBlue
     * @see #getAlpha
     * @see #getRGB
     */
    @ConstructorProperties({"red", "green", "blue", "alpha"})
    public Color(int r, int g, int b, int a) {
        value = ((a & 0xFF) << 24) |
                ((r & 0xFF) << 16) |
                ((g & 0xFF) << 8)  |
                ((b & 0xFF) << 0);
        testColorValueRange(r,g,b,a);
    }

java裏頭的color不指定alpha的話，默認其值爲255，也就是沒有透明度。

opacity

color對象裏頭的alpha實際上是指不透明度，其值範圍爲0-255，越大越不透明。
其一般對應opacity，這個就是單詞語義表達的不透明度，其值範圍[0,1.0f]，值越大，越不透明。

opacity與alpha的映射

opacity與alpha之間的主要關係列表以下

100% — FF
95% — F2
90% — E6
85% — D9
80% — CC
75% — BF
70% — B3
65% — A6
60% — 99
55% — 8C
50% — 80
45% — 73
40% — 66
35% — 59
30% — 4D
25% — 40
20% — 33
15% — 26
10% — 1A
5% — 0D
0% — 00

這個怎麼轉義呢，以下

int alpha = Math.round(opacity * 255);

再將int輸出爲十六進制的表示方式

String hex = Integer.toHexString(alpha).toUpperCase();
        if (hex.length() == 1){
            hex = "0" + hex;
        }

不足兩位往前不零

doc

• Understanding colors on Android (six characters)
• Hex transparency in colors