Android的计量单位px/dip/dp/sp/pt/in/mm，及相互转换_android dp和mm换算

px   ：原始像素点（raw Pixel）。
dip  ：设备独立点（Device Independent Pixel），会根据屏幕的密度（density）进行缩放。
dp   ：同dip，是其简写。
sp   ：缩放像素点（Scaled Pixel），会根据用户选择的字体大小（scaledDensity）进行缩放。

pt    ：英磅（Point），英寸的1/72。

in    ：英寸（Inch）。

mm ：毫米（Millimeter）。

建议：文字尺寸使用sp，其他尺寸使用dp。

PPI = Pixels per inch，每英寸上的像素数,即 "像素密度"

• xhdpi: 2.0
• hdpi: 1.5
• mdpi: 1.0 (baseline) 160
• ldpi: 0.75

    //android.util.TypedValue
 
    /**  complex unit: Value is raw pixels. */
    public static final int COMPLEX_UNIT_PX = 0;
    /**  complex unit: Value is Device Independent Pixels. */
    public static final int COMPLEX_UNIT_DIP = 1;
    /**  complex unit: Value is a scaled pixel. */
    public static final int COMPLEX_UNIT_SP = 2;
    /**  complex unit: Value is in points. */
    public static final int COMPLEX_UNIT_PT = 3;
    /**  complex unit: Value is in inches. */
    public static final int COMPLEX_UNIT_IN = 4;
    /**  complex unit: Value is in millimeters. */
    public static final int COMPLEX_UNIT_MM = 5;
 
    /** 
     * Converts an unpacked complex data value holding a dimension to its final
     * floating point value. The two parameters unit and value are as in dimension.
     *  
     * @param unit The unit to convert from.
     * @param value The value to apply the unit to.
     * @param metrics Current display metrics to use in the conversion -- 
     *                supplies display density and scaling information.
     * 
     * @return The complex floating point value multiplied by the appropriate 
     * metrics depending on its unit. 
     */
    public static float applyDimension(int unit, float value,
                                       DisplayMetrics metrics)
    {
        switch (unit) {
        case COMPLEX_UNIT_PX: //原始像素点
            return value;
        case COMPLEX_UNIT_DIP: //设备独立点
            return value * metrics.density;
        case COMPLEX_UNIT_SP: //缩放像素点
            return value * metrics.scaledDensity;
        case COMPLEX_UNIT_PT: //英磅
            return value * metrics.xdpi * (1.0f/72);
        case COMPLEX_UNIT_IN: //英寸
            return value * metrics.xdpi;
        case COMPLEX_UNIT_MM: //毫米
            return value * metrics.xdpi * (1.0f/25.4f);
        }
        return 0;
    }