Android xdpi and ydpi

[Android beginner]

Hello All,

I am new to android platform and would like to get help regarding the following information:

1)DisplayMetrics has members xdpi and ydpi. How are these values calculated?Will these both values be same as densitydpi [ro.sf.lcd_density] always even when the resolution is different across the x and y dimensions?

2)Where are these values set and read from?

3)I would like to change these values for my application.Where can I change these values?

Thank You in advance.