where for cell , denotes the inverse compressibilityof the cell, is the number of pixels in the cell (its volume), and is the cell’s target volume. This constraint defines as the pressure inside the cell. A cell with has a positive internal pressure, while a cell with has a negative internal pressure.
Since many cells have nearly fixed amounts of cell membrane, we often use a surface- area constraint of form:
,
, 
denotes the inverse compressibility 
is the number of pixels in the cell (its volume), and 
is the cell’s target volume. This constraint defines 
as the pressure inside the cell. 
has a positive internal pressure, 
has a negative internal pressure.