error in converting image to binary

I have a rgb image inwhich the image consists of two persons,i want to make the background black and persons in white colour,i tried im2bw but i get only whit backgroubf,no image duisplayed ,please help

Answer 1

[Pugazhendhi]

First convert to grayscale(rgb2gray) then use im2bw

[FIR]

I tried but getting same result

Answer 2

[Walter Roberson]

See graythresh()