As far as I know.. 2 programs cannot access to camera hardware at sametime.
but if developer has control in kernel or module or camera hardware(which could be modified by him)
and when developer access camera hardware directly and seeing through camera(just seeing, not controling focus, light etc..) in kernel level
can user also use camera app at sametime?
I searched, and found some say its possible if modify camera driver.
*Assuming user installed developer’s custom kernel or ROM
