uploading mask generated outside photoscan

[ronan]

Hi,
I have masks (one mask per image) that were generated independently from photoscan.
is there a way to upload them to be used in photoscan?
 
I tried "import mask" in the File menu but could not get the 'from file' option to work.
my mask are black and white png images.

Thanks for any help
   

[ronan]

I reply to my own question.
mask needs to be saved as grayscale image not rgb even if it is black and white.