Prevent automatic depth reduction

[paolopazziani]

Hi, I'm working on a program that needs to make changes to images (resizing, cropping, ...) without altering the total depth of each pixel. I have noticed that as a result of a disk read by any of the following methods:
- // Construct from image files or image specification
     Image (const std :: string & imageSpec_);
- // Read single image frame into current object
     void read (const std :: string & imageSpec_);
the total depth is reduced in some case when number_of_colors <2 ^ total_depth
Is there a way to prevent this behavior? Or restore the original depth?