depth buffer

Hello I have a huge problem,

I draw my scene in the depth buffer, then i set with the function glDepthMask(0) that the Depth buffer is readonly.

than i draw the depth buffer in an array and save it in an bitmap.

but there is only a grey rectangle?

when i draw the depth buffer in the bitmap without setting it readonly i can see the depth buffer in the image

thank you for help

helda