Help,about rendertarget

well,I need some Help,the problem is that,How can I to take the pixel’s color data from a Rendertarget2D? and How to render the appointed characters with appointed material into the Rendertarget2D?I want to make a system about Bullet and Armor by this way. the Armor had accurate thickness data.I want to put the thickness data into a texture,It’s a bit like the WOT and WOWS,so How can I do.