OpenGL ES 2.0:带有浮动或半浮动纹理的glReadPixels()

I am writing an OpenGL ES 2.0 app for the iPhone (iOS 4.1). At the end of the computations, which are done in the shaders, i need to write back some data to the CPU. As far as I know this can be done by glReadPixels(). In order to keep precision, i want to use half_float or float textures between the shaders, which seem to be supported by extensions.I am writing an OpenGL ES 2.0 app for the iPhon