i wan’t to render a normalized cude with opengl ,the minimized coordinate is (0,0,0),the largest is(1,1,1),the depth test is enabled ,the value of the result color of the view plane(u,v) is the coordinate of the vertex(px,py,pz), ,and the z value of the view plane is vz, now assume the eyepostion is in eyepos,eyepos is a float3 value

so we can calculate a direction1 =normalized(u-eyepos.x,v-eyepos.y,vz-eyepos.z),and another direction2=normalized(px-eyepos.x,py-eyepos.y,pz-eyepos.z). theoretically ,direction1==direction2

now my question is in my program ,direction1<>direction2,why?

how much does the precision of the opengl rendering is?