This might be silly or a gap in understading, pl. help. In the classic derivation of special relativity - Lorentz transformations we see the diagram (say from HRW )

Frame S' (moving with velocity v in direction x w.r.t S)

http://en.wikipedia.org/wiki/File:Time-dilation-001.svg

Frame S

http://en.wikipedia.org/wiki/File:Time-dilation-002.svg

1. Derivation is based on postulate that speed of light is constant in any direction and is not affected by the speed of the source or the observer. Now, isnt it true that in the second diagram (frame S) we have just constructed D with angle Theta - which means the light is affected by the moving source when looked at from frame S ?

2. If starting event was a emission of a single photon in Y direction, (and light is not affected as the postulate says ) would it reach the mirror ?

What is wrong in the arguments ? pl. help.