Project CD (a very old project, still ongoing)

 You may be wondering why I'm interested in SLM or computer-generated holograms (CGH). Why all this?
The answer is somehow complicated, but it all started many, many years ago.
There was an article in the news that some scientist wrote a hologram on a CD (compact disc - if you know what it is ;). When I heard about this news I thought maybe I should try to make it also, and project CD was born (originally called CD project but I think I can not use this name ;( ).
The idea was simple - write a hologram on a CD.
It took some time but I managed to write something visible on a CD (on the data side). Then I started to read de documentation of a compact disc and tried to overcome all protection of data on the CD.
After a long time, I managed to write a picture.
Then I forgot about the project and lost all programs that I wrote for it. After some time I started again. With some background on what works and what doesn't. Eventually, I made the same progress as before (but I think the approach was somehow different) but still just wrote a picture that was not that good visible. An example is shown below.

Ok, there is a picture not that visible but still a picture. But if you think about it, the picture changes colors like a rainbow... eureka, this is a hologram.

So, why the CGH and SLM, if you write to a CD you have to overcome the protection of the data if you do that you are not sure if you did it correctly if you don't see a hologram, or maybe the hologram computation was wrong, so a better approach is to test the hologram (CGH) on an SLM and if it is correct than write it to the CD and if you don't see it than you messed up with the data protection on a CD.
How to check that you bypass the protection correctly. This is another topic that I will cover briefly in the future..