Display Drawing with true scale

There is no generic way for a display application to know what the size of the screen is. Especially across different computer systems. 1:1 and 100% does not even generically mean 1:1 reproduction size but rather one pixel is one pixel whatever that pixels size happens to be on your screen.

Sure you can measure the screen and compensate. Its easy to do if you can control all the hardware. But you can not make sure every device everywhere does this.