Question says it all really. I want to be able to display something at actual size, so, for example, if something is displayed in the browser as 20cm, I want to be able to measure it and it to actually be 20cm.
So far I have come across solutions which take into account the diagonal resolution of the screen (such as 21.5 inches) and the screen resolution to calculate how big a pixel is.
Another approach is to ask the user to put up a standard sized card such as a bank card to the screen and measure the edges to calculate the size.
Is there a way to do this without a physical object on the screen or without the user needing to know their diagonal screen size?
Please also let me know if you know of any different solutions other than the ones I have mentioned.