Shader get content from screen buffer


I am now starting playong with the shaders in codea.
At first i want to try something like the game of life. Therefore i need to get the content from the screen.
I know how to write on the screen in the main function of the fragment:
gl_FragColor = …
In this function i want to get the content of the screen at first?

Can you please help!

Thank you!

do a search for @AndrewStacey - he has written a GoL in shaders.

The key is to draw your mesh within a setContext and then pass the image to the shader.