What is the CoronaTexelSize good for? What can I calculate with it?
Maybe the pixel size of the image texture?
In essence, they enable you to create effects that are based on actual screen (or content) pixel distances, e.g. a checkerboard pattern, that’s independent of the pixel resolution of the texture.
I’ve updated the docs with more explanation:
http://docs.coronalabs.com/daily/guide/graphics/customEffects.html#environment-variables
Thank you for the update in the docs.
It is not 100% clear to me but I will play around to get the last percent 
In essence, they enable you to create effects that are based on actual screen (or content) pixel distances, e.g. a checkerboard pattern, that’s independent of the pixel resolution of the texture.
I’ve updated the docs with more explanation:
http://docs.coronalabs.com/daily/guide/graphics/customEffects.html#environment-variables
Thank you for the update in the docs.
It is not 100% clear to me but I will play around to get the last percent