Well, now I have the math base needed, but how I do it? I can use some software package to solve this integral and I found some after some day of web search. But there is also another big problem. The calculation is time consuming, for today computers it means nothing, but if we want to map meshes with tens or hundreds thousands polygons it will take some time to do it, well rendering an image takes much more, but I wanted to find another and faster way to do it.

My idea was to approximate the solution, I don't need an exact solution, I am not going to land in Mars with this. An error of one percent would be something beautiful and nobody will see any difference in the texturing. I wanted to approximate this integral with something basic mathematical operations and basic mathematical functions.

After two or three weeks of work I found something very simple with an error of less than 1.5% and it took me half of an hour to code and include into a software app.

Stupidity also evolves!