If I create a surface using a triangle, I do not see Warping as with a square.
Capture d'écran 2019-12-03 21.50.24.png
Sorry, this isn't possible.
In fact, you cannot subdivise a triangle into several rectangle to create a warping grid (at best, the grid would be made both of triangles and rectangles), so it's not that obvious.
In your situation, I think the best solution is to use a triangle mask, then apply some warping.
Thank You Philippe. I will use your solution