Painting dream

I am sitting in a group of several people at a hillside in a rural countryside.
We have a sheet of drawing paper on our knees. A teacher is coming.
He stands a big easel with a large white paper sheet in front of us
and starts to sketch the landscape in rough outline.
A big old deciduous tree is placed in foreground of the picture.
Immediately, a sharply shining greenish point appears on the tree trunk
below the tree top in a late afternoon dusk.
The teacher touch the shining point with a point of his pencil.
All the picture, including each of leaves of the tree and grass stalks,
starts to shine by the greenish intensive light.
The teacher asks all of us to repeat whole the procedure ourselves.