Hi Gary,
Thank you for your answer.
If fact, that I want to do is this :
I want to create a polygon which is 5 m (for example) on one side of the selected polygon.
I joined some screens shot so that it is more explicit.
1) I have a polygon and I select it (ie : Polygon1.png in the file Fic1.png)
2) I edit the polygon, the user clicks on vertices, and then they are marked as selected (ie : PolygonSelectVertice.PNG .. green vertices in the file Fic1.png).
3) I use a buffer to create the polygon which is 5 m of the line (=select vertices) (ie. : PolygonBuffer.png in the file Fic1.png)
The user can select more than 2 vertices, and the buffer is built according to the selected vertices (ie. PolygonBufffer1.png in the file Fic2.png)
4) And the last step is to create the 2 polygons (ie PolygonResult.png in the file Fic2.png)
These differents steps work.
But the last thing which I want to make and which I did not succeed is Instead of selecting vertices to define where the buffer is applied, the user selects the side of the polygon and the side of the polygon is marked as selected or the click on the side of the polygon select the vertices.
I hope I describe enough my problem, and let me know if you want more information.
Thanks.
Regards.
Stéph.