hello,
when modeling ( for ex a 3D sphere in Edit object mode )if you press ctrl key you can apply a mask.
is it possible to apply this mask using an alph ?
thank you…
yes, there are four ways to do this that I know of.
1: In edit mode, select your alpha then click and drag from outside your model.
2: Use TextureMaster to paint a black and white texture. Apply that texture as an alpha by pressing Tool 
modifiers selection intensity .
3: Select your alpha and press Alpha Make Stencil. Resize/position the stencil over your model and ctrl+click as normal to create a mask.
4: Simply press Tool modifiers selection alpha .
The accuracy of an alpha applied as a mask, depends on the number of polygons your object is made of and the resolution of the alpha.
TVeyes, saves me more time AGAIN… 
I was going from texture to alpha to selection.
Now I’ll go straight from texture to alpha.
Thanks,
Andre Khromov
your third method is very interesting…
but when the stencil is created, i positionned it, but how you draw the mask and how you put the stencil off to continue modeling ?
Yes, the third method is very very interesting… because it does not work 
Sorry, I must have been dreaming again. You can’t create a mask on an object in edit mode using a stencil.
Stencils are turned on/off via the Stencils palette.
Aurick explains how to use stencils for deformations/masking in this thread, Tutorial: Stencil-Based Deformations .
Ahkromov: You can also get selection masks from the textures saturation and hue, as I’m sure you’ve found out.
Actually, you can model through a stencil and the editing brush will take it into account, but you can’t use the stencil to control deformations.
TVeyes thanks very much for highlight this point about the use of stencil (a dark area among other in my learning process¡¡)and for the Aurick tutorial. I will download and try to understand.
Thanks again your generous sharing your knowledge¡¡
In debt with you¡¡
i can’t understand what Aurick wants to say?
what is the differance between " you can model through a stencil" and " but you can’t use the stencil to control deformations "
Putting a mask using stencil, is modeling or controling ?
A stencil is not a mask. A mask applies directly to your model on a polygon-by-polygon basis. When a deformation is applied to a masked model, the masked areas are unaffected. When you sculpt a masked model using the sculpting tools (Edit>Move, Edit>Draw, Edit>Scale), the masked areas are also unaffected.
The stencil works differently. It floats over top of your model. When you sculpt through it using the sculpting tools, it will constrain your sculpting the same way that a mask would. But if you apply a deformation, the entire model will be affected as if the stencil was not there.
Only a mask can restrict deformations. A stencil cannot.
Hope that clears things up a little.