I succeeded getting the boundaries by reclassifying pixels values (outside raster values seems to have value 1) and then vectorize the raster. I tried to cut my first raster with those boundaries, but it's not working and I still get the rectangle when i try
Vector -> Research -> Polygon from Layer.
When I try the saga reclassification, and set values from 0.9 to 1 = 0/0 , I just get the rectangle filled with black.
Is there a way to tatally remove those values and to make them as the rest of the canvas ? (I don't want to give them another value because it would interfere with the grass:neighbors algorithm). Is ther a place where I can find what is the nodata value of my raster?