[Gvsig_english] About NDVI and Map Calculator in Sextante Toolbox

[Antonio Falciano]

Hi list,
has anyone experience about computation of NDVI on Quickbird images?
gvSIG recognizes Quickbird .hdr thanks to 'gvSIG Image Driver' and
allows to choose three bands to represent in the view (RGB). This is great!
I also decided to make some tests using the Sextante Toolbox.
I tried to calculate NDVI using the tool contained in Vegetation Indexes
on a small view extent, but the result is a null layer.
I thinked about a bug or something else due to my mistakes. I decided to
do that with the Map Calculator using the following expression:
( (qb Band 4) -  (qb Band 3)) /  ( (qb Band 4) +  (qb Band 3))
where qb is the name of my layer/image (Band 4 is NIR and Band 3 is Red),
but the result is a null layer too.
Suggestions are welcome.

Regards,
Antonio