Image Boundary plugin did not work for me either, therefore I used the same approach with GDAL. Nevertheless it only worked for me after changing the first step to:

step 1: gdalwarp -srcnodata 0 -dstalpah -of GTiff foo1 foo2

I am working with Landsat8 band (where no data=0) and when using the -dstnodata function I get:

band1 with no data = 'no data'
band2 (Alpha band) = '255' for the entire scene/extent

whereas with -srcnodatafunction I get:

band1 with no data = 'no data'
band2 (Alpha band) with no data = 'no data' and valid data = '255' which then allows to extract polygon for valid data only.

I couldn't fully understand the reason behind this behavior (how alpha is computed?) , but I hope this might help others facing the same problem

Image Boundary plugin did not work for me either, therefore I used the same approach with GDAL. Nevertheless it only worked for me after changing the first step to:

step 1: gdalwarp -srcnodata 0 -dstalpha -of GTiff foo1 foo2

I am working with Landsat8 band (where no data=0) and when using the -dstnodata function I get:

band1 with no data = 'no data'
band2 (Alpha band) = '255' for the entire scene/extent

whereas with -srcnodatafunction I get:

band1 with no data = 'no data'
band2 (alpha band) with no data = 'no data' and valid data = '255' which then allows to extract polygon for valid data only.

I couldn't fully understand the reason behind this behavior (how alpha is computed?) , but I hope this might help others facing the same problem.