My version of QGIS is 2.8.
I want to convert a RGB raster into a polygon. I just need the boundary of it (not the rectangle extent but the boundary of the real feature). I have tried raster calculator but it says SAGA does not support multiband raster.
I also have tried the Vectorize raster layer tool but it takes a very long time because the resolution is very high. It gives me around 3000000 polygon for an image like below and I have a few hundreds this size of images.