Alpha-channel PNG from gdaldem

Question

I was hoping that the following command would give me an alpha-channel PNG which was transparent where the DEM has "nodata":

gdaldem color-relief -alpha -of png dem.tif palette.dat out.png

I get an RGBA PNG alright, but the alpha channel is completely opaque! Why?

I am reasonably convinced that there are "nodata" pixels in dem.tif, because if I add an nv entry to the palette (with some garish colour), I get a lot of this colour in my output, and right where I expect it to be.

gdaldem --version says GDAL 2.2.1, released 2017/06/23.

Answer 1

Answering my own question: Make sure the nv entry in the palette has four channels (R, G, B, A) with a zero for the alpha value, and it works!

I had mistakenly assumed that the alpha channel would be deduced from the "nodata-ness" of the pixels, but apparently, you have to provide the alpha data explicitly in the palette.