[gdal-dev] What happens by default when writing RGBA into jpeg?

[Jukka Rahkonen]

Hi,

I would like to know what happens if I convert an original RGBA tiff image
into jpeg as 

gdal_translate -of jpeg rgba_in.tif jpeg_out.jpg

I know that for getting a good result I can add "-b 1 -b 2 -b 3" but I am
curious to know what happens and how GDAL computes sort of a transparent
sepia coloured mask when run without selecting bands.

-Jukka Rahkonen-