ImageMagick extend canvas with transparent background

convert input.png -extent 100x100 -gravity center -background white output.png

If the input is 50x50 the surrounding background is white. Can I somehow set this to transparent without declaring any color within input as transparent?

Use this instead:

convert               \
      input.png       \
     -background none \
     -gravity center  \
     -extent 100x100  \
      output.png

Note well: The order of the parameters is significant! (To convince yourself, just put -background none at the end of the parameters instead of the start...)

---

Updated: Thanks to @jesmith who noticed that the commandline I originally provided does no longer work as intended. More recent versions of convert additionally require that the -gravity center is called before -extent 100x100. (This was one of the changes introduced to one ImageMagick's most recent versions [at the time of originally writing this answer]).

Kurt's note is ironically spot on, order matters greatly. Kurt's command results in gravity not being applied to the extent, so the transparent 'border' will all be to the bottom and/or right of the image.

Moving gravity before extent will correctly create equal transparent 'borders' on all applicable sides.

> convert input.jpg -background none -gravity Center -extent 100x50 > output.png