Ahhh, that will be a problem, as far as I know, there is no HW resize in gstreamer, unless you write one.
So you may well need OpenMAX (or MMAL, which is an easier to use interface layer which uses OMX, but is only used on the Pi).
However, you are going to need to check whether the Pi can actually do the job you want. You are decoding 1080p25, resizing, and re-encoding to 480p25, which might be a little bit more than the HW can cope with.
There are people who have done transcoding on the Pi, try Google - a lot of the work may already have been done.
Principal Software Engineer at Raspberry Pi (Trading) Ltd.
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