1. An apparatus for compressing data representing a sequence of images in dependence upon data defining at least one region of particular interest within at least one image in the sequence, the apparatus being configured to provide data representing an image region outside the at least one region of particular interest that is more highly compressed than data representing an image region inside the at least one region of particular interest, the apparatus comprising: a video input; pre-processor apparatus which receives images from the video input and low pass filters data representing image regions outside the region or regions of particular interest; a first coder which receives images from the pre-processor and in a prediction, transform and quantization process provides intra-coded frames and prediction coded frames of transform coefficients; a variable length coder which receives transform coefficients and provides a compressed video data output; a switch; and a second coder which receives images from the video input and in a transform and quantization process provides intra-coded frames to the switch; wherein the switch receives an intra frame identification signal from the first coder and operates to provide the variable length coder with prediction coded frames of transform coefficients from the first coder and intra-coded frames of transform coefficients from the second coder.