1. In a video encoder comprising motions estimation means (22) providing a predicted block for each predefined block based upon estimating the motion between said predefined block of the current image and the corresponding block in the previous image, transform means (16) for transforming a prediction error resulting from the difference between said predicted block and said predefined block into the frequency domain, and quantizing means (20) for quantizing the coefficients of the prediction error in the frequency domain and providing the quantized coefficients to a video multiplex coding unit (30), wherein said quantized coefficients are de-quantized (24) and inverse transformed (26) to give back said prediction error and add it to said predicted block whereby the result is provided to said motion estimation means in order to get a new current predicted block; a conditional replenishment device characterized in that it comprises: change detection means for producing a segmentation map in which each pixel of said predefined block is marked as moving or stationary, said segmentation map being obtained by using an iterative minimization algorithm to solve a problem formalized as a statistical Maximum Aposteriori Problem; motion determination means for determining whether there is a group of moving pixels the number of which exceeds a predetermined threshold; and, coding means for encoding only blocks containing said group of moving pixels.