The present invention relates to the field of motion estimation in video processing. Specifically, the invention relates to a video-processing method and device for ascertaining motion vectors for a plurality of first pixel blocks forming a currently processed image region of a currently processed image of an image sequence. The invention addresses the problem of the impact of borders between neighboring image regions in region-based motion estimation on the quality of the video output in video applications like picture-rate up conversion. The video-processing device (100) of the invention comprises a processing unit (104), which is adapted to perform motion estimation on an image according to a fragmentation of the image into a number of image regions, each image a region containing the pixel blocks shared by a first number of pixel-block lines and a second number of pixel-block columns in accordance with an adjustable value of an aspect ratio of the image region, and to set a different aspect-ratio value for processing a next image of the image sequence, such that the number of image regions per image remains constant. The dynamic change of the aspect ratio of the image regions implemented in the motion estimation device of the invention reduces the impact of the borders between neighboring image regions and thus improves the quality of region-based motion estimation.