A 3D graphics rendering pipeline is used to carry out data comparisons for motion estimation in video data encoding. Video data for the pixel block of the video frame currently being encoded is loaded into the output buffers of the rendering pipeline. The video data for the comparison pixel blocks from the reference video frame is stored as texture map values in the texture cache of the rendering pipeline. Once the sets of pixel data for comparison have been stored, the rendering pipeline is controlled to render a primitive having fragment positions and texture coordinates corresponding to the data values that it is desired to compare. As each fragment is rendered, the stored and rendered fragment data is compared by fragment compare unit and the determined differences in the data values are accumulated in an error term register.