Moving images present a unique set of scaling challenges. In NTSC TV, fields alternate every 16.6 ms. Any object that moves significantly between field-refresh times will appear distorted. If an image is scaled in the Y direction by assembling two fields into a single frame, the distortion is even more exaggerated. When the full frame is scaled down using decimation (line-dropping), a group of lines from one field can end up adjacent to a group of lines from another field, causing a jagged, stepped appearance in the scaled image. This distortion is often more noticeable than the distortion in the original TV image. Therefore, a general rule for scaling down is to use either the even or odd field from each frame. If the final image is to be less than one-half the size of the original, scale the single field down to the required size. If the final image is to be greater than one-half the size of the original, use one field, then increase image to the required number of lines with line replication.