Rotate a video without losing quality

An MP4 or MOV file can carry a display instruction alongside its compressed frames. It asks the player to turn the picture by a quarter or half turn as it shows it. Changing that instruction leaves the frames alone, so there is nothing to compress again and nothing to lose.

A re-encode works differently. Each frame is decoded, drawn again with the change applied, and compressed afresh. Video compression discards detail it expects you won't notice, so each new pass costs a little more, even at a generous bitrate.

The flag approach has one side effect worth knowing about. The frames are still stored in their original shape, so some tools report a turned clip as 1920 × 1080 with a rotation value attached, while players show it as 1080 × 1920.

The tool switches to redrawing frames for any of the following, and the note under the controls names the reason:

• Fine angles. A turn such as 3° moves pixels off the original grid, so new ones have to be calculated. See straightening a tilted video.
• Flips. This tool redraws every frame to mirror or flip, so flipping a video always means a re-encode.
• Crop and Fill. Both keep the original frame shape, so the turned picture has to be reframed inside it.
• Maximum compatibility. It bakes the turn into the frames for players that skip the flag. For an old TV or editor that is worth one re-encode.
• Trims, a smaller size, WebM, or High or Smaller file quality. Each of these changes the encoded video itself, so it can't simply be copied.

• Choose High under Quality. It raises the bitrate, which keeps more detail in exchange for a bigger file.
• Keep Size at the original resolution. Scaling down throws pixels away for good.
• Make every change in a single export. Rotating, trimming and flipping in three separate passes compresses the video three times over.
• Work from the original recording, not a copy that a messaging app or another editor has already squeezed.
• When a job needs one lossless step and one re-encoded step, do the lossless one first, as in the two-pass method for vertical video. That way the video is compressed only once.

• Read the note. Under the controls, the box says Lossless and instant when your settings allow a copy. Otherwise it says Re-encodes the video, followed by the reason.
• Compare file sizes. A flag change leaves the new file very close in size to the original. A re-encoded copy can come out noticeably larger or smaller.
• Notice the time. A copy finishes almost at once, even for a long clip. A re-encode takes longer the longer the video is.
• Look inside the file. MediaInfo shows a Rotation entry for the video track. ffprobe, which comes with ffmpeg, reports a display matrix with a rotation value, while the stored width and height stay as recorded.
• Compare a frame. Pause both files at the same moment and zoom in on fine detail such as hair, grass or small text. A copy matches exactly, whereas a re-encode may look a touch softer.