A video that shows green or gray blocks when it plays on a Mac

Green or gray blocks, smeared patches that drag across the picture, or a frame that seems to melt into the next are what a player shows when it is decoding data it cannot make sense of. Either the file has damaged or missing data at that point, or the file is fine and the playback is the problem. A quick test tells you which, and whether a repair is worth trying.

Why damage looks like blocks and smears

Video does not store every frame as a complete picture. It stores an occasional full frame, called a keyframe, and then a run of frames that only describe what changed since the one before.

It also explains the look of the damage. Pictures are divided into square blocks, and each block is decoded from its own slice of data. When a slice is missing or wrong, the decoder fills that block with whatever it can. Data made of zeros turns into flat green in the color format video uses, and a neutral fill turns into gray. Every frame after that builds on the broken picture, so the error smears and drifts along with any movement until the next keyframe arrives and resets everything. That is why the glitch often clears on its own a second or a few seconds later, and why it looks worse in scenes with a lot of motion.

First, find out whether it is the file or the playback

1. Note the timestamp and play it again. If the blocks appear at exactly the same moment every time, the damage is in the file. If they come and go at different moments, suspect playback.
2. Try another way of viewing. Select the file in Finder and press Space for Quick Look, then open it in QuickTime Player, then drag it onto a Safari window.
3. Try another video. If every video on the Mac shows blocks, including ones that played cleanly last week, the files are not the problem. Restart the Mac, and check System Settings, General, Software Update if it persists.
4. Check the copy. If the video was downloaded or copied from somewhere, compare its size with the original. A file that is smaller than its source is missing data. Verifying a file arrived intact shows how to check properly rather than by eye.

Where the damage usually comes from

Damage inside the file usually comes from one of these:

• An interrupted copy or download. Part of the file was never written, or was written as zeros, which is the classic source of solid green.
• A failing memory card or drive. Worn storage can hand back wrong data without reporting an error, so the file copies without complaint and glitches on playback.
• A recording of a broadcast or live stream. Packets lost while recording leave holes in the data. Transport stream files are especially prone to this, and converting a TS file to MP4 covers what else is odd about them.
• A file brought back by recovery software. Carved files sometimes have another file’s data stitched into the middle. Recovered videos that will not play explains why.

What you can do about it

Work through these in order.

Get a fresh copy

The best repair is an undamaged original. Copy the clip off the camera card again, download it again, or ask the sender to resend it by a route that does not recompress video.

Repair the structure

Some glitches come from the file’s structure rather than from the picture data. A video file carries an index that says where each frame sits, and if that index points to the wrong places, the player decodes the wrong bytes and draws garbage. A structural repair fixes that kind of fault.

Manatee does this in Repair mode. Drag the video onto its menu bar icon or into its window, or right-click it in Finder and choose Services, Fix with Manatee. It rebuilds a playable file around the frames that survive and writes the result as a new copy beside the original, which it never modifies. Everything happens on your Mac, with no upload.

Be clear about the limits. The outcome is reported honestly as Fixed, Partial or Not fixable. If the blocks came from data that is simply not in the file, no repair can invent it. What a repair can do in that case is give you a file that plays past the damaged stretch, seeks properly and keeps the audio aligned, with the glitch still visible where the data was lost.

Convert, knowing what it does

Converting does not heal a damaged frame. A conversion decodes each frame, glitches included, and encodes the result into a new file, so the blocks become ordinary picture content. That still has a use: a player that stutters or freezes at the bad spot will usually play a converted copy straight through, and an app that refuses the original may accept the copy. A video that freezes partway while the audio keeps playing goes into that case.

Cut the damaged stretch out

If the damage is a few seconds in an otherwise good clip, remove it. Open the file in QuickTime Player, move the playhead to just before the glitch and choose Edit, Split Clip. Split again just after it, click the damaged segment and press Delete, then choose File, Export As. This needs QuickTime to open the file in the first place, so repair or convert first if it will not.

When it is the playback after all

If the file tested clean in step 1, the fault is in how the Mac is playing it:

• Quit and reopen the player, or restart the Mac.
• Try a different kind of copy. Very demanding video, such as high bitrate 4K in HEVC on an older Mac, is more likely to stutter than to turn green, but a converted H.264 copy at a lower resolution rules it out. H.264 vs HEVC explains which Macs decode what.
• Check the display path. If blocks also appear over the desktop or in apps, it is the screen, the cable or the graphics, and no file will fix that.

Questions

Will a repair remove the green blocks? Only if they come from a structural fault, such as an index pointing at the wrong data. If the picture data itself is missing, the repaired copy plays properly around the damage but the blocks stay where the data was lost.

Why does the glitch affect only part of the screen? Each block of the picture is decoded from its own slice of data, so a short stretch of damage spoils only the blocks it covers. Movement then carries the broken area across the frame until the next keyframe.

The clip played perfectly on my phone. Why does it glitch on the Mac? Usually because the copy on the Mac is not identical to the one on the phone. Compare the sizes, and send it again if they differ. iPhone video won’t play on Mac after AirDrop covers the transfer side.