Letterboxing

What is letterboxing?

Letterboxing in digital video advertising is the black bars that appear above and below a video ad when the creative's aspect ratio is narrower than the placement's player container. A 16:9 landscape ad rendered inside a 9:16 mobile Stories player leaves large black strips at the top and bottom of the frame, shrinking the visible creative and weakening its stopping power in the feed.

Letterboxing can be caused two ways. The first is a runtime mismatch, where the media file is a standard aspect ratio but the player is something else. The second is "burned in" letterboxing, where the video file itself already contains black bars baked into the encoded pixels. Both reduce the usable creative surface and both look unprofessional on modern mobile and CTV inventory.

What is pillarboxing?

Pillarboxing is the same problem on the opposite axis. It is the black bars that appear on the left and right sides of a video ad when the creative's aspect ratio is wider than the player container. A 9:16 vertical ad rendered inside a 16:9 landscape player leaves vertical strips on either side of the frame.

Pillarboxing is most common on CTV and desktop inventory, where vertical-first social creatives get reused without a dedicated landscape cut. Like letterboxing, it can be a runtime mismatch or it can be burned into the media file itself.

Why letterboxing and pillarboxing hurt campaigns

Both consume less of the available screen than the format allows. On a vertical mobile player, a letterboxed 16:9 spot can leave 40 percent or more of the viewport as black bars. On a landscape CTV player, a pillarboxed 9:16 spot can waste even more. Less pixel real estate for the brand message, fewer readable captions, and weaker stopping power in the feed.

On connected-TV the problem is especially costly. CTV inventory is premium, screens are large, and viewers notice the difference between a native full-screen spot and a creative that looks like it was pulled from a social feed and padded with bars. This is why every major publisher, social platform, and the IAB publish placement-specific aspect ratio specifications that treat letterboxing as a delivery failure, not a stylistic choice.

IAB guidelines on letterboxing and pillarboxing

The IAB Digital Video In-Stream Ad Format Guidelines and the IAB Vertical Video Best Practices both address the issue directly:

• Creatives should be submitted without burned-in letterboxing or pillarboxing.
• Ad units should maintain their original aspect ratio and scale to fit the container, not be padded to fit it.
• Creative produced for a horizontal format should not be repurposed into vertical placements by adding bars, and vice versa. Produce a dedicated cut for each aspect ratio.
• Preferred aspect ratios are 16:9 for landscape, 1:1 for square, and 9:16 for vertical. Pick the one the inventory expects.

Buyers and sellers that enforce these guidelines at intake catch repurposed creative before it enters rotation.

How to detect letterboxing in a video file

Three ways to check whether a video file carries burned-in letterboxing or pillarboxing:

• Visual inspection: play the file at full screen and look for black strips. Fast for a spot check, impossible at volume.
• FFmpeg cropdetect (command line): `ffmpeg -i input.mp4 -vf cropdetect -f null -`. The output lists suggested crop values like `crop=1920:800:0:140`. If the suggested crop height is smaller than the source height, the file contains horizontal bars. If the suggested crop width is smaller than the source width, it contains vertical bars. See the FFmpeg cropdetect filter documentation for the full flag reference.
• Advalidation: automated pixel analysis as part of every video scan. Runs across every media file in a VAST tag and every direct upload. No command line required, no manual review.

Visual inspection works for one-off checks. FFmpeg works for engineering teams willing to script it. For ad spec validation at volume, across every creative and every VAST variation, automated scanning is the only approach that scales.

How to fix letterboxed video creative

Burned-in letterboxing cannot be removed cleanly after the fact. The bars are part of the encoded pixels. The fix is always to re-export from the master file at the target aspect ratio:

• Produce a dedicated cut for each target aspect ratio. A horizontal master reused in vertical placements always needs a vertical re-cut. A vertical master reused on CTV always needs a landscape re-cut. Do not pad at export.
• Recompose the frame, do not pad it. If the subject does not fit the target ratio, reposition, crop in on the action, or rebuild the shot, but do not add bars.
• Reject padded exports from creative agencies. When a creative is delivered with burned-in bars, send it back. Re-encoding a letterboxed file by cropping the bars reduces the effective resolution, so there is no clean in-platform fix.
• For legacy 4:3 assets, re-encode from the master into a 16:9 delivery frame with cropping or digital zoom, rather than letterboxing into the new container.

Common root causes worth watching for at intake: single-master workflows reused across placements, editing software exporting with the source aspect ratio instead of the target container aspect ratio, and stitched creative that keeps the aspect ratio of the background plate rather than the final delivery format.

How Advalidation detects letterboxing and pillarboxing

Advalidation analyzes each video media file during scanning and flags creatives where letterboxing or pillarboxing is present. The check catches burned-in black bars that are baked into the encoded video, which metadata-only checks would miss.

For VAST tags, the check applies to each media file in the tag, including the variations that rotate behind a single VAST response.

Enabling the check

Letterboxing detection is a single toggle in the ad specification editor. When it is on, Advalidation flags any creative where letterboxing or pillarboxing is detected. There are no thresholds or severity levels to configure. The check is on or off.

For strict CTV and premium mobile inventory, leave it on so every creative that ships with black bars is caught. For ad specs that expect occasional aspect ratio mismatches and handle them through visual QA, the test can be left off.

Frequently asked questions

Is letterboxing the same as pillarboxing?

No. They are the two axes of the same underlying aspect ratio mismatch. Letterboxing produces horizontal black bars at the top and bottom, caused by a wider source in a narrower container (for example 16:9 content in a 9:16 frame). Pillarboxing produces vertical black bars on the left and right, caused by a narrower source in a wider container (for example 9:16 content in a 16:9 frame). Advalidation detects and flags both.

What aspect ratios should my video ads use?

Match the ratio of the placement. 9:16 for full-screen vertical placements (Stories, Reels, TikTok), 1:1 for square feed placements, 16:9 for landscape (YouTube in-stream, CTV, desktop). Any other ratio forces the player to pad the frame with bars.

Can letterboxing be removed from a video without re-encoding?

Not cleanly. Burned-in bars are encoded into the pixels. Cropping a letterboxed file removes the bars but re-encodes the content at a lower effective resolution. The only lossless fix is re-exporting from the master file at the target aspect ratio.

Does letterboxing actually affect ad performance?

Yes. Letterboxed ads waste viewport real estate, reducing the usable creative surface on mobile and CTV placements. Brand messages read smaller, captions become harder to read, and the creative looks out of place next to native-ratio content. This is why every major publisher and platform publishes aspect ratio specifications per placement.

How does Advalidation detect burned-in letterboxing?

By analyzing the actual pixel content of each video media file during scanning. Files with persistent black bars at the edges of the frame are flagged. Metadata-only checks miss burned-in bars because the declared dimensions of the file do not reveal the padding.

Related video tests

• Test for video quality: overall quality scoring that flags visual issues at a glance
• H.264 profile: encoding profile validation on the underlying video file
• VAST required media files: confirm every required rendition ships for each aspect ratio target

See every check on the video tests page.

Start detecting black bars in your pipeline

Enable letterboxing detection in your ad specification and run any video or VAST tag through Advalidation to catch black bars before launch. Get in touch for a walk-through tailored to your inventory.