You generated a 16:9 clip in Runway and exported a 9:16 vertical version for TikTok or Reels. The platform’s auto-crop took the center column and now the subject’s head is cropped off, or two people in the wide shot are now framed half-off-screen. The video is unusable for the target platform without manual re-framing. Fix it by generating natively at the target ratio when possible, padding with smart bars when not, or using Premiere’s Auto Reframe to track the subject.
Common causes
Ordered by hit rate.
1. Generated at one aspect, delivering to another
Runway Gen-3 native is 16:9 at 1280x768. TikTok wants 9:16 at 1080x1920. Cropping a 16:9 frame to 9:16 keeps only the center 28 percent of the width, which usually clips the subject if the subject was anywhere off-center.
How to spot it: Source aspect ratio is different from delivery aspect ratio.
2. Tool’s vertical mode produces lower quality
Some tools (early Runway, Sora) generate vertical clips at lower internal resolution. Composition is OK but detail is worse than the horizontal equivalent.
How to spot it: Generate the same prompt in both ratios. If vertical looks visibly softer, the tool’s vertical mode is undercooked.
3. Center-crop assumption when subject is off-center
Composition put the subject on the rule-of-thirds, but auto-crop assumes center. The subject ends up at the edge or fully cropped.
How to spot it: Pause the source clip; if subject’s eyes are not on the vertical center line, center-crop will misframe.
4. Letterbox bars baked into the source
Source clip has black bars top and bottom from a previous step. Re-crop ignores the bars and frames partially on black.
How to spot it: Inspect the source clip; if there are visible bars, they need to be cropped out before re-frame.
5. Animated subject moves out of center during the clip
Subject starts centered then walks left. Static center-crop captures the empty right side at the end of the clip.
How to spot it: Watch the source; if subject moves significantly, no static crop will work.
6. Wrong aspect set in delivery tool
Premiere sequence is 1920x1080 but you exported to 1080x1920 — the sequence math is wrong and the export comes out stretched or with bars.
How to spot it: Check sequence settings match delivery target.
Before you start
- Save the original generated clip at full quality.
- Identify the source aspect and the target aspect.
- Determine if subject is centered, off-center, or moves during the clip.
- Decide whether to regenerate at target aspect or to re-frame the existing clip.
- Note the target platform’s exact resolution (TikTok 1080x1920, Reels 1080x1920, YouTube Shorts 1080x1920, Square 1080x1080).
Information to collect
- Source clip resolution, fps, and aspect.
- Target platform resolution and aspect.
- Subject position (centered, left third, right third, moving).
- Whether the source has letterbox bars.
- The delivery tool (CapCut, Premiere, Resolve, FCPX, native platform editor).
Step-by-step fix
Step 1: Generate natively at the target aspect when possible
Tool support for vertical generation:
# Runway Gen-3 Alpha
- Supports 9:16, 16:9, 1:1
- Pick at generation time, do not crop later
# Pika 2.0
- Supports 9:16, 16:9, 5:4, 1:1
- Aspect ratio dropdown in generation panel
# Kling 1.6
- Supports 9:16, 16:9, 1:1, 4:3
- Native generation at all standard ratios
# Sora
- Tier-dependent; check current support
- Letterbox fallback at unsupported ratiosNative generation at target ratio always beats post-crop.
Step 2: Compose for the most restrictive aspect first
If you must deliver in multiple ratios:
- Generate 9:16 first (most restrictive vertical)
- Subject in the top two-thirds, never the bottom
- Center the face vertically at 40 percent from top
- Generate 16:9 separately with wider composition
- Do not assume a single generation will work in bothStep 3: Use Premiere Auto Reframe
# Premiere Pro
- Drag clip to a new sequence at target aspect (e.g., 1080x1920)
- Right-click clip -> Auto Reframe
- Motion preset: Slower Motion (locked), Default, or Faster Motion
- For talking head: Slower Motion
- For walking subject: Default
# DaVinci Resolve
- Inspector -> Smart Reframe (Studio version only)
- Object detection -> Subject
- Match Aspect RatioAuto Reframe keypoints follow the detected subject; it beats static center-crop in 90 percent of cases.
Step 4: Add smart bars when re-frame is not possible
For two people side-by-side in a 16:9 needing 9:16 delivery:
# Option A: Blurred bars
- Resize to fill width at 9:16
- Background: same clip, scaled to fill height, blurred 50 px
- Foreground: original at full width, vertically centered
# Option B: Solid bars
- Less polished but cleaner for product / text content
- Top and bottom bars in brand color
# Option C: Hero crop
- Pick the most important person, center on them
- Other subject is sacrificedStep 5: Re-frame manually with keyframes
For moving subjects:
# Premiere
- Place clip in target-aspect sequence
- Effect Controls -> Position
- At t=0, position so subject is centered
- At every shot where subject moves > 10 percent of frame width, add a keyframe
- Use Bezier easing between keyframes10 to 20 keyframes for a 10-second clip is normal.
Step 6: Crop letterbox bars first if present
# ffmpeg crop black bars
ffmpeg -i input.mp4 -vf "crop=in_w:in_h-2*60:0:60" -c:a copy cropped.mp4Where 60 is the bar height in pixels. Use ffprobe and cropdetect to find the right value:
ffmpeg -i input.mp4 -vf "cropdetect=24:16:0" -t 5 -f null -The output suggests a crop value.
Step 7: Verify on the actual target platform
- Upload to TikTok / Reels / Shorts as a draft
- Preview in the platform's editor
- Check that subject is fully visible and text-safe zones are clear
- TikTok safe zone: top 250 px (caption), bottom 350 px (UI)
- Reels safe zone: bottom 250 px (UI)Verify
- Open delivery file at native resolution; subject should be fully visible.
- Play through; subject should not drift toward an edge.
- Test on the actual target platform before finalizing.
- Three different reviewers should not flag composition issues.
Long-term prevention
- Decide target aspect before generating, not after.
- Build a project-level “vertical first” policy if delivery is mostly vertical.
- Standardize subject positioning: face at 40 percent from top for vertical, rule-of-thirds for horizontal.
- Always generate at native aspect ratio when the tool supports it.
- Keep a Premiere Auto Reframe preset library for common conversions.
Common pitfalls
- Center-cropping a rule-of-thirds composition.
- Assuming the platform’s auto-crop is smart enough to track the subject.
- Forgetting safe zones for captions and UI on TikTok / Reels.
- Re-encoding through multiple aspect changes; each step loses quality.
FAQ
Can I just regenerate at the new ratio? Yes, and that is usually the best option. Generation at native aspect always beats post-crop.
Why is Premiere Auto Reframe not perfect? It uses motion tracking; if subject leaves frame or there are two equal subjects, it picks one and may switch mid-clip.
Is blurred-bar fill considered low quality? TikTok and Reels viewers tolerate it for repurposed content; native vertical is still preferred for new content.