Box Crop Through Black — effect skill for transitions

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A shrinks into a rectangle on black, B grows out of one. A boxy cousin of the iris.

What it does

The frame closes as a rectangle to black, then opens as a rectangle on the next shot.

When: Tech or UI-flavoured edits, framing a screen inside a screen.

Watch out: Leaving the black too long between the boxes.

Timing: 0.6–1 s.

Also called: Box in / box out, Rectangle mask

In FFmpeg

Both inputs need the same size, frame rate, timebase, SAR and pixel format (settb=AVTB fixes the timebase error that fps alone leaves). offset is where the transition starts on the first clip's timeline (its length minus the overlap).

ffmpeg -i a.mp4 -i b.mp4 -filter_complex \
 "[0:v]fps=30,scale=1920:1080,setsar=1,settb=AVTB,format=yuv420p[a];[1:v]fps=30,scale=1920:1080,setsar=1,settb=AVTB,format=yuv420p[b];\
  [a][b]xfade=transition=rectcrop:duration=0.5:offset=4.5[v];[0:a][1:a]acrossfade=d=0.5[a]" \
 -map "[v]" -map "[a]" out.mp4

In HyperFrames

Tween clipPath: inset(50% 50% 50% 50%) to inset(0) on B after the same move closes A.

In CapCut

Transitions → Mask → Rectangle.

Where this comes from

Use when

Not for

Install

Example output

https://s3.varg.ai/how-to/previews/2cce3deb076f/take-xfade-rect-crop.mp4 — rectcrop between two shots, with the timeline strip of the edit. Footage: WING IT! (2023), Blender Studio | studio.blender.org, CC BY 4.0; Sprite Fright (2021), Blender Studio | studio.blender.org, CC BY 4.0. (credit: Rendered by varg · WING IT! (2023) + Sprite Fright (2021))

More transitions skills