Resolution Scaling

~10 min · scale, resolution, aspect-ratio

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The scale filter

Resolution changes go through -vf scale=W:H. Width and height accept literal pixels (1280:720), the special value -1 (preserve aspect ratio, must be divisible by 2), or expressions (iw/2:ih/2).

Aspect-ratio safe scaling

Hard-coding both dimensions can squish video. Two patterns to remember:

scale=1280:-2 — width is 1280, height auto-computed and rounded to even (H.264/H.265 require even dimensions).
scale=-2:720 — height is 720, width auto.
scale=iw/2:ih/2 — half size, exact ratio preserved.

Upscaling vs downscaling

Downscaling is lossless in perceptual terms — you're throwing pixels away. Upscaling is lossy in perceptual terms — you're inventing pixels. scale uses bicubic by default, which is fine for downscaling and acceptable for moderate upscaling. For aggressive upscaling (480p → 4K) consider AI-based tools (Topaz, esrgan) or the realesrgan_ncnn filter on Linux.

Code

Common scale recipes·bash

# To 720p, preserve aspect
ffmpeg -i in.mp4 -vf scale=1280:-2 -c:v libx264 -crf 22 -c:a copy out_720.mp4

# To 1080p
ffmpeg -i in.mp4 -vf scale=1920:-2 -c:v libx264 -crf 20 -c:a copy out_1080.mp4

# Half size
ffmpeg -i in.mp4 -vf scale=iw/2:ih/2 -c:v libx264 -crf 23 -c:a copy out_half.mp4

# Force exact dimensions (will distort if aspect ratio differs)
ffmpeg -i in.mp4 -vf scale=1024:768 -c:v libx264 -crf 23 -c:a copy out_squish.mp4

Better scaling algorithm — Lanczos·bash

# For downscale work where every pixel matters
ffmpeg -i in.mp4 \
  -vf scale=1280:-2:flags=lanczos \
  -c:v libx264 -crf 20 -c:a copy out_lanczos.mp4

# Lanczos is a sharper sinc-based filter than bicubic. Sharper = good
# for content with text/lines, slightly more aliasing for soft footage.

External links

Exercise

Take a 4K source. Downscale to 1080p two ways: default bicubic (scale=1920:-2) and Lanczos (scale=1920:-2:flags=lanczos). Look at high-frequency areas (text, foliage). Can you see the sharpness difference at 100% pixel-zoom? Save your two outputs and the source for the rest of the track.

Progress

Progress is local-only — sign in to sync across devices.

← PreviousQuality Control — CRF, Bitrate, and the Tradeoff Next →Frame Rate — Changing FPS Without Breaking Sync

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