Learn FFmpeg - Basic Filters

#Introduction

After mastering video encoding in episode 4 — codec, -crf, bitrate, and GOP — this episode adds one new dimension: filters. If encoding determines how the video is compressed, filters determine what the video looks like before it's compressed.

Remember the pipeline from episode 2: Demux -> Decode -> Filter -> Encode -> Mux. Filters live in the middle, after the video is decoded into raw frames and before it's encoded again. That's where you can resize, crop, rotate, or change speed — all before encoding.

#What Is a Video Filter

A filter is a single-frame processor: it receives one frame, processes it, and produces one output frame. libavfilter — the library we met in episode 2 — is the engine behind it.

Think of a studio photographer touching each photo one by one before printing: resize, crop, rotate, adjust lighting. FFmpeg does that for thousands of frames per second with a single command.

#Filter Syntax: Parameters and Options

A filter is written in the form filter_name=options. Options can be:

Form	Example	Meaning
Single value	`scale=640`	Set the first parameter only
Two values	`scale=640:360`	Parameters separated by colon
Name=value	`scale=w=640:h=360`	Options named explicitly
Expression	`rotate=45*PI/180`	Values can be expressions

Important

Be careful with two separators that are easy to confuse: colon (:) separates options within one filter, while comma (,) separates filters. Swapping them will make the filtergraph fail to parse.

Filters are attached to the -vf (video filter) option on the output side:

Filter paling sederhana

ffmpeg -i sample.mp4 -vf scale=640:360 out-640.mp4

This command changes all frames to 640x360 before encoding — regardless of the original resolution.

#Basic Filters You Must Master

#scale: Resizing

Scale dengan proporsi otomatis

ffmpeg -i sample.mp4 -vf scale=480:-1 out-480.mp4

-1 makes FFmpeg compute the other side so the aspect ratio is preserved: a width of 480 produces a height that follows the video's original ratio. This is far safer than forcing scale=480:360, which would distort the picture.

#crop: Cropping

Crop 640x360 dari tengah video

ffmpeg -i sample.mp4 -vf crop=640:360 out-crop.mp4

The format is crop=width:height:x:y. The x:y coordinates mark the top-left corner of the crop area; without coordinates, FFmpeg crops from the center. Cropping is like cutting a photo with scissors — pixels outside the area are discarded.

#rotate: Rotating

Putar 90 derajat searah jarum jam

ffmpeg -i sample.mp4 -vf rotate=PI/2 out-rotated.mp4

rotate accepts values in radians — PI/2 equals 90 degrees. The value is an expression that FFmpeg computes, so rotate=45*PI/180 will rotate exactly 45 degrees.

#fps: Setting the Frame Rate

Paksa 15 frame per detik

ffmpeg -i sample.mp4 -vf fps=15 out-fps15.mp4

The fps filter recomputes the frame rate: frames are duplicated or dropped to match the target. The difference from the -r option in episode 4: -vf fps affects frames before encoding, while -r works at the output stage.

#trim and -ss: Cutting Duration

There are two approaches to cutting duration. The quick way uses the input option -ss, the precise way uses the trim filter:

Potong 5 detik pertama

ffmpeg -ss 00:00:00 -t 5 -i sample.mp4 -c copy potong.mp4

Potong dengan filter presisi

ffmpeg -i sample.mp4 -vf trim=start=2:end=8,setpts=PTS-STARTPTS potong-presisi.mp4

Both change the duration, but in different ways. -ss before -i makes FFmpeg jump directly to second 0 then grab 5 seconds — fast, good for rough cuts. trim processes frames and needs setpts to reset the timestamps after cutting.

#setpts: Setting Timestamps

Mempercepat video 2 kali lipat

ffmpeg -i sample.mp4 -vf setpts=PTS/2,scale=640:-1 out-fast.mp4

setpts shifts the frame timestamps. PTS/2 speeds up by 2x (half timestamp = frames appear faster); PTS*2 slows down. This filter almost always appears paired with trim to clean up timestamps.

#Filter Chains: Building a Chain

Filters can be joined with commas — the first filter's output becomes the next filter's input:

Tiga filter dalam satu rantai

ffmpeg -i sample.mp4 -vf scale=480:-1,fps=15,setpts=PTS/2 out-chain.mp4

The order is read left to right: resize to 480p, force 15 fps, then speed up 2x. Order matters: resize first then process, so each filter works on frames at the expected size. A filter chain is like an assembly line — each station does one job before passing on.

Tip

Check what filters are available with ffmpeg -filters — the list has hundreds. For now, master the five filters above until they're second nature, because they're the foundation for the complex filtergraphs in later episodes.

#Conclusion

In episode 5, you've mastered the basic video filters: filter syntax with parameters and options, six important filters — scale, crop, rotate, fps, trim, and setpts — and building filter chains with -vf.

Key takeaways:

Filters are attached with -vf on the output side.
Colon separates options within a filter; comma separates filters.
scale for sizing, crop for cutting an area, rotate for rotating.
trim/-ss cut duration; setpts sets timestamps and speed.
Filter chains are read left to right; the order determines the final result.

In the next episode 6, we'll cover audio processing — audio filters like volume, fade, aecho, and amix, resampling, normalization, and how to swap and combine audio tracks. See you in episode 6!