How conversion works · Concept
How Does File Conversion Work?
Under the hood, conversion is decode → (optional transform) → encode, or a lighter remux when you only change the wrapper. Understanding that pipeline explains quality, speed, and file-size changes.
Quick take: Converters decode the source into an intermediate representation, then encode that into the target format — or remux compressed packets when codecs already match.
1. Identify the source
The tool inspects bytes (and sometimes the extension) to choose a decoder: JPEG decoder, HEVC stills for HEIC, H.264 demuxer for MP4, and so on.
2. Decode or demux
Decoding expands compressed media into raw samples/frames/text. Demuxing separates tracks inside a container without necessarily decompressing them.
3. Transform (optional)
Resize, rotate, strip metadata, change color space, resample audio — anything that is not just “same data, new wrapper.”
4. Encode or remux into the target
An encoder writes a new format (JPG, WebP, AAC…). A remuxer can copy compressed packets into MP4/MKV when codecs are already compatible.
In the browser
Format Convertly performs these steps in WASM/JS on your device. You download the result; the intermediate data never needs a remote upload.
Key points
- Decode/encode is the heavy path; remux is lighter.
- Quality settings mainly affect the encode step.
- Wrong target format (not wrong tool) causes many “failed” conversions.
Examples
PNG → WebP
Decode PNG pixels, encode WebP (lossy or lossless).
MKV → MP4 (compatible codecs)
Often remux H.264/AAC into an MP4 container.
Try it in a converter
Concepts stick better with a real file — processing stays in your browser.
Related concepts
FAQ
Why is some conversion slow?
Re-encoding video/images (especially AVIF/HEVC) is CPU-heavy. Remuxes are usually much faster.
Browse all conversion concepts or jump to converters.