Dynamic Media

How URL-based image transformation works

Most dynamic media systems put the instructions in the image URL. A request has three parts:

Part	Example	Role
The asset	an ID or path	Which master to start from
The options	w=800,f=webp	Width, height, format, quality, crop mode
A signature or token	a hash at the end of the URL	Proves the URL was issued by the system and has not been edited

The server checks the signature, picks the closest stored version of the image, resizes and encodes it, and returns it with caching headers. A CDN in front of the server keeps the result, so the next visitor gets the cached copy without any processing.

Signatures also protect the server: without them, anyone can request thousands of variants (every width from 1 to 5,000 pixels, in every format), filling the cache and using processor time. Many systems also round widths to a fixed set of sizes for the same reason.

Format negotiation and responsive images

Two techniques let one image serve every device:

• Format negotiation. Browsers send an Accept header listing the image formats they support. The server returns AVIF to browsers that accept it, WebP to those that accept WebP, and JPEG to the rest, from the same URL. The response carries Vary: Accept so caches keep one copy per format. AVIF and WebP files are usually much smaller than JPEG at similar visual quality.
• Responsive images. The HTML srcset and sizes attributes list several widths of the same image, and the browser downloads the one that suits the screen size and pixel density. Dynamic media supplies those widths without anyone exporting them.

<img src="…/w=1024/…" srcset="…/w=512/… 512w, …/w=1024/… 1024w, …/w=2048/… 2048w"
     sizes="(max-width: 600px) 100vw, 50vw" alt="Ceramic vase, 23 cm">

Dynamic media in marketing work

• Websites and webshops. The web team links product images from the library instead of uploading copies to the CMS. A retouched packshot replaces the old one everywhere it is linked.
• Dealer and partner sites. Partners embed approved images by link, so an image withdrawn for legal reasons disappears from their pages too.
• Email and landing pages. The campaign team picks the width per template; nobody keeps a folder of "hero_600px_final" files.
• Page speed. A 40 MB TIFF never reaches a browser; a 120 KB AVIF at the displayed size does.

Static exports vs dynamic media

	Exported files	Dynamic media
Sizes	Made by hand for each use	Generated on request
Copies	One per CMS, site and channel	One master, many URLs
Updates	Re-export and re-upload everywhere	Replace the master once
Withdrawal	Find and delete every copy	Revoke the link or the publication
Rights	Unknown once the file is downloaded	Can be checked on every request

Common mistakes:

• Publishing the original file instead of a sized version.
• Letting long-lived public links outlive the image's licence. See usage rights.
• Using a different width for every page, so the CDN caches dozens of near-identical variants.
• Treating a crop by URL as art direction: a URL crop does not know where the subject is. See smart crop.

Dynamic media in Freedam

• Freedam keeps every image as renditions at fixed widths from 128 to 5,120 pixels, and serves resized versions from them through one image URL path. Images are never enlarged beyond their real size.
• In the app and through the API, image URLs accept a width and height, a quality, a device pixel ratio, a fit mode (contain, cover or fill) and a format: JPEG, PNG, WebP or AVIF. Without a format, the browser's Accept header decides: AVIF, then WebP, then JPEG. URLs returned by the API are signed, expire after one to two hours, and re-check the user's access, download rights and watermark on every request. See image transformations.
• Publication channels give websites and CMSs image links that do not expire. An administrator sets conditions, such as a status of Approved or membership of a collection, a use and territory, a largest size and an optional watermark. People copy the links from the asset's Publishing tab. See publishing.
• Published links take a width and a format. A width between two stored sizes is served the next size up, so a CDN keeps one copy per size and format. Responses can be cached for five minutes and loaded from any website.
• The image's rights policy is checked on every request. When an image leaves the channel's conditions, its rights expire or the channel is switched off, the link stops serving it. Replace links invalidates every link of a channel at once.
• The original file is never published. Videos are shared through embed codes.

Frequently asked questions

What is an image CDN?

An image CDN is a content delivery network that also transforms images: it resizes, converts and compresses them on request according to the URL, then caches each version on servers close to the visitor.

Is dynamic media the same as a DAM?

No. A DAM stores, describes and governs the master files. Dynamic media is a way of delivering them. Some DAMs include it; otherwise a DAM feeds a separate image server or CDN.

Does image transformation reduce quality?

Each request is encoded from a high-quality stored version, not from a previous transformation, so quality depends on the quality setting and format chosen. Enlarging an image beyond its real size does reduce quality, which is why many systems refuse to upscale.