Information on transformations made by Image Optimizer.

Fastly writes this header into responses. It is proprietary to Fastly.

This header is written by Fastly when a response is transformed by Image Optimizer. It provides detail on the input format (ifmt), dimensions (idim), and size in bytes (ifsz), and also the output format (ofmt), dimensions (odim), and size in bytes (ofsz).


The header's value is a space-delimited string of key/value pairs. These pairs can appear in any order.

ifsz=<input size in bytes>
idim=<input dimensions - width x height>
ifmt=<input format>
ofsz=<output size in bytes>
odim=<output dimensions - width x height>
ofmt=<output format>


ifsz=108501 idim=827x788 ifmt=jpeg ofsz=13066 odim=300x286 ofmt=jpeg