Module sniff 

Sniffs containers for animation.

A picture names its own format in its first bytes and carries its frame sequence, if it has one, at a place each format fixes. Reading those is what lets a request for a still be honored against a file whose declared content type says otherwise, or says nothing useful, as image/png does for an APNG.

A walk answers one of three states, because the two questions asked of it take opposite defaults. Whether a picture may be served fails closed, so an unreadable one is withheld and a wrong answer costs a needless re-encode rather than a violation; what a picture is names nothing the walk did not settle, since a guess there would be written down as fact.

Enums

Sequence 🔒

What a container’s header settles about a frame sequence.

Constants

GIF_COLOR_TABLE 🔒

GIF_EXTENSION 🔒

GIF_HEADER_LEN 🔒

Bytes the signature and the logical screen descriptor occupy together.

GIF_IMAGE 🔒

GIF_MAGIC 🔒

GIF_TABLE_SIZE 🔒

GIF_TRAILER 🔒

PNG_ANIMATION 🔒

PNG_DATA 🔒

PNG_MAGIC 🔒

RIFF_MAGIC 🔒

WEBP_ANIMATION 🔒

WEBP_EXTENDED 🔒

WEBP_LOSSLESS 🔒

WEBP_LOSSY 🔒

WEBP_MAGIC 🔒

Functions

animated_type 🔒

The container type a picture’s own bytes name.

animates 🔒

Whether these bytes may carry more than one frame.

color_table_len 🔒

Size in bytes of the colour table a descriptor announces.

gif_sequence 🔒

Whether a GIF holds more than one image descriptor.

image_data_start 🔒

Offset of an image descriptor’s sub-block chain.

png_sequence 🔒

Whether a PNG holds the control chunk that makes it an APNG.

sequence 🔒

Walks the container its first bytes name.

skip_sub_blocks 🔒

Offset past the sub-block chain starting here, when it ends.

webp_sequence 🔒

Whether a WebP announces animation in its opening chunk.