gif-encoder

Description

A minimal Clojure library for encoding gifs.

Notes

This is a low-level implementation of the GIF spec. It may be useful to those who would like to generate gifs from code or include the ability to render gifs from an application. It does not include any additional features, such as image format decoding, or drawing or compositing tools.

The Netcape Looping Application Extension is included.

The Plain Text Extension is not implemented.

Usage

See api.cljc for an example user-friendly interface built-on the low-level implementation, and examples.cljc for examples of creating gifs using that API.

Reference

To do

Optimize gifs.
Add options for handling color palettes.
Optimize code.

Implementation

The gif spec is encoded as a set of data structures interpreted as a schema. Several functions are provided which implement necessary algorithms, such as the LZW compression algorithm.

Default values are provided for some fields. No distinction is made between default values and fixed values. And no validation is performed. And nothing is done to prevent users from changing fixed values. Higher-level code should provide a better interface.

Several data representations are employed:

A declarative specification of the format as a schema. This includes the layout for the various components of the data stream with labels and types and default values.
An instantiation of a schema, which is essentially the same as the schema, but with all values provided. Values are something that can be interpreted to match the given type as determined in the field->pseudo-binary function. Usually values are an int, an instantiation, or a sequence of one of those.
A pseudo-binary representation: [bits int], where bits is the number of bits to extract from int.
The final binary output format.

Finally, anyone using this library will probably want to create a user-friendly layer that provides domain-specific data structures and functions, and then implement those by instantiating the schema and calling the functions convert the instantions to binary. See api.cljc for an example.

Schema Grammar

Entities are either primitive values (bit, byte, unsigned, and rgb) or one of the compound types defined in gif_encoder.clj (and in the official gif spec). Compound types are represented as vectors of fields, and fields are vectors like [label type default-value].

Type is either a single value or a vector like [type count]. If type is a single value, then count is assumed to be 1. Count can be an int or one of :?, :+, :* with the usual meanings.

Thanks

Thanks to Zach Beane for Skippy, which I referenced heavily while writing this library.

License

Distributed under the Eclipse Public License either version 1.0 or (at your option) any later version.

austinhaas/gif-encoder