# Image

`Image` is an image processing library for Elixir. It is based upon the fabulous [vix]( library that provides a [libvips]( wrapper for Elixir.

`Image` is intended to provide well-documented common image processing functions in an idiomatic Elixir functional style as a layer above the very comprehensive set of functions in `Vix` and `libvips`.

In a very simple image resizing [benchmark](, `Image` is approximately 2 to 3 times faster than `Mogrify` and uses about 5 times less memory.

## Installation

`Image` can be installed by adding `image` to your list of dependencies in `mix.exs`:

def deps do
    {:image, "~> 0.1.0"}

The documentation can be found at [](

### Installing Dependencies

Installing `Vix` requires `libvips` with development headers. Installation is platform dependent however the common platform installations are:

* macOS: Install using [homebrew]( with `brew install libvips`
* Linux: Install with `apt install libvips-dev`

For more details see

In addition the following will be required (and would normally be installed by the steps above):

* [pkg-config](
* A `C` compiler

### Configurration

`Vix` and `libvips` offer various configuration parameters that affect debug output, image caching, concurrency of imaging operations and memory leak detection. Each of these options has reasonable defaults so no action is required in order to start using the library. 

#### Vix NIF Error Logging

`Vix` NIF code writes logs to stderr on certain errors. This is disabled by default. To enable logging set the `VIX_LOG_ERROR` environment variable to `true`.

#### GLib Debug Output

The platform upon which `Image` and `Vix` stand is [libvips](, a `C` library that performs the image manipulation. Its `libvips` that delivers the speed, memory efficiency and functionality.

`libvips` uses the [GLib]( library which has configurable debug output. This output depends on the setting of the environment variable `G_DEBUG`.  The initial value will depend on the installation method of `libvips` for a given system. It can be changed by setting the `G_DEBUG` environment variable to one of the following:

* **fatal-warnings** which causes GLib to abort the operation at the first call to g_warning() or g_critical().

* **fatal-criticals** causes GLib to abort the operation at the first call to g_critical().

* **gc-friendly** causes newly allocated memory that isn't directly initialized, as well as memory being freed to be reset to 0. The point here is to allow memory checkers and similar programs that use Boehm GC alike algorithms to produce more accurate results.

* **resident-modules** causes all modules loaded by GModule will be made resident. This can be useful for tracking memory leaks in modules which are later unloaded; but it can also hide bugs where code is accessed after the module would have normally been unloaded.

* **bind-now-modules** causes all modules loaded by GModule to bind their symbols at load time, even when the code uses %G_MODULE_BIND_LAZY.

To produce debug output for only the most critical issues, set `G_DEBUG` as follows (in bash and compatible shells):

export G_DEBUG=fatal-criticals

#### Memory Leak Detection

The environment variable `VIPS_LEAK` determines whether `libvips` reports possible memory leaks. To enable leak detection (on `bash` compatible systems):
export VIPS_LEAK=true

To stop leak detection:

#### Concurrency

`Image` (because of `Vix` and `libvips`) will execute concurrent image operations using a number of system native threads (not BEAM processes). The number of threads available for concurrent image operations is configurable by either setting the environment variable `VIPS_CONCURRENCY` or through a call to `Image.put_concurrency/1`.  The current number of configured threads is returned from `Image.get_concurrency/0`.

The default number of threads is equal to the number of cores detected on the running system. This may create CPU contention with other workloads given that image processing is CPU intensive.  Therefore it may be prudent to reduce the number of threads if overall system throughput is being affected.

### Security Cosiderations

There are several considerations in the use of any image processing library and any NIF-based library:

1. If a NIF crashes it will likely bring down the BEAM virtual machine. `libvips` is a robust, time-tested library however this risk cannot be eliminated.

2. Image processing is CPU intensive with its concurrent pipeling model and default concurrency level equal to the number of cores in the host machine, CPU starvation for other parts of the application is a possibility. In such cases, reducing the `libvips` concurrency is recommended.

3. Image processing by its nature operates on external data and there have been exploits based upon maliciously crafted images. The two primary vectors are:
    * An invalid image format that causes the image parser to crash and therefore crash the NIF and the BEAM
    * Executable code embedded in image metadata (such as EXIF data) that if passed un-escaped to a web browser may result in arbitraty code execution.
In comparison to `Imagemagick` that has a reported [638]( CVEs, there have been only  [8]( CVE's reported for `libvips`, each resolved in a very timely manner.