# [libimagequant](https://pngquant.org/lib/) — Image Quantization Library

Imagequant library converts RGBA images to palette-based 8-bit indexed images, *including* alpha component. It's ideal for generating tiny PNG images and [nice-looking GIFs](https://gif.ski).

Image encoding/decoding isn't handled by the library itself, bring your own encoder. If you're looking for a command-line tool, see [pngquant](https://pngquant.org).

## License

Libimagequant is dual-licensed:

* For Free/Libre Open Source Software it's available under GPL v3 or later with additional [copyright notices](https://raw.github.com/ImageOptim/libimagequant/master/COPYRIGHT) for historical reasons.
* For use in closed-source software, AppStore distribution, and other non-GPL uses, you can [obtain a commercial license](https://supso.org/projects/pngquant). Feel free to ask kornel@pngquant.org for details and custom licensing terms if you need them.


## Rust documentation

Add to `Cargo.toml`:

```toml
[dependencies]
imagequant = "4.0"
```

[See docs.rs for the library API documentation](https://docs.rs/imagequant).

## C documentation

### Building for C

1. Get Rust 1.70 or later via [rustup](https://rustup.rs) and run `rustup update`.
2. `cd imagequant-sys`

   The C API is exposed by a separate package called [`imagequant-sys`](https://github.com/ImageOptim/libimagequant/tree/main/imagequant-sys).
3. Run `cargo build --release`

   This will build `target/release/libimagequant_sys.a` or `target\release\libimagequant_sys.lib` that you can use for static linking.
   Please don't worry about the size of the `.a` file. It includes a few unused objects. It only adds 500KB when linked. Use Link-Time-Optimization (LTO) option in your compiler, and/or add `-Wl,--as-needed` to linker flags if necessary. You can make the binary a bit smaller by building with `cargo +nightly build --target $TARGET_ARCH --release -Z build-std=std,panic_abort -Z build-std-features=panic_immediate_abort`.

The repository includes an Xcode project file that can be used on iOS and macOS.

If you want to build a C dynamic library (DLL, so, dylib), then:

1. `cargo install cargo-c`
2. `cd imagequant-sys`
3. `cargo cinstall --destdir=.`
   This will build `./usr/local/lib/libimagequant.0.0.0.{so,dylib,dll}`

### C API usage

1. Create attributes object and configure the library.
2. Create image object from RGBA pixels or data source.
3. Perform quantization (generate palette).
4. Store remapped image and final palette.
5. Free memory.

Please note that libimagequant only handles raw uncompressed arrays of pixels in memory and is completely independent of any file format.

    /* See imagequant-sys/example.c for the full code! */

    #include "libimagequant.h"

    liq_attr *attr = liq_attr_create();
    liq_image *image = liq_image_create_rgba(attr, example_bitmap_rgba, width, height, 0);
    liq_result *res;
    liq_image_quantize(image, attr, &res);

    liq_write_remapped_image(res, image, example_bitmap_8bpp, example_bitmap_size);
    const liq_palette *pal = liq_get_palette(res);

    // Save the image and the palette now.
    for(int i=0; i < pal->count; i++) {
        example_copy_palette_entry(pal->entries[i]);
    }
    // You'll need a PNG library to write to a file.
    example_write_image(example_bitmap_8bpp);

    liq_result_destroy(res);
    liq_image_destroy(image);
    liq_attr_destroy(attr);

Functions returning `liq_error` return `LIQ_OK` (`0`) on success and non-zero on error.

It's safe to pass `NULL` to any function accepting `liq_attr`, `liq_image`, `liq_result` (in that case the error code `LIQ_INVALID_POINTER` will be returned). These objects can be reused multiple times.

There are 3 ways to create image object for quantization:

  * `liq_image_create_rgba()` for simple, contiguous RGBA pixel arrays (width×height×4 bytes large bitmap).
  * `liq_image_create_rgba_rows()` for non-contiguous RGBA pixel arrays (that have padding between rows or reverse order, e.g. BMP).
  * `liq_image_create_custom()` for RGB, ABGR, YUV and all other formats that can be converted on-the-fly to RGBA (you have to supply the conversion function).

Note that "image" here means raw uncompressed pixels. If you have a compressed image file, such as PNG, you must use another library (e.g. libpng or lodepng) to decode it first.

You'll find full example code in "example.c" file in the library source directory.

## C Functions

----

    liq_attr* liq_attr_create(void);

Returns object that will hold initial settings (attributes) for the library. The object should be freed using `liq_attr_destroy()` after it's no longer needed.

Returns `NULL` in the unlikely case that the library cannot run on the current machine (e.g. the library has been compiled for SSE-capable x86 CPU and run on VIA C3 CPU).

----

    liq_error liq_set_max_colors(liq_attr* attr, int colors);

Specifies maximum number of colors to use. The default is 256. Instead of setting a fixed limit it's better to use `liq_set_quality()`.

The first argument is attributes object from `liq_attr_create()`.

Returns `LIQ_VALUE_OUT_OF_RANGE` if number of colors is outside the range 2-256.

----

    int liq_get_max_colors(liq_attr* attr);

Returns the value set by `liq_set_max_colors()`.

----

    liq_error liq_set_quality(liq_attr* attr, int minimum, int maximum);

Quality is in range `0` (worst) to `100` (best) and values are analoguous to JPEG quality (i.e. `80` is usually good enough).

Quantization will attempt to use the lowest number of colors needed to achieve `maximum` quality. `maximum` value of `100` is the default and means conversion as good as possible.

If it's not possible to convert the image with at least `minimum` quality (i.e. 256 colors is not enough to meet the minimum quality), then `liq_image_quantize()` will fail. The default minimum is `0` (proceeds regardless of quality).

Quality measures how well the generated palette fits image given to `liq_image_quantize()`. If a different image is remapped with `liq_write_remapped_image()` then actual quality may be different.

Regardless of the quality settings the number of colors won't exceed the maximum (see `liq_set_max_colors()`).

The first argument is attributes object from `liq_attr_create()`.

Returns `LIQ_VALUE_OUT_OF_RANGE` if target is lower than minimum or any of them is outside the 0-100 range.
Returns `LIQ_INVALID_POINTER` if `attr` appears to be invalid.

    liq_attr *attr = liq_attr_create();
    liq_set_quality(attr, 50, 80); // use quality 80 if possible. Give up if quality drops below 50.

----

    int liq_get_min_quality(liq_attr* attr);

Returns the lower bound set by `liq_set_quality()`.

----

    int liq_get_max_quality(liq_attr* attr);

Returns the upper bound set by `liq_set_quality()`.

----

    liq_image *liq_image_create_rgba(liq_attr *attr, void* pixels, int width, int height, double gamma);

Creates an object that represents the image pixels to be used for quantization and remapping. The pixel array must be contiguous run of RGBA pixels (alpha is the last component, 0 = transparent, 255 = opaque).

The first argument is attributes object from `liq_attr_create()`. The same `attr` object should be used for the entire process, from creation of images to quantization.

The `pixels` array must not be modified or freed until this object is freed with `liq_image_destroy()`. See also `liq_image_set_memory_ownership()`.

`width` and `height` are dimensions in pixels. An image 10x10 pixel large will need a 400-byte array.

If the `gamma` argument is `0`, then the default of 1/2.2 [gamma](https://en.wikipedia.org/wiki/Gamma_correction) is assumed, which is good for most sRGB images.
Otherwise `gamma` must be > 0 and < 1, e.g. `0.45455` (1/2.2) or `0.55555` (1/1.8). Generated palette will use the same gamma unless `liq_set_output_gamma()` is used. If `liq_set_output_gamma` is not used, then it only affects whether brighter or darker areas of the image will get more palette colors allocated.

Returns `NULL` on failure, e.g. if `pixels` is `NULL` or `width`/`height` is <= 0.

----

    liq_image *liq_image_create_rgba_rows(liq_attr *attr, void* rows[], int width, int height, double gamma);

Same as `liq_image_create_rgba()`, but takes an array of pointers to rows of pixels. This allows defining images with reversed rows (like in BMP), "stride" different than width or using only fragment of a larger bitmap, etc.

The `rows` array must have at least `height` elements, and each row must be at least `width` RGBA pixels wide.

    unsigned char *bitmap = …;
    void *rows = malloc(height * sizeof(void*));
    int bytes_per_row = width * 4 + padding; // stride
    for(int i=0; i < height; i++) {
        rows[i] = bitmap + i * bytes_per_row;
    }
    liq_image *img = liq_image_create_rgba_rows(attr, rows, width, height, 0);
    // …
    liq_image_destroy(img);
    free(rows);

The row pointers and pixels must not be modified or freed until this object is freed with `liq_image_destroy()` (you can change that with `liq_image_set_memory_ownership()`).

See also `liq_image_create_rgba()` and `liq_image_create_custom()`.

----

    liq_error liq_image_quantize(liq_image *const input_image, liq_attr *const attr, liq_result **out_result);

Performs quantization (palette generation) based on settings in `attr` (from `liq_attr_create()`) and pixels of the image.

Returns `LIQ_OK` if quantization succeeds and sets `liq_result` pointer in `out_result`. The last argument is used for receiving the `result` object:

    liq_result *result;
    if (LIQ_OK == liq_image_quantize(img, attr, &result)) { // Note &result
        // result pointer is valid here
    }

Returns `LIQ_QUALITY_TOO_LOW` if quantization fails due to limit set in `liq_set_quality()`.

See `liq_write_remapped_image()`.

If you want to generate one palette for multiple images at once, see `liq_histogram_create()`.

----

    liq_error liq_set_dithering_level(liq_result *res, float dither_level);

Enables/disables dithering in `liq_write_remapped_image()`. Dithering level must be between `0` and `1` (inclusive). Dithering level `0` enables fast non-dithered remapping. Otherwise a variation of Floyd-Steinberg error diffusion is used.

Precision of the dithering algorithm depends on the speed setting, see `liq_set_speed()`.

Returns `LIQ_VALUE_OUT_OF_RANGE` if the dithering level is outside the 0-1 range.

----

    liq_error liq_write_remapped_image(liq_result *result, liq_image *input_image, void *buffer, size_t buffer_size);

Remaps the image to palette and writes its pixels to the given buffer, 1 pixel per byte.

The buffer must be large enough to fit the entire image, i.e. width×height bytes large. For safety, pass the size of the buffer as `buffer_size`.

For best performance call `liq_get_palette()` *after* this function, as palette is improved during remapping (except when `liq_histogram_quantize()` is used).

Returns `LIQ_BUFFER_TOO_SMALL` if given size of the buffer is not enough to fit the entire image.

    int buffer_size = width*height;
    char *buffer = malloc(buffer_size);
    if (LIQ_OK == liq_write_remapped_image(result, input_image, buffer, buffer_size)) {
        liq_palette *pal = liq_get_palette(result);
        // save image
    }

See `liq_get_palette()`.

The buffer is assumed to be contiguous, with rows ordered from top to bottom, and no gaps between rows. If you need to write rows with padding or upside-down order, then use `liq_write_remapped_image_rows()`.

Please note that it only writes raw uncompressed pixels to memory. It does not perform any PNG compression. If you'd like to create a PNG file then you need to pass the raw pixel data to another library, e.g. libpng or lodepng. See `rwpng.c` in `pngquant` project for an example how to do that.

----

    const liq_palette *liq_get_palette(liq_result *result);

Returns pointer to palette optimized for image that has been quantized or remapped (final refinements are applied to the palette during remapping).

It's valid to call this method before remapping, if you don't plan to remap any images or want to use same palette for multiple images.

`liq_palette->count` contains number of colors (up to 256), `liq_palette->entries[n]` contains RGBA value for nth palette color.

The palette is **temporary and read-only**. You must copy the palette elsewhere *before* calling `liq_result_destroy()`.

Returns `NULL` on error.

----

    void liq_attr_destroy(liq_attr *);
    void liq_image_destroy(liq_image *);
    void liq_result_destroy(liq_result *);
    void liq_histogram_destroy(liq_histogram *);

Releases memory owned by the given object. Object must not be used any more after it has been freed.

Freeing `liq_result` also frees any `liq_palette` obtained from it.

## Advanced Functions

----

    liq_error liq_set_speed(liq_attr* attr, int speed);

Higher speed levels disable expensive algorithms and reduce quantization precision. The default speed is `4`. Speed `1` gives marginally better quality at significant CPU cost. Speed `10` has usually 5% lower quality, but is 8 times faster than the default.

High speeds combined with `liq_set_quality()` will use more colors than necessary and will be less likely to meet minimum required quality.

<table><caption>Features dependent on speed</caption>
<tr><th>Noise-sensitive dithering</th><td>speed 1 to 5</td></tr>
<tr><th>Forced posterization</th><td>8-10 or if image has more than million colors</td></tr>
<tr><th>Quantization error known</th><td>1-7 or if minimum quality is set</td></tr>
<tr><th>Additional quantization techniques</th><td>1-6</td></tr>
</table>

Returns `LIQ_VALUE_OUT_OF_RANGE` if the speed is outside the 1-10 range.

----

    int liq_get_speed(liq_attr* attr);

Returns the value set by `liq_set_speed()`.

----

    liq_error liq_set_min_opacity(liq_attr* attr, int min);

This was a workaround for Internet Explorer 6, but because this browser is not used any more, this option has been deprecated and removed.

----

    int liq_get_min_opacity(liq_attr* attr);

This function has been deprecated.

----

    liq_set_min_posterization(liq_attr* attr, int bits);

Ignores given number of least significant bits in all channels, posterizing image to `2^bits` levels. `0` gives full quality. Use `2` for VGA or 16-bit RGB565 displays, `4` if image is going to be output on a RGB444/RGBA4444 display (e.g. low-quality textures on Android).

Returns `LIQ_VALUE_OUT_OF_RANGE` if the value is outside the 0-4 range.

----

    int liq_get_min_posterization(liq_attr* attr);

Returns the value set by `liq_set_min_posterization()`.

----

    liq_set_last_index_transparent(liq_attr* attr, int is_last);

`0` (default) makes alpha colors sorted before opaque colors. Non-`0` mixes colors together except completely transparent color, which is moved to the end of the palette. This is a workaround for programs that blindly assume the last palette entry is transparent.

----

    liq_image *liq_image_create_custom(liq_attr *attr, liq_image_get_rgba_row_callback *row_callback, void *user_info, int width, int height, double gamma);

<p>

    void image_get_rgba_row_callback(liq_color row_out[], int row_index, int width, void *user_info) {
        for(int column_index=0; column_index < width; column_index++) {
            row_out[column_index] = /* generate pixel at (row_index, column_index) */;
        }
    }

Creates image object that will use callback to read image data. This allows on-the-fly conversion of images that are not in the RGBA color space.

`user_info` value will be passed to the callback. It may be useful for storing pointer to program's internal representation of the image.

The callback must read/generate `row_index`-th row and write its RGBA pixels to the `row_out` array. Row `width` is given for convenience and will always equal to image width.

The callback will be called multiple times for each row. Quantization and remapping require at least two full passes over image data, so caching of callback's work makes no sense — in such case it's better to convert entire image and use `liq_image_create_rgba()` instead.

To use RGB image:

    void rgb_to_rgba_callback(liq_color row_out[], int row_index, int width, void *user_info) {
        unsigned char *rgb_row = ((unsigned char *)user_info) + 3*width*row_index;

        for(int i=0; i < width; i++) {
            row_out[i].r = rgb_row[i*3];
            row_out[i].g = rgb_row[i*3+1];
            row_out[i].b = rgb_row[i*3+2];
            row_out[i].a = 255;
        }
    }
    liq_image *img = liq_image_create_custom(attr, rgb_to_rgba_callback, rgb_bitmap, width, height, 0);

The library doesn't support RGB bitmaps "natively", because supporting only single format allows compiler to inline more code, 4-byte pixel alignment is faster, and SSE instructions operate on 4 values at once, so alpha support is almost free.

----

    liq_error liq_image_set_memory_ownership(liq_image *image, int ownership_flags);

Passes ownership of image pixel data and/or its rows array to the `liq_image` object, so you don't have to free it yourself. Memory owned by the object will be freed at its discretion with `free` function specified in `liq_attr_create_with_allocator()` (by default it's stdlib's `free()`).

* `LIQ_OWN_PIXELS` makes pixel array owned by the object. The pixels will be freed automatically at any point when it's no longer needed. If you set this flag you must **not** free the pixel array yourself. If the image has been created with `liq_image_create_rgba_rows()` then the starting address of the array of pixels is assumed to be the lowest address of any row.

* `LIQ_OWN_ROWS` makes array of row pointers (but not the pixels pointed by these rows) owned by the object. Rows will be freed when object is deallocated. If you set this flag you must **not** free the rows array yourself. This flag is valid only if the object has been created with `liq_image_create_rgba_rows()`.

These flags can be combined with binary *or*, i.e. `LIQ_OWN_PIXELS | LIQ_OWN_ROWS`.

This function must not be used if the image has been created with `liq_image_create_custom()`.

Returns `LIQ_VALUE_OUT_OF_RANGE` if invalid flags are specified or the image object only takes pixels from a callback.

----

    liq_error liq_image_set_background(liq_image *image, liq_image *background_image);

Analyze and remap this image with assumption that it will be always presented exactly on top of this background.

When this image is remapped to a palette with a fully transparent color (use `liq_image_add_fixed_color()` to ensure this) pixels that are better represented by the background than the palette will be made transparent. This function can be used to improve quality of animated GIFs by setting previous animation frame as the background.

This function takes full ownership of the background image, so you should **not** free the background object. It will be freed automatically together with the foreground image.

Returns `LIQ_BUFFER_TOO_SMALL` if the background image has a different size than the foreground.

----

    liq_error liq_image_set_importance_map(liq_image *image, unsigned char map[], size_t buffer_size, liq_ownership ownership);

Impotance map controls which areas of the image get more palette colors. Pixels corresponding to 0 values in the map are completely ignored. The higher the value the more weight is placed on the given pixel, giving it higher chance of influencing the final palette.

The map is one byte per pixel and must have the same size as the image (width×height bytes). `buffer_size` argument is used to double-check that.

If the `ownership` is `LIQ_COPY_PIXELS` then the `map` content be copied immediately (it's up to you to ensure the `map` memory is freed).

If the `ownership` is `LIQ_OWN_PIXELS` then the `map` memory will be owned by the image and will be freed automatically when the image is freed. If a custom allocator has been set using `liq_attr_create_with_allocator()`, the `map` must be allocated using the same allocator. This option is deprecated. Use the Rust API or `LIQ_COPY_PIXELS` instead.

Returns `LIQ_INVALID_POINTER` if any pointer is `NULL`, `LIQ_BUFFER_TOO_SMALL` if the `buffer_size` does not match the image size, and `LIQ_UNSUPPORTED` if `ownership` isn't a valid value.

----

    liq_error liq_write_remapped_image_rows(liq_result *result, liq_image *input_image, unsigned char **row_pointers);

Similar to `liq_write_remapped_image()`. Writes remapped image, at 1 byte per pixel, to each row pointed by `row_pointers` array. The array must have at least as many elements as height of the image, and each row must have at least as many bytes as width of the image. Rows must not overlap.

For best performance call `liq_get_palette()` *after* this function, as remapping may change the palette (except when `liq_histogram_quantize()` is used).

Returns `LIQ_INVALID_POINTER` if `result` or `input_image` is `NULL`.

----

    double liq_get_quantization_error(liq_result *result);

Returns mean square error of quantization (square of difference between pixel values in the source image and its remapped version). Alpha channel, gamma correction and approximate importance of pixels is taken into account, so the result isn't exactly the mean square error of all channels.

For most images MSE 1-5 is excellent. 7-10 is OK. 20-30 will have noticeable errors. 100 is awful.

This function may return `-1` if the value is not available (this happens when a high speed has been requested, the image hasn't been remapped yet, and quality limit hasn't been set, see `liq_set_speed()` and `liq_set_quality()`). The value is not updated when multiple images are remapped, it applies only to the image used in `liq_image_quantize()` or the first image that has been remapped. See `liq_get_remapping_error()`.

----

    double liq_get_remapping_error(liq_result *result);

Returns mean square error of last remapping done (square of difference between pixel values in the remapped image and its remapped version). Alpha channel and gamma correction are taken into account, so the result isn't exactly the mean square error of all channels.

This function may return `-1` if the value is not available (this happens when a high speed has been requested or the image hasn't been remapped yet).

----

    double liq_get_quantization_quality(liq_result *result);

Analoguous to `liq_get_quantization_error()`, but returns quantization error as quality value in the same 0-100 range that is used by `liq_set_quality()`.

It may return `-1` if the value is not available (see note in `liq_get_quantization_error()`).

This function can be used to add upper limit to quality options presented to the user, e.g.

    liq_attr *attr = liq_attr_create();
    liq_image *img = liq_image_create_rgba(…);
    liq_result *res;
    liq_image_quantize(img, attr, &res);
    int max_attainable_quality = liq_get_quantization_quality(res);
    printf("Please select quality between 0 and %d: ", max_attainable_quality);
    int user_selected_quality = prompt();
    if (user_selected_quality < max_attainable_quality) {
        liq_set_quality(user_selected_quality, 0);
        liq_result_destroy(res);
        liq_image_quantize(img, attr, &res);
    }
    liq_write_remapped_image(…);

----

    double liq_get_remapping_quality(liq_result *result);

Analoguous to `liq_get_remapping_error()`, but returns quantization error as quality value in the same 0-100 range that is used by `liq_set_quality()`.

----

    void liq_set_log_callback(liq_attr*, liq_log_callback_function*, void *user_info);

<p>

    void log_callback_function(const liq_attr*, const char *message, void *user_info) {}

----

    void liq_set_log_flush_callback(liq_attr*, liq_log_flush_callback_function*, void *user_info);
<p>

    void log_flush_callback_function(const liq_attr*, void *user_info) {}

Sets up callback function to be called when the library reports status or errors. The callback must not call any library functions.

`user_info` value will be passed through to the callback. It can be `NULL`.

`NULL` callback clears the current callback.

In the log callback the `message` is a zero-terminated string containing informative message to output. It is valid only until the callback returns, so you must copy it.

`liq_set_log_flush_callback()` sets up callback function that will be called after the last log callback, which can be used to flush buffers and free resources used by the log callback.

----

    void liq_set_progress_callback(liq_attr*, liq_progress_callback_function*, void *user_info);
    void liq_result_set_progress_callback(liq_result*, liq_progress_callback_function*, void *user_info);

<p>

    int progress_callback_function(const liq_attr*, float progress_percent, void *user_info) {}

Sets up callback function to be called while the library is processing images. The callback may abort processing by returning `0`.

Setting callback to `NULL` clears the current callback. `liq_set_progress_callback` is for quantization progress, and `liq_result_set_progress_callback` is for remapping progress (currently only dithered remapping reports progress).

`user_info` value will be passed through to the callback. It can be `NULL`.

The callback must not call any library functions.

`progress_percent` is a value between 0 and 100 that estimates how much of the current task has been done.

The callback should return `1` to continue the operation, and `0` to abort current operation.

----

    liq_attr* liq_attr_create_with_allocator(void* (*malloc)(size_t), void (*free)(void*));

This function is deprecated. Same as `liq_attr_create`, but specifies `free` to use for `liq_image_set_memory_ownership`. The `malloc` argument is not used.

The library will use Rust's [global allocator](https://doc.rust-lang.org/std/alloc/index.html).

----

    liq_attr* liq_attr_copy(liq_attr *orig);

Creates an independent copy of `liq_attr`. The copy should also be freed using `liq_attr_destroy()`.

---

    liq_error liq_set_output_gamma(liq_result* res, double gamma);

Sets gamma correction for generated palette and remapped image. Must be > 0 and < 1, e.g. `0.45455` for gamma 1/2.2 in PNG images. By default output gamma is same as gamma of the input image.

----

    int liq_image_get_width(const liq_image *img);
    int liq_image_get_height(const liq_image *img);
    double liq_get_output_gamma(const liq_result *result);

Getters for `width`, `height` and `gamma` of the input image.

If the input is invalid, these all return -1.

---

    liq_error liq_image_add_fixed_color(liq_image* img, liq_color color);
    liq_error liq_histogram_add_fixed_color(liq_histogram* hist, liq_color color, double gamma);

Reserves a color in the output palette created from this image. It behaves as if the given color was used in the image and was very important.

RGB values of `liq_color` are assumed to have the same gamma as the image. For the histogram function, the `gamma` can be `0` (see `liq_image_create_rgba()`).

It must be called before the image is quantized.

Returns error if more than 256 colors are added. If image is quantized to fewer colors than the number of fixed colors added, then excess fixed colors will be ignored.

For histograms see also a more flexible `liq_histogram_add_colors()`.

---

    int liq_version();

Returns version of the library as an integer. Same as `LIQ_VERSION`. Human-readable version is defined as `LIQ_VERSION_STRING`.

## Multiple images with the same palette

It's possible to efficiently generate a single palette that is optimal for multiple images, e.g. for an APNG animation. This is done by collecting statistics of images in a `liq_histogram` object.

    liq_attr *attr = liq_attr_create();
    liq_histogram *hist = liq_histogram_create(attr);

    liq_image *image1 = liq_image_create_rgba(attr, example_bitmap_rgba1, width, height, 0);
    liq_histogram_add_image(hist, attr, image1);

    liq_image *image2 = liq_image_create_rgba(attr, example_bitmap_rgba2, width, height, 0);
    liq_histogram_add_image(hist, attr, image2);

    liq_result *result;
    liq_error err = liq_histogram_quantize(attr, hist, &result);
    if (LIQ_OK == err) {
        // result will contain shared palette best for both image1 and image2
    }

---

    liq_histogram *liq_histogram_create(liq_attr *attr);

Creates histogram object that will be used to collect color statistics from multiple images. It must be freed using `liq_histogram_destroy()`.

All options should be set on `attr` before the histogram object is created. Options changed later may not have effect.

---

    liq_error liq_histogram_add_image(liq_histogram *hist, liq_attr *attr, liq_image* image);

"Learns" colors from the image, which will be later used to generate the palette.

After the image is added to the histogram it may be freed to save memory (but it's more efficient to keep the image object if it's going to be used for remapping).

Fixed colors added to the image are also added to the histogram. If total number of fixed colors exceeds 256, this function will fail with `LIQ_BUFFER_TOO_SMALL`.

---

    liq_error liq_histogram_add_colors(liq_histogram *hist, liq_attr *attr, liq_histogram_entry entries[], int num_entries, double gamma);

Alternative to `liq_histogram_add_image()`. Intead of counting colors in an image, it directly takes an array of colors and their counts (see `liq_histogram_entry` in `libimagequant.h`). This function is only useful if you already have a histogram of the image from another source.

For description of gamma, see `liq_image_create_rgba()`.

---

    liq_error liq_histogram_quantize(liq_histogram *const hist, liq_attr *const attr, liq_result **out_result);

Generates palette from the histogram. On success returns `LIQ_OK` and writes `liq_result*` pointer to `out_result`. Use it as follows:

    liq_result *result;
    liq_error err = liq_histogram_quantize(attr, hist, &result);
    if (LIQ_OK == err) {
        // Use result here to remap and get palette
    }

Returns `LIQ_QUALITY_TOO_LOW` if the palette is worse than limit set in `liq_set_quality()`. One histogram object can be quantized only once.

Palette generated using this function won't be improved during remapping. If you're generating palette for only one image, it's better to use `liq_image_quantize()`.

## Multithreading

* Different threads can perform unrelated quantizations/remappings at the same time (e.g. each thread working on a different image).
* The same `liq_attr`, `liq_result`, etc. can be accessed from different threads, but not at the same time (e.g. you can create `liq_attr` in one thread and free it in another).
* By default, this library uses threads internally. You can set `RAYON_NUM_THREADS` environmental variable to control the number of threads used. You can disable threads completely by compiling with `--no-default-features`.

## Working with GIF

The library can generate palettes for GIF images. To ensure correct transparency is used you need to preprocess the image yourself and replace alpha values other than 0 or 255 with one of these.

For animated GIFs see `liq_image_set_background()` which remaps images for GIF's "keep" frame disposal method. See [gif.ski](https://gif.ski).

## Cross-compilation

You can compile the library for other platforms via `cargo build --target=…`. See `rustup target list` for the list of platforms.

When compiling for WASM, you need to disable default features of this library (compile with `--no-default-features` flag). Otherwise it will use mult-threading, which requires [special handling in WASM](https://github.com/GoogleChromeLabs/wasm-bindgen-rayon).

If you're cross-compiling a dynamic library (so/dylib/DLL), you may need to [configure a linker](https://doc.rust-lang.org/cargo/reference/config.html#target) for Cargo. For building for Android see [this tutorial](https://mozilla.github.io/firefox-browser-architecture/experiments/2017-09-21-rust-on-android.html) and [cargo-ndk](https://lib.rs/crates/cargo-ndk).
