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# Image Quality & Options

Image quality is set per-format, using `format_quality`.

## Constant Quality

  _Format:_

  ```yaml
  format_quality:
    (format): 0 <= integer <= 100
    (...)
  ```

  _Example:_

  ```yaml
  format_quality:
    jpg: 75
    png: 65
    webp: 55
  ```

  _Defaults:_ 

  - webp - 50
  - avif - 30
  - jp2 - 30
  - all others - 75

## Variable Quality

  ```yaml
    format_quality: 
      (format):
        (image width): (quality setting)
        (image width): (quality setting) 
  ```

  _Example:_

  ```yaml
    format_quality: 
      jpg:
        1000: 65
        300: 100
  ```

Set a variable image quality, based on image width. Provide exactly 2 image widths and associated
quality settings. Quality will be calculated as follows:

  * For images smaller than the lowest image width, the setting for the lowest width is used. 
  * For images larger than the highest image width, the setting for the highest width is used. 
  * For images in between the 2, the quality setting will be linearly interpolated to some value in
    between.

  ![](quality_width_graph.png)

Using this setting, you can get away with more compression on high pixel density screens without
sacrificing image quality for low-density screens. Taking the example settings above:

  * A 1500px image will use a quality of 65.
  * A 200px image will use a quality of 100.
  * A 500px image will use a quality of 90.

## Strip Metadata

_Format:_ `strip_metadata: (true|false)`

_Example:_ `strip_metadata: false`

_Default:_ `true`

Remove EXIF data, which can save a few tens of kilobytes per image. This is set for all formats.

## Image Format Options

  _Format:_

  ```yaml
  image_options: 
    format:
      setting1: value
      setting2: value
      (...)
    (...)
  ```

  _Example:_

  ```yaml
  image_options:
    webp:
      lossless: true
  ```

  _Default:_

  ```yaml
  image_options:
    avif: 
      compression: av1
      speed: 8
  ```

Control the write options passed to libvips for each image output format. To see a list of options
available for a given image format, search for the method `vips_(format)save` in
[this](https://libvips.github.io/libvips/API/current/VipsForeignSave.html) API documentation. For
example, png options can be found by searching for `vips_pngsave()`, leading
[here](https://libvips.github.io/libvips/API/current/VipsForeignSave.html#vips-pngsave). See the
optional arguments. `avif`'s options are under `vips_heifsave()`,
[here](https://libvips.github.io/libvips/API/current/VipsForeignSave.html#vips-heifsave).

For all formats, note that
- `Q:` (quality) is handled by the settings above.
- `strip` is handled by the `strip_metadata` setting documented above.