# `JpegPresets` Module

JPEG quality settings equivalent to the Photoshop settings. Can be used when saving JPEG files.

The following presets are available by default: `web_low`, `web_medium`, `web_high`, `web_very_high`, `web_maximum`, `low`, `medium`, `high`, `maximum`. More presets can be added to the `presets` dict if needed.

To apply the preset, specify:

    quality="preset_name"

To apply only the quantization table:

    qtables="preset_name"

To apply only the subsampling setting:

    subsampling="preset_name"

Example:

    im.save("image_name.jpg", quality="web_high")

## Subsampling

Subsampling is the practice of encoding images by implementing less resolution for chroma information than for luma information. (ref.: https://en.wikipedia.org/wiki/Chroma_subsampling)

Possible subsampling values are 0, 1 and 2 that correspond to 4:4:4, 4:2:2 and 4:2:0.

You can get the subsampling of a JPEG with the `JpegImagePlugin.get_sampling()` function.

In JPEG compressed data a JPEG marker is used instead of an EXIF tag. (ref.: https://www.exiv2.org/tags.html)

## Quantization tables

They are values use by the DCT (Discrete cosine transform) to remove unnecessary information from the image (the lossy part of the compression). (ref.: https://en.wikipedia.org/wiki/Quantization_matrix#Quantization_matrices, https://en.wikipedia.org/wiki/JPEG#Quantization)

You can get the quantization tables of a JPEG with:

    im.quantization

This will return a dict with a number of lists. You can pass this dict directly as the qtables argument when saving a JPEG.

The quantization table format in presets is a list with sublists. These formats are interchangeable.

Libjpeg ref.: https://web.archive.org/web/20120328125543/http://www.jpegcameras.com/libjpeg/libjpeg-3.html

PIL.JpegPresets.presets: dict

  A dictionary of all supported presets.