12.4 kB

	"""
	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 :py:data:`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
	:func:`.JpegImagePlugin.get_sampling` function.

	In JPEG compressed data a JPEG marker is used instead of an EXIF tag.
	(ref.: https://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

	"""

	from __future__ import annotations

	# fmt: off
	presets = {
	'web_low': {'subsampling': 2, # "4:2:0"
	'quantization': [
	[20, 16, 25, 39, 50, 46, 62, 68,
	16, 18, 23, 38, 38, 53, 65, 68,
	25, 23, 31, 38, 53, 65, 68, 68,
	39, 38, 38, 53, 65, 68, 68, 68,
	50, 38, 53, 65, 68, 68, 68, 68,
	46, 53, 65, 68, 68, 68, 68, 68,
	62, 65, 68, 68, 68, 68, 68, 68,
	68, 68, 68, 68, 68, 68, 68, 68],
	[21, 25, 32, 38, 54, 68, 68, 68,
	25, 28, 24, 38, 54, 68, 68, 68,
	32, 24, 32, 43, 66, 68, 68, 68,
	38, 38, 43, 53, 68, 68, 68, 68,
	54, 54, 66, 68, 68, 68, 68, 68,
	68, 68, 68, 68, 68, 68, 68, 68,
	68, 68, 68, 68, 68, 68, 68, 68,
	68, 68, 68, 68, 68, 68, 68, 68]
	]},
	'web_medium': {'subsampling': 2, # "4:2:0"
	'quantization': [
	[16, 11, 11, 16, 23, 27, 31, 30,
	11, 12, 12, 15, 20, 23, 23, 30,
	11, 12, 13, 16, 23, 26, 35, 47,
	16, 15, 16, 23, 26, 37, 47, 64,
	23, 20, 23, 26, 39, 51, 64, 64,
	27, 23, 26, 37, 51, 64, 64, 64,
	31, 23, 35, 47, 64, 64, 64, 64,
	30, 30, 47, 64, 64, 64, 64, 64],
	[17, 15, 17, 21, 20, 26, 38, 48,
	15, 19, 18, 17, 20, 26, 35, 43,
	17, 18, 20, 22, 26, 30, 46, 53,
	21, 17, 22, 28, 30, 39, 53, 64,
	20, 20, 26, 30, 39, 48, 64, 64,
	26, 26, 30, 39, 48, 63, 64, 64,
	38, 35, 46, 53, 64, 64, 64, 64,
	48, 43, 53, 64, 64, 64, 64, 64]
	]},
	'web_high': {'subsampling': 0, # "4:4:4"
	'quantization': [
	[6, 4, 4, 6, 9, 11, 12, 16,
	4, 5, 5, 6, 8, 10, 12, 12,
	4, 5, 5, 6, 10, 12, 14, 19,
	6, 6, 6, 11, 12, 15, 19, 28,
	9, 8, 10, 12, 16, 20, 27, 31,
	11, 10, 12, 15, 20, 27, 31, 31,
	12, 12, 14, 19, 27, 31, 31, 31,
	16, 12, 19, 28, 31, 31, 31, 31],
	[7, 7, 13, 24, 26, 31, 31, 31,
	7, 12, 16, 21, 31, 31, 31, 31,
	13, 16, 17, 31, 31, 31, 31, 31,
	24, 21, 31, 31, 31, 31, 31, 31,
	26, 31, 31, 31, 31, 31, 31, 31,
	31, 31, 31, 31, 31, 31, 31, 31,
	31, 31, 31, 31, 31, 31, 31, 31,
	31, 31, 31, 31, 31, 31, 31, 31]
	]},
	'web_very_high': {'subsampling': 0, # "4:4:4"
	'quantization': [
	[2, 2, 2, 2, 3, 4, 5, 6,
	2, 2, 2, 2, 3, 4, 5, 6,
	2, 2, 2, 2, 4, 5, 7, 9,
	2, 2, 2, 4, 5, 7, 9, 12,
	3, 3, 4, 5, 8, 10, 12, 12,
	4, 4, 5, 7, 10, 12, 12, 12,
	5, 5, 7, 9, 12, 12, 12, 12,
	6, 6, 9, 12, 12, 12, 12, 12],
	[3, 3, 5, 9, 13, 15, 15, 15,
	3, 4, 6, 11, 14, 12, 12, 12,
	5, 6, 9, 14, 12, 12, 12, 12,
	9, 11, 14, 12, 12, 12, 12, 12,
	13, 14, 12, 12, 12, 12, 12, 12,
	15, 12, 12, 12, 12, 12, 12, 12,
	15, 12, 12, 12, 12, 12, 12, 12,
	15, 12, 12, 12, 12, 12, 12, 12]
	]},
	'web_maximum': {'subsampling': 0, # "4:4:4"
	'quantization': [
	[1, 1, 1, 1, 1, 1, 1, 1,
	1, 1, 1, 1, 1, 1, 1, 1,
	1, 1, 1, 1, 1, 1, 1, 2,
	1, 1, 1, 1, 1, 1, 2, 2,
	1, 1, 1, 1, 1, 2, 2, 3,
	1, 1, 1, 1, 2, 2, 3, 3,
	1, 1, 1, 2, 2, 3, 3, 3,
	1, 1, 2, 2, 3, 3, 3, 3],
	[1, 1, 1, 2, 2, 3, 3, 3,
	1, 1, 1, 2, 3, 3, 3, 3,
	1, 1, 1, 3, 3, 3, 3, 3,
	2, 2, 3, 3, 3, 3, 3, 3,
	2, 3, 3, 3, 3, 3, 3, 3,
	3, 3, 3, 3, 3, 3, 3, 3,
	3, 3, 3, 3, 3, 3, 3, 3,
	3, 3, 3, 3, 3, 3, 3, 3]
	]},
	'low': {'subsampling': 2, # "4:2:0"
	'quantization': [
	[18, 14, 14, 21, 30, 35, 34, 17,
	14, 16, 16, 19, 26, 23, 12, 12,
	14, 16, 17, 21, 23, 12, 12, 12,
	21, 19, 21, 23, 12, 12, 12, 12,
	30, 26, 23, 12, 12, 12, 12, 12,
	35, 23, 12, 12, 12, 12, 12, 12,
	34, 12, 12, 12, 12, 12, 12, 12,
	17, 12, 12, 12, 12, 12, 12, 12],
	[20, 19, 22, 27, 20, 20, 17, 17,
	19, 25, 23, 14, 14, 12, 12, 12,
	22, 23, 14, 14, 12, 12, 12, 12,
	27, 14, 14, 12, 12, 12, 12, 12,
	20, 14, 12, 12, 12, 12, 12, 12,
	20, 12, 12, 12, 12, 12, 12, 12,
	17, 12, 12, 12, 12, 12, 12, 12,
	17, 12, 12, 12, 12, 12, 12, 12]
	]},
	'medium': {'subsampling': 2, # "4:2:0"
	'quantization': [
	[12, 8, 8, 12, 17, 21, 24, 17,
	8, 9, 9, 11, 15, 19, 12, 12,
	8, 9, 10, 12, 19, 12, 12, 12,
	12, 11, 12, 21, 12, 12, 12, 12,
	17, 15, 19, 12, 12, 12, 12, 12,
	21, 19, 12, 12, 12, 12, 12, 12,
	24, 12, 12, 12, 12, 12, 12, 12,
	17, 12, 12, 12, 12, 12, 12, 12],
	[13, 11, 13, 16, 20, 20, 17, 17,
	11, 14, 14, 14, 14, 12, 12, 12,
	13, 14, 14, 14, 12, 12, 12, 12,
	16, 14, 14, 12, 12, 12, 12, 12,
	20, 14, 12, 12, 12, 12, 12, 12,
	20, 12, 12, 12, 12, 12, 12, 12,
	17, 12, 12, 12, 12, 12, 12, 12,
	17, 12, 12, 12, 12, 12, 12, 12]
	]},
	'high': {'subsampling': 0, # "4:4:4"
	'quantization': [
	[6, 4, 4, 6, 9, 11, 12, 16,
	4, 5, 5, 6, 8, 10, 12, 12,
	4, 5, 5, 6, 10, 12, 12, 12,
	6, 6, 6, 11, 12, 12, 12, 12,
	9, 8, 10, 12, 12, 12, 12, 12,
	11, 10, 12, 12, 12, 12, 12, 12,
	12, 12, 12, 12, 12, 12, 12, 12,
	16, 12, 12, 12, 12, 12, 12, 12],
	[7, 7, 13, 24, 20, 20, 17, 17,
	7, 12, 16, 14, 14, 12, 12, 12,
	13, 16, 14, 14, 12, 12, 12, 12,
	24, 14, 14, 12, 12, 12, 12, 12,
	20, 14, 12, 12, 12, 12, 12, 12,
	20, 12, 12, 12, 12, 12, 12, 12,
	17, 12, 12, 12, 12, 12, 12, 12,
	17, 12, 12, 12, 12, 12, 12, 12]
	]},
	'maximum': {'subsampling': 0, # "4:4:4"
	'quantization': [
	[2, 2, 2, 2, 3, 4, 5, 6,
	2, 2, 2, 2, 3, 4, 5, 6,
	2, 2, 2, 2, 4, 5, 7, 9,
	2, 2, 2, 4, 5, 7, 9, 12,
	3, 3, 4, 5, 8, 10, 12, 12,
	4, 4, 5, 7, 10, 12, 12, 12,
	5, 5, 7, 9, 12, 12, 12, 12,
	6, 6, 9, 12, 12, 12, 12, 12],
	[3, 3, 5, 9, 13, 15, 15, 15,
	3, 4, 6, 10, 14, 12, 12, 12,
	5, 6, 9, 14, 12, 12, 12, 12,
	9, 10, 14, 12, 12, 12, 12, 12,
	13, 14, 12, 12, 12, 12, 12, 12,
	15, 12, 12, 12, 12, 12, 12, 12,
	15, 12, 12, 12, 12, 12, 12, 12,
	15, 12, 12, 12, 12, 12, 12, 12]
	]},
	}
	# fmt: on

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