""" 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://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 arrays. You can pass this dict directly as the qtables argument when saving a JPEG. The tables format between im.quantization and quantization in presets differ in 3 ways: 1. The base container of the preset is a list with sublists instead of dict. dict[0] -> list[0], dict[1] -> list[1], ... 2. Each table in a preset is a list instead of an array. 3. The zigzag order is remove in the preset (needed by libjpeg >= 6a). You can convert the dict format to the preset format with the :func:`.JpegImagePlugin.convert_dict_qtables()` function. Libjpeg ref.: https://web.archive.org/web/20120328125543/http://www.jpegcameras.com/libjpeg/libjpeg-3.html """ # 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