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249 lines
12 KiB
249 lines
12 KiB
2 weeks ago
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"""
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JPEG quality settings equivalent to the Photoshop settings.
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Can be used when saving JPEG files.
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The following presets are available by default:
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``web_low``, ``web_medium``, ``web_high``, ``web_very_high``, ``web_maximum``,
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``low``, ``medium``, ``high``, ``maximum``.
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More presets can be added to the :py:data:`presets` dict if needed.
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To apply the preset, specify::
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quality="preset_name"
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To apply only the quantization table::
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qtables="preset_name"
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To apply only the subsampling setting::
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subsampling="preset_name"
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Example::
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im.save("image_name.jpg", quality="web_high")
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Subsampling
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-----------
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Subsampling is the practice of encoding images by implementing less resolution
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for chroma information than for luma information.
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(ref.: https://en.wikipedia.org/wiki/Chroma_subsampling)
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Possible subsampling values are 0, 1 and 2 that correspond to 4:4:4, 4:2:2 and
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4:2:0.
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You can get the subsampling of a JPEG with the
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:func:`.JpegImagePlugin.get_sampling` function.
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In JPEG compressed data a JPEG marker is used instead of an EXIF tag.
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(ref.: https://www.exiv2.org/tags.html)
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Quantization tables
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-------------------
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They are values use by the DCT (Discrete cosine transform) to remove
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*unnecessary* information from the image (the lossy part of the compression).
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(ref.: https://en.wikipedia.org/wiki/Quantization_matrix#Quantization_matrices,
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https://en.wikipedia.org/wiki/JPEG#Quantization)
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You can get the quantization tables of a JPEG with::
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im.quantization
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This will return a dict with a number of arrays. You can pass this dict
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directly as the qtables argument when saving a JPEG.
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The tables format between im.quantization and quantization in presets differ in
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3 ways:
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1. The base container of the preset is a list with sublists instead of dict.
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dict[0] -> list[0], dict[1] -> list[1], ...
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2. Each table in a preset is a list instead of an array.
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3. The zigzag order is remove in the preset (needed by libjpeg >= 6a).
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You can convert the dict format to the preset format with the
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:func:`.JpegImagePlugin.convert_dict_qtables()` function.
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Libjpeg ref.:
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https://web.archive.org/web/20120328125543/http://www.jpegcameras.com/libjpeg/libjpeg-3.html
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"""
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# fmt: off
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presets = {
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'web_low': {'subsampling': 2, # "4:2:0"
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'quantization': [
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[20, 16, 25, 39, 50, 46, 62, 68,
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16, 18, 23, 38, 38, 53, 65, 68,
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25, 23, 31, 38, 53, 65, 68, 68,
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39, 38, 38, 53, 65, 68, 68, 68,
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50, 38, 53, 65, 68, 68, 68, 68,
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46, 53, 65, 68, 68, 68, 68, 68,
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62, 65, 68, 68, 68, 68, 68, 68,
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68, 68, 68, 68, 68, 68, 68, 68],
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[21, 25, 32, 38, 54, 68, 68, 68,
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25, 28, 24, 38, 54, 68, 68, 68,
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32, 24, 32, 43, 66, 68, 68, 68,
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38, 38, 43, 53, 68, 68, 68, 68,
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54, 54, 66, 68, 68, 68, 68, 68,
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68, 68, 68, 68, 68, 68, 68, 68,
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68, 68, 68, 68, 68, 68, 68, 68,
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68, 68, 68, 68, 68, 68, 68, 68]
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]},
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'web_medium': {'subsampling': 2, # "4:2:0"
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'quantization': [
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[16, 11, 11, 16, 23, 27, 31, 30,
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11, 12, 12, 15, 20, 23, 23, 30,
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11, 12, 13, 16, 23, 26, 35, 47,
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16, 15, 16, 23, 26, 37, 47, 64,
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23, 20, 23, 26, 39, 51, 64, 64,
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27, 23, 26, 37, 51, 64, 64, 64,
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31, 23, 35, 47, 64, 64, 64, 64,
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30, 30, 47, 64, 64, 64, 64, 64],
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[17, 15, 17, 21, 20, 26, 38, 48,
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15, 19, 18, 17, 20, 26, 35, 43,
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17, 18, 20, 22, 26, 30, 46, 53,
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21, 17, 22, 28, 30, 39, 53, 64,
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20, 20, 26, 30, 39, 48, 64, 64,
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26, 26, 30, 39, 48, 63, 64, 64,
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38, 35, 46, 53, 64, 64, 64, 64,
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48, 43, 53, 64, 64, 64, 64, 64]
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]},
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'web_high': {'subsampling': 0, # "4:4:4"
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'quantization': [
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[6, 4, 4, 6, 9, 11, 12, 16,
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4, 5, 5, 6, 8, 10, 12, 12,
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4, 5, 5, 6, 10, 12, 14, 19,
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6, 6, 6, 11, 12, 15, 19, 28,
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9, 8, 10, 12, 16, 20, 27, 31,
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11, 10, 12, 15, 20, 27, 31, 31,
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12, 12, 14, 19, 27, 31, 31, 31,
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16, 12, 19, 28, 31, 31, 31, 31],
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[7, 7, 13, 24, 26, 31, 31, 31,
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7, 12, 16, 21, 31, 31, 31, 31,
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13, 16, 17, 31, 31, 31, 31, 31,
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24, 21, 31, 31, 31, 31, 31, 31,
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26, 31, 31, 31, 31, 31, 31, 31,
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31, 31, 31, 31, 31, 31, 31, 31,
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31, 31, 31, 31, 31, 31, 31, 31,
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31, 31, 31, 31, 31, 31, 31, 31]
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]},
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'web_very_high': {'subsampling': 0, # "4:4:4"
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'quantization': [
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[2, 2, 2, 2, 3, 4, 5, 6,
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2, 2, 2, 2, 3, 4, 5, 6,
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2, 2, 2, 2, 4, 5, 7, 9,
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2, 2, 2, 4, 5, 7, 9, 12,
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3, 3, 4, 5, 8, 10, 12, 12,
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4, 4, 5, 7, 10, 12, 12, 12,
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5, 5, 7, 9, 12, 12, 12, 12,
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6, 6, 9, 12, 12, 12, 12, 12],
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[3, 3, 5, 9, 13, 15, 15, 15,
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3, 4, 6, 11, 14, 12, 12, 12,
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5, 6, 9, 14, 12, 12, 12, 12,
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9, 11, 14, 12, 12, 12, 12, 12,
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13, 14, 12, 12, 12, 12, 12, 12,
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15, 12, 12, 12, 12, 12, 12, 12,
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15, 12, 12, 12, 12, 12, 12, 12,
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15, 12, 12, 12, 12, 12, 12, 12]
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]},
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'web_maximum': {'subsampling': 0, # "4:4:4"
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'quantization': [
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[1, 1, 1, 1, 1, 1, 1, 1,
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1, 1, 1, 1, 1, 1, 1, 1,
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1, 1, 1, 1, 1, 1, 1, 2,
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1, 1, 1, 1, 1, 1, 2, 2,
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1, 1, 1, 1, 1, 2, 2, 3,
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1, 1, 1, 1, 2, 2, 3, 3,
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1, 1, 1, 2, 2, 3, 3, 3,
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1, 1, 2, 2, 3, 3, 3, 3],
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[1, 1, 1, 2, 2, 3, 3, 3,
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1, 1, 1, 2, 3, 3, 3, 3,
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1, 1, 1, 3, 3, 3, 3, 3,
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2, 2, 3, 3, 3, 3, 3, 3,
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2, 3, 3, 3, 3, 3, 3, 3,
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3, 3, 3, 3, 3, 3, 3, 3,
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3, 3, 3, 3, 3, 3, 3, 3,
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3, 3, 3, 3, 3, 3, 3, 3]
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]},
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'low': {'subsampling': 2, # "4:2:0"
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'quantization': [
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[18, 14, 14, 21, 30, 35, 34, 17,
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14, 16, 16, 19, 26, 23, 12, 12,
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14, 16, 17, 21, 23, 12, 12, 12,
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21, 19, 21, 23, 12, 12, 12, 12,
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30, 26, 23, 12, 12, 12, 12, 12,
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35, 23, 12, 12, 12, 12, 12, 12,
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34, 12, 12, 12, 12, 12, 12, 12,
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17, 12, 12, 12, 12, 12, 12, 12],
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[20, 19, 22, 27, 20, 20, 17, 17,
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19, 25, 23, 14, 14, 12, 12, 12,
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22, 23, 14, 14, 12, 12, 12, 12,
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27, 14, 14, 12, 12, 12, 12, 12,
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20, 14, 12, 12, 12, 12, 12, 12,
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20, 12, 12, 12, 12, 12, 12, 12,
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17, 12, 12, 12, 12, 12, 12, 12,
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17, 12, 12, 12, 12, 12, 12, 12]
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]},
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'medium': {'subsampling': 2, # "4:2:0"
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'quantization': [
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[12, 8, 8, 12, 17, 21, 24, 17,
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8, 9, 9, 11, 15, 19, 12, 12,
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8, 9, 10, 12, 19, 12, 12, 12,
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12, 11, 12, 21, 12, 12, 12, 12,
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17, 15, 19, 12, 12, 12, 12, 12,
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21, 19, 12, 12, 12, 12, 12, 12,
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24, 12, 12, 12, 12, 12, 12, 12,
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17, 12, 12, 12, 12, 12, 12, 12],
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[13, 11, 13, 16, 20, 20, 17, 17,
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11, 14, 14, 14, 14, 12, 12, 12,
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13, 14, 14, 14, 12, 12, 12, 12,
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16, 14, 14, 12, 12, 12, 12, 12,
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20, 14, 12, 12, 12, 12, 12, 12,
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20, 12, 12, 12, 12, 12, 12, 12,
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17, 12, 12, 12, 12, 12, 12, 12,
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17, 12, 12, 12, 12, 12, 12, 12]
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]},
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'high': {'subsampling': 0, # "4:4:4"
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'quantization': [
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[6, 4, 4, 6, 9, 11, 12, 16,
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4, 5, 5, 6, 8, 10, 12, 12,
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4, 5, 5, 6, 10, 12, 12, 12,
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6, 6, 6, 11, 12, 12, 12, 12,
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9, 8, 10, 12, 12, 12, 12, 12,
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11, 10, 12, 12, 12, 12, 12, 12,
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12, 12, 12, 12, 12, 12, 12, 12,
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16, 12, 12, 12, 12, 12, 12, 12],
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[7, 7, 13, 24, 20, 20, 17, 17,
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7, 12, 16, 14, 14, 12, 12, 12,
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13, 16, 14, 14, 12, 12, 12, 12,
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24, 14, 14, 12, 12, 12, 12, 12,
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20, 14, 12, 12, 12, 12, 12, 12,
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20, 12, 12, 12, 12, 12, 12, 12,
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17, 12, 12, 12, 12, 12, 12, 12,
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17, 12, 12, 12, 12, 12, 12, 12]
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]},
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'maximum': {'subsampling': 0, # "4:4:4"
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'quantization': [
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[2, 2, 2, 2, 3, 4, 5, 6,
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2, 2, 2, 2, 3, 4, 5, 6,
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2, 2, 2, 2, 4, 5, 7, 9,
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2, 2, 2, 4, 5, 7, 9, 12,
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3, 3, 4, 5, 8, 10, 12, 12,
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4, 4, 5, 7, 10, 12, 12, 12,
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5, 5, 7, 9, 12, 12, 12, 12,
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6, 6, 9, 12, 12, 12, 12, 12],
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[3, 3, 5, 9, 13, 15, 15, 15,
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3, 4, 6, 10, 14, 12, 12, 12,
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5, 6, 9, 14, 12, 12, 12, 12,
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9, 10, 14, 12, 12, 12, 12, 12,
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13, 14, 12, 12, 12, 12, 12, 12,
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15, 12, 12, 12, 12, 12, 12, 12,
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15, 12, 12, 12, 12, 12, 12, 12,
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15, 12, 12, 12, 12, 12, 12, 12]
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]},
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}
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# fmt: on
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