72 lines
7.9 KiB
Plaintext
72 lines
7.9 KiB
Plaintext
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Ë
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ß¼j[0 ã óÎ — d Z ddlmZ dg d¢g d¢gdœdg d¢g d¢gdœdg d ¢g d
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¢gdœdg d ¢g d ¢gdœdg d
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¢g d¢gdœdg d¢g d¢gdœdg d¢g d¢gdœdg d¢g d¢gdœdg d ¢g d¢gdœdœ Zy)u
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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 EXIFÂ tag.
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(ref.: https://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 lists. You can pass this dict
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directly as the qtables argument when saving a JPEG.
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The quantization table format in presets is a list with sublists. These formats
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are interchangeable.
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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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r é r" r r
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