imagick: document the image types - #5826
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The IMGTYPE_* constants had empty descriptions and Imagick::setImageType only repeated its own name, leaving readers to guess what an image type is and what the MATTE variants mean. Each constant now states the conversion SetImageType() performs, and the group notes that MATTE is the ImageMagick 6 name for the alpha channel, with the ALPHA spellings available as of ImageMagick 7. setType() is distinguished from setImageType().
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IMGTYPE_*constants had empty descriptions, andImagick::setImageType()only repeated its own name with an empty parameter entry, so nothing on either page said what an image type is or what theMATTEvariants mean.Each constant now states the conversion
SetImageType()actually performs, taken from the switch inMagickCore/attribute.c: which colorspace the image is transformed to, whether it is quantised, and whether the alpha channel is kept or discarded. Two of them convert nothing at all —IMGTYPE_UNDEFINEDandIMGTYPE_OPTIMIZEfall through to a barebreak.The group introduction answers the question the issue asks:
MATTEis the ImageMagick 6 name for the alpha channel, and Imagick registers anALPHAspelling with the same value when built against ImageMagick 7.Imagick::setType()is also given a sentence distinguishing it fromImagick::setImageType(): it sets the type used for images read or written afterwards, whereassetImageType()converts the current image. Both previously carried nearly the same description.Fixes: #2990