[png-mng-implement] Web images with 16 bits per component

[John B. <joh...@gm...>]

Out of curiosity I examined by test set of 127357 real world PNG
images randomly selected from images.google.com for images which have
16-bit components.  There were 3350 such images (about 2.6% of the
total).  Of these images:

937 have a gAMA chunk, of the remainder:
none have an sRGB chunk and
349 have an iCCP chunk (but no gAMA chunk), of the remainder:
2055 were written by ImageMagick (they have a vpAg chunk) and, of the remainder:
9 have (just) a cHRM chunk and, of the remainder:
197 have no colorspace information or vpAg tag

So only 28% of the files have gamma information.   In fact 2119 files
have no color space information at all (not even a lone cHRM tag).

Also of the total 2721 files have a vpAg tag, indicating that they
were written by ImageMagick.  Of the remaining 629 files 389 have an
ICC profile and another 34 have a gAMA tag.

The 197 files that have no colorspace information at all many of these
are simple graphics, consequently it is almost impossible to guess at
the true gamma.  There are about 40 photographic images, all
thumbnails, and these appear to be gamma encoded.  The gamma encoding
seems to vary, but this could just be a consequence of bad
photography.

It seems pretty clear from visual examination that none of those 197
images should have been encoded with 16-bit samples.  Indeed there are
a lot of monochrome images, or even black+transparent images in there!

-- 
John Bowler <joh...@gm...>
+1 (541) 450-9885
PO BOX 3151
KERBY OR 97531-3151
USA