Fwd: [libpng:feature-requests] #32 Modern compressor

On Fri, Dec 30, 2016 at 10:24 PM, Cosmin Truta <[email protected]>
wrote:

> On 30 December 2016 at 16:41, John Bowler
> <[email protected]> wrote:
>
> I would be happy enough to see our present-day PNG format being used
> in digital photography. Never mind exceeding 24-bit. Even for plain
> true-color images, deflate-compressed TIFF is more suitable than PNG,
> because TIFF can hold EXIF and PNG can't. For digital photographers
> (myself included), EXIF is super-precious.
>
> In theory, PNG can hold any metadata. In practice, there have been two
> or three attempts by our group to incorporate EXIF into PNG, back in
> early 2000's, all of which failed.
>
> My attempt to make a special PNG EXIF chunk did fail, but on the
other hand, ImageMagick and GraphicsMagick have been keeping EXIF
data successfully as "raw profiles" for a long time (at least since the
ImageMagick/GraphicsMagick fork of 2004).

Try this, with the current ImageMagick:

    magick DSCnnnn.JPG DSCnnnn.png
    pngcheck -v DSCnnnn.png
    magick DSCnnnn.png DSCnnnn_copy.jpg
    magick identify -verbose DSCnnnn_copy.jpg

and the same with GraphicsMagick:

    gm convert DSCnnnn.JPG DSCnnnn_gm.png
    pngcheck -v DSCnnnn_gm.png
    gm convert DSCnnnn_gm.png DSCnnnn_gm_copy.jpg
    gm identify -verbose DSCnnnn_gm_copy.jpg

You'll get something like this:

pngcheck -v DSCnnnn.png:
  ...
  chunk IDAT at offset 0x9b897b, length 31513
  chunk tEXt at offset 0x9c04a0, length 37, keyword: date:create
  chunk tEXt at offset 0x9c04d1, length 37, keyword: date:modify
  chunk tEXt at offset 0x9c0502, length 27, keyword: exif:BrightnessValue
  chunk tEXt at offset 0x9c0529, length 17, keyword: exif:ColorSpace
  ...

and

magick identify -verbose DSCnnnn_copy.jpg:

  Properties:
    date:create: 2016-12-31T08:28:46-05:00
    date:modify: 2016-12-31T08:28:46-05:00
    exif:BrightnessValue: 50/100
    exif:ColorSpace: 1
  Profiles:
    Profile-exif: 53679 bytes

pngcheck -v DSCnnnn_gm.png:
  chunk zTXt at offset 0x00025, length 31365, keyword: Raw profile type EXIF

GraphicsMagick doesn't display the internals of the EXIF chunk, but
the recovered JPG still has it:

gm identify -verbose DSCnnnn_gm_copy.jpg:

    ....
    User Comment: \000\000\000\000...
    Flash Pix Version: 0100
    Color Space: 1
    Exif Image Width: 3872
    Exif Image Length: 2592
    ...

Or, using ImageMagick to show the GraphicsMagick result:

  magick identify -verbose DSCnnnn_gm_copy.jpg:

  Properties:
    date:create: 2016-12-31T08:39:06-05:00
    date:modify: 2016-12-31T08:39:06-05:00
    exif:BrightnessValue: 50/100
    exif:ColorSpace: 1
    exif:ComponentsConfiguration: 1, 2, 3, 0
    exif:CompressedBitsPerPixel: 8/1
    exif:Contrast: 0
    exif:CustomRendered: 0
    exif:DateTime: 2016:12:02 16:46:32
    exif:DateTimeDigitized: 2016:12:02 16:46:32
    exif:XResolution: 72/1
    exif:YCbCrPositioning: 2
    exif:YResolution: 72/1

Glenn

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