Re: EXIF support in PNG

On Sun, Jan 1, 2017 at 11:06 PM, Willem van Schaik <[email protected]>
wrote:

>
> but how can we have a PNG image with an EXIF chunk that says that the
> format of the image is JPEG ??
>
> again, not talking about the thumbnail
>
> yep, coordination of this will be very complicated, but it needs to be
> resolved


Strictly speaking, the EXIF chunk says that the image *used* to be JPEG.

So, it's OK if the person reading the EXIF treats it as historical rather
than current information about the image.
We should say something to that effect in the PNG EXIF chunk spec.

Most of the EXIF data remains relevant after a format conversion.

Here's the exif data from my new camera, according to ImageMagick:

Image: DSCN0068.JPG
  Format: JPEG (Joint Photographic Experts Group JFIF format)
  Mime type: image/jpeg
  Geometry: 5184x3888+0+0
  Compression: JPEG
  Quality: 98
  Orientation: TopLeft
  Properties:
    date:create: 2017-01-01T15:33:06-05:00
    date:modify: 2017-01-01T10:10:12-05:00
    exif:Artist:
    exif:ColorSpace: 1
    exif:ComponentsConfiguration: 1, 2, 3, 0
    exif:CompressedBitsPerPixel: 4/1
    exif:Contrast: 0
    exif:Copyright:
    exif:CustomRendered: 0
    exif:DateTime: 2017:01:01 15:10:13
    exif:DateTimeDigitized: 2017:01:01 15:10:13
    exif:DateTimeOriginal: 2017:01:01 15:10:13
    exif:DigitalZoomRatio: 0/100
    exif:ExifImageLength: 3888
    exif:ExifImageWidth: 5184
    exif:ExifOffset: 364
    exif:ExifVersion: 48, 50, 51, 48
    exif:ExposureBiasValue: 0/10
    exif:ExposureMode: 0
    exif:ExposureProgram: 2
    exif:ExposureTime: 10/2500
    exif:FileSource: 3
    exif:Flash: 16
    exif:FlashPixVersion: 48, 49, 48, 48
    exif:FNumber: 65/10
    exif:FocalLength: 2580/10
    exif:FocalLengthIn35mmFilm: 1440
    exif:GainControl: 1
    exif:GPSInfo: 942
    exif:GPSVersionID: 2, 3, 0, 0
    exif:ImageDescription:
    exif:InteroperabilityOffset: 1054
    exif:ISOSpeedRatings: 220
    exif:LightSource: 0
    exif:Make: NIKON
    exif:MakerNote: 78, 105, 107, [...]
    exif:MaxApertureValue: 34/10
    exif:MeteringMode: 5
    exif:Model: COOLPIX B700
    exif:Orientation: 1
    exif:ResolutionUnit: 2
    exif:Saturation: 0
    exif:SceneCaptureType: 0
    exif:SceneType: 1
    exif:Sharpness: 0
    exif:Software: COOLPIX B700   V1.0
    exif:SubjectDistanceRange: 0
    exif:thumbnail:Compression: 6
    exif:thumbnail:InteroperabilityIndex: R98
    exif:thumbnail:InteroperabilityVersion: 48, 49, 48, 48
    exif:thumbnail:JPEGInterchangeFormat: 14740
    exif:thumbnail:JPEGInterchangeFormatLength: 8592
    exif:thumbnail:ResolutionUnit: 2
    exif:thumbnail:XResolution: 300/1
    exif:thumbnail:YResolution: 300/1
    exif:UserComment: 65, 83, [...]
    exif:WhiteBalance: 0
    exif:XResolution: 300/1
    exif:YCbCrPositioning: 1
    exif:YResolution: 300/1
    jpeg:colorspace: 2
    jpeg:sampling-factor: 2x1,1x1,1x1
  Profiles:
    Profile-exif: 24994 bytes
  Filesize: 9.371MB
  Number pixels: 20.16M

Note that the "Filesize", "Compression" and "Quality" items aren't from
EXIF but are generated by ImageMagick.

Glenn

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