Re: EXIF support in PNG

John Bowler <[email protected]>
Newsgroups gmane.comp.graphics.png.general
Message-ID <CAP7U399cnfPo6=-DtUBKxaku2d0gWL24m1a_=488fPHfVxd_4g@mail.gmail.com> 
On Mon, Jan 2, 2017 at 9:10 AM, Willem van Schaik <[email protected]> wrote:
>here another argument in the "compression or not" debate
>photo images stored in PNG is to replace RAW, which is
>typically only used by high-end photographers

Well, not just that; any image processing pipeline which uses digital
camera image data and, for whatever reason, uses PNG as an intermediate
format.  PNG isn't a good replacement for RAW, but under some circumstances
might be useful where JFIF hasn't enough range for the data.

Ok, so I have a Sony RX-100 (the original one, actually it's a Hasselblad
re-badge but it really is an RX-100).  This camera has a setting where it
records both a RAW file and an Exif file using JPEG compression.  Taking
just one image:

DSC00552.ARW (RAW): 5504x3672, 21,021,184 bytes
DSC00552.JPG (Exif): 5472x3648, 6,606,865 bytes

The latter file has an APP1 Exif chunk (it really is an Exif file, not a
JFIF one):

 APP1: Application marker [segment: 37566 bytes]
     Thumbnail length: 10608 bytes
     MakerNote length: 25717 bytes
     Remainder (by subraction): 1241 bytes

In that file there is a second smaller (1616x1080) JPEG after the main one;
I don't understand what that is.  The full dump file is attached (this is
from my own program and that isn't very up-to-date), I'm happy to upload
the images to one of the binary dump sites.  (They're copyright either me
or my wife, I can't remember who took the photo, it's not very good ;-)

Anyway even in the JPEG version the APP1 marker only takes 0.6% of the
total and of the raw file that would be 0.2%.

Meanwhile compressing the two files with 'gzip -c9' does the following:

DSC00552.JPG 6606865 -> 6,584,713 [22152 bytes less]
DSC00552.ARW 21,021,184 -> 20,818,660 [202,524 bytes less]

and (using ImageMagick):

DSC00552.JPG -> DSC00552.PNG: 21,447,864 bytes
     Compressed Exif profile size: 30,248 bytes
     Compressed MakerNote tag: 540 bytes (weird, a bug?)
     Uncompressed Exif profile size: 37,564 bytes (not 37566; no APP1 code)
     deflate(default) of Exif profile: 24,956 bytes
     gzip -9 of Exif profile: 25,303 bytes (has gzip header)

So deflate of this profile reduces it to 83% of its original size, saving
12,608 bytes.

Of course other cameras or even other photographs may produce different
results; I suspect most of the compression here comes from the MakerNote.
All the same, 60-80% compression is typical for deflate on byte-aligned
data and very good for non-byte-aligned data like JPEG.  Since any any
likely image will be 1MPixel or more having 3MByte+ of RGB data PNG might
on really good day reduce it o 1.5MByte I can't imagine there being any
issue about lack of compression of the 40kByte or so of  Exif APP1 data.

-- 
John Bowler <[email protected]>
+1 (541) 450-9885
PO BOX 3151
KERBY OR 97531-3151
USA

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DSC00552.txt (text/plain, 4.1 KB) 
 SOI: Start of image [marker]
 APP1: Application marker [segment: 37566 bytes]
  Exif(2.0 0)
   little-endian
  IFD[0] base: 8 entries: 12
   ImageDescription ASCII[32] "                               [00]"
   Make ASCII[11] "HASSELBLAD[00]"
   Model ASCII[8] "Stellar[00]"
   Orientation USHORT[1] TopLeft
   XResolution URATIONAL[1] 350/1 [350]
   YResolution URATIONAL[1] 350/1 [350]
   ResolutionUnit USHORT[1] DPI
   Software ASCII[14] "Stellar v1.00[00]"
   DateTime ASCII[20] "2016:10:31 13:38:56[00]"
   YCbCrPositioning USHORT[1] Co-sited
   Exif IFD Offset ULONG[1]         288
   UNKNOWN[50341] UNDEFINED[28]
  IFD[1] base: 26686 entries: 13
   Compression USHORT[1] JPEG
   ImageDescription ASCII[32] "                               [00]"
   Make ASCII[11] "HASSELBLAD[00]"
   Model ASCII[8] "Stellar[00]"
   Orientation USHORT[1] TopLeft
   XResolution URATIONAL[1] 72/1 [72]
   YResolution URATIONAL[1] 72/1 [72]
   ResolutionUnit USHORT[1] DPI
   Software ASCII[14] "Stellar v1.00[00]"
   DateTime ASCII[20] "2016:10:31 13:38:56[00]"
   JPEGInterchangeFormat (Thumbnail offset) ULONG[1]       26950
   JPEGInterchangeFormatLength (Thumbnail length) ULONG[1]       10608
   YCbCrPositioning USHORT[1] Co-sited
  IFD[Exif] base: 288 entries: 37
   ExposureTime[s] URATIONAL[1] 1/125 [0.008]
   FNumber URATIONAL[1] 45/10 [4.5]
   ExposureProgram USHORT[1] NormalProgram
   ISOSpeedRating[ISO12232] USHORT[1]   800
   UNKNOWN[34864] USHORT[1]
   UNKNOWN[34866] ULONG[1]
   ExifVersion "0230"
   DateTimeOriginal ASCII[20] "2016:10:31 13:38:56[00]"
   DateTimeDigitized ASCII[20] "2016:10:31 13:38:56[00]"
   ComponentsConfiguration UNDEFINED[4] Y Cb Cr absent
   CompressedBitsPerPixel URATIONAL[1] 2/1 [2]
   Brightness[APEX] SRATIONAL[1] 10884/2560 [4.25156]19.0479footLambert(!)
   ExposureBias[APEX] SRATIONAL[1] 0/10 [0]
   MaxAperture[APEX] URATIONAL[1] 1110/256 [4.33594] f/4.4939
   MeteringMode[255=other] USHORT[1] Pattern
   LightSource[255=other] USHORT[1] unknown
   Flash USHORT[1] ERROR:    16
   FocalLength[mm] URATIONAL[1] 3285/100 [32.85]
   MakerNote UNDEFINED[25717]
   UserComment UNDEFINED[64] "[00][00][00][00][00][00][00][00][00][00][00]
     [00][00][00][00][00][00][00][00][00][00][00][00][00][00][00][00][00]
     [00][00][00][00][00][00][00][00][00][00][00][00][00][00][00][00][00]
     [00][00][00][00][00][00][00][00][00][00][00][00][00][00][00][00][00]
     [00][00]"
   FlashPixVersion "0100"
   ColorSpace USHORT[1] sRGB
   PixelXDimension ULONG[1]        5472
   PixelYDimension ULONG[1]        3648
   UNKNOWN[40965] ULONG[1]
   FileSource UNDEFINED[1] DigitalCamera
   SceneType UNDEFINED[1] DirectCapture
   UNKNOWN[41985] USHORT[1]
   UNKNOWN[41986] USHORT[1]
   UNKNOWN[41987] USHORT[1]
   UNKNOWN[41988] URATIONAL[1]
   UNKNOWN[41989] USHORT[1]
   UNKNOWN[41990] USHORT[1]
   UNKNOWN[41992] USHORT[1]
   UNKNOWN[41993] USHORT[1]
   UNKNOWN[41994] USHORT[1]
   UNKNOWN[42034] URATIONAL[4]
 APP2: Application marker [segment: 316 bytes]
 DQT: Define quantization table [segment: 132 bytes]
 DHT: Define Huffman tables [segment: 418 bytes]
 SOF: Non differential Huffman; Baseline DCT [segment: 17 bytes]
  SOF 5472x3648 8 bits per 3 components
   component 0: 1 sampling: 2x1 Tq(0)
   component 1: 2 sampling: 1x1 Tq(1)
   component 2: 3 sampling: 1x1 Tq(1)
 SOS: Start of scan [segment: 12 bytes]
 EOI: End of image [marker]
 skipped 1011 bytes
 SOI: Start of image [marker]
 APP1: Application marker [segment: 160 bytes]
  Exif(2.0 0)
   little-endian
  IFD[0] base: 8 entries: 4
   XResolution URATIONAL[1] 72/1 [72]
   YResolution URATIONAL[1] 72/1 [72]
   ResolutionUnit USHORT[1] DPI
   Exif IFD Offset ULONG[1]          78
  IFD[Exif] base: 78 entries: 2
   PixelXDimension ULONG[1]        1616
   PixelYDimension ULONG[1]        1080
 DQT: Define quantization table [segment: 132 bytes]
 DHT: Define Huffman tables [segment: 418 bytes]
 SOF: Non differential Huffman; Baseline DCT [segment: 17 bytes]
  SOF 1616x1080 8 bits per 3 components
   component 0: 1 sampling: 2x1 Tq(0)
   component 1: 2 sampling: 1x1 Tq(1)
   component 2: 3 sampling: 1x1 Tq(1)
 SOS: Start of scan [segment: 12 bytes]
 EOI: End of image [marker]
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