Pixel Dimension tags

[eponymous]

Just starting using a new photo scanner (Epson V600) and when I use Imagemagick to grab the info, it reports this error (using grep to get just the relevant line which describe the image width):

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> identify -verbose Pictures/image.tif | grep -i 5584
  Geometry: 3998x5584+0+0
  Page geometry: 3998x5584+0+0
identify: Incorrect value for "RichTIFFIPTC"; tag ignored. `TIFFFetchNormalTag' @ warning/tiff.c/TIFFWarnings/960.
identify: Wrong data type 3 for "PixelXDimension"; tag ignored. `TIFFReadCustomDirectory' @ warning/tiff.c/TIFFWarnings/960.
identify: Wrong data type 3 for "PixelYDimension"; tag ignored. `TIFFReadCustomDirectory' @ warning/tiff.c/TIFFWarnings/960.


When I look up the Pixel Dimension tags, I find that they're perhaps needed only for compressed images (which this isn't; see below).

However, I find that exiftool seems to call this same tag by another name:

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> exiftool -s Pictures/image.tif | grep -i 5584
ImageHeight                     : 5584
ExifImageHeight                 : 5584
ImageSize                       : 3998x5584


I find this post from a few years ago:

There is no ExifTool tag called PixelXDimension.  ExifTool calls it ExifImageWidth, because it is the image width, duplicated in the EXIF information.

But that tag is described is several places in the exif spec. (If I understand it correctly, it's to make clear the dimensions of images in compression schemes (like jpeg) where the image edge falls somewhere not on an iMCU boundary.)

So two questions:

• Are PixelXDimension and ExifImageWidth actually the same tags?
• Is it correct that these tags shouldn't be present in uncompressed files?

TIA.

[StarGeek]

Are PixelXDimension and ExifImageWidth actually the same tags?

Yes.  If you look up the entries for ExifImageWidth/ExifImageHeight on the EXIF tags page, you'll see that it is mentioned in the Notes column.

Is it correct that these tags shouldn't be present in uncompressed files?

I would say that they are required by the EXIF specs because when you run exiftool on a file and include the -Validate option, you'll see this response

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C:\>exiftool -g1 -a -s -warning -validate y:\!temp\Test4.jpg
---- ExifTool ----
Warning                         : Missing required JPEG ExifIFD tag 0xa002 ExifImageWidth
Warning                         : Missing required JPEG ExifIFD tag 0xa003 ExifImageHeight
Validate                        : 2 Warnings

[eponymous]

Thanks for the help!

Is it correct that these tags shouldn't be present in uncompressed files?

I would say that they are required by the EXIF specs because when you run exiftool on a file and include the -Validate option, you'll see this response

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C:\>exiftool -g1 -a -s -warning -validate y:\!temp\Test4.jpg
---- ExifTool ----
Warning                         : Missing required JPEG ExifIFD tag 0xa002 ExifImageWidth
Warning                         : Missing required JPEG ExifIFD tag 0xa003 ExifImageHeight
Validate                        : 2 Warnings

Right, but this is a jpeg file which has the issue with image borders not lining up with an iMCU boundary, which is maybe a better way to describe where it's needed though perhaps the fact that jpegs are almost always compressed makes it as good as required. In any case the file coming from my scan software is a tiff.

[StarGeek]

Right, but this is a jpeg file which has the issue with image borders not lining up with an iMCU boundary, which is maybe a better way to describe where it's needed though perhaps the fact that jpegs are almost always compressed makes it as good as required.

While it's required by the EXIF standard, it's pretty much universally ignored.  The actual image dimensions are taken from the Jpeg SOF segment.  For example, if you use the -v3 (-verbose3) option on a jpg, you'll get the hex dump of each section.  In this example, bytes 0x04b5 is the 1205 heigh and 06db is the 1749 width.

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JPEG SOF0 (15 bytes):
    0dbe: 08 04 b5 06 d5 03 01 22 00 02 11 01 03 11 01    [.......".......]
  ImageWidth = 1749
  ImageHeight = 1205
  EncodingProcess = 0
  BitsPerSample = 8
  ColorComponents = 3
  YCbCrSubSampling = 2 2

And that doesn't even take into account the fact that the EXIF data is completely optional in a jpeg file.

But going back to original question 2, a quick test showed me that it probably is needed in some form in a TIFF file, compressed or not.  I had exiftool forcibly remove ImageHeight and ImageWidth from a uncompressed tiff and the file was no longer renderable, at least by Irfanview.

[Phil Harvey]

• Are PixelXDimension and ExifImageWidth actually the same tags?
• Is it correct that these tags shouldn't be present in uncompressed files?

• Yes.
• Yes.

- Phil