DPI is a tuple

[hugovk]

Fix merged PR #2449, re: 977f319#commitcomment-21546844

DPI should be an (xres, yres) tuple.

Confirmed in code and docs:

The open() method may set the following info properties if available:
...
dpi
A tuple representing the reported pixel density in pixels per inch, if the file is a jfif file and the units are in inches.
...
The save() method supports the following options:
...
dpi
A tuple of integers representing the pixel density, (x,y).

Note: one test image (Tests/images/exif_gps.jpg) only has an X value, and so raised exceptions, so if there's no Y value, use the X twice.

[wiredfool]

This could fail with a keyerror?

[hugovk]

Theoretically, yes. I'll wrap in another try/except later.

[hugovk]

Updated, but see below.

[hugovk]

It's better to check the spec before implementing things...

http://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/EXIF.html says:

• Exif.Image.XResolution: "The number of pixels per <ResolutionUnit> in the <ImageWidth> direction. When the image resolution is unknown, 72 [dpi] is designated."
• Exif.Image.YResolution: "The number of pixels per <ResolutionUnit> in the <ImageLength> direction. The same value as <XResolution> is designated."
• Exif.Image.ResolutionUnit: "The unit for measuring <XResolution> and <YResolution>. The same unit is used for both <XResolution> and <YResolution>. If the image resolution is unknown, 2 (inches) is designated."

And http://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/EXIF.html gives these for resolution unit:

(the value 1 is not standard EXIF)
1 = None
2 = inches
3 = cm

So I'll update this again:

• we don't need to check for YResolution, it's the same as XResolution - is that correct?
• if no value, default to 72,72 dpi
• should also check the unit: if inches, do nothing; if cm, convert to inches; else default to inches

[wiredfool]

@hugovk , you happy with this?

[hugovk]

@wiredfool Yes.

Feel free to squash and merge if you'd like to tidy up the commits.

[wiredfool]

someone is screen shotting on a high dpi monitor...

[hugovk]

Yes, that was a surprisingly large image!