How to read ISO Speed of an image in Octave? - matlab

Question

In MATLAB I used to go:
info = imfinfo('image.jpg');
iso = info.DigitalCamera.ISOSpeedRatings;
And that's it. Octave has an imfinfo function that reads many parameters from the image header, but apparently not the ISO speed. That looks strange to me (why would it read dozens of params and let out ISO speed?).
Is there something I am missing?
Example image is here, shot with a DSLR (as you can see with imfinfo). With MATLAB, I can clearly see that imfinfo('image.jpg').DigitalCamera.ISOSpeedRatings is 200.

Answer 1

tl;dr
The value you want is under the field PhotographicSensitivity. Use:
iso = imfinfo ("image.jpg").DigitalCamera.PhotographicSensitivity;
See Octave bug #47706.
Octave only considers tags that follow Exif version 2.3.
Workaround
Old versions of the Octave image package had a function readexif (the function was deprecated in image package version 2.2.0 and removed in version 2.4.0) which may be able to read them. The last version to include that function was 2.2.2
Discussion
The details you are accessing are the EXIF values on your image file. Basically, there's a table on your file that maps tags, such as ExposureTime or FNumber, to their actual values. However, the tag names are not encoded in the file. Instead, each tag has an integer value e.g., ExposureTime or FNumber will appear as 33437 and 33434 respectively. It's the job of a program/library to map those integer values to the tag name.
The tag you are looking for, ISOSpeedRatings is not a valid EXIF tag per the EXIF specs version 2.3 (the latest specs, released in 2010). It was renamed to PhotographicSensitivity. If you read the specs under that tag, you will find this note:
Note that this tag was referred to as “ISOSpeedRatings” in versions of this standard up to Version 2.21.
If you read even more of the specs, you will find the following explanation on Annex G:
[...] the PhotographicSensitivity tag whose ID was 34855 was previously referred to as ISOSpeedRating, which, in reality, led to confused interpretations of sensitivity that resulted in many recordings that differed from ISO speed as defined under the ISO 12232 standard.
And if you read even more of the specs, you will notice that the meaning of the value with the tag 34855 was slightly changed when its name changed. This means that it's not only a tag name issue, its meaning is different if your camera follows a different version of the EXIF specs (assuming that your camera recorded it correctly, remember that the reason for the change was that cameras were recording the wrong thing).
I'm not sure what is the right technical answer. I'm guessing a program should check the recorded ExifVersion (which is the EXIF tag 36864) and change the tag names according to that.
Edit
Octave uses GraphicsMagick to read images, including its metadata. GraphicsMagick does not accept the EXIF tag numbers, programs need to query using the tag names. Octave then has a hardcoded list of tag names but only includes the tags for Exif specs 2.3. It seems that GraphicsMagick recognizes your file is an older version of Exif and names the tag old way which Octave does not ask for.

Answer 2

I did the following and can now use readexif from 2.2.2 in image 2.6.1:
Download Image 2.2.2 as suggested earlier in this thread
Add the readexif.m from Image 2.2.2 among image 2.6.1
Reload package pkg load image
Now you can call readexif. To get ISOSpeedRating:
readexif(JPGPath).Exif_IFD_Pointer.ISOSpeedRatings