I've found that on my El Capitan Retina MacBook, a screenshot will contain a chunk with identifier “iDOT” containing 28 bytes of data. Given its designation, that chunk type should be public, but in a question about this on Stack Overflow, Glenn Randers-Pehrson stated in a comment that no such chunk type had been registered so far. So I wonderWhat information does that chunk contain?Is this in some way related to the pixel density at the time the screenshot was taken?Where is the specification for this?Are there any plans to register the chunk type?How can that data be accessed programmatically?What's the meaning of the often reported error messageImageIO: PNG invalid PNG file: iDOT doesn't point to valid IDAT chunk

iDOT PNG chunk | Apple Developer Forums

iDOT PNG chunk

I've found that on my El Capitan Retina MacBook, a screenshot will contain a chunk with identifier “iDOT” containing 28 bytes of data. Given its designation, that chunk type should be public, but in a question about this on Stack Overflow, Glenn Randers-Pehrson stated in a comment that no such chunk type had been registered so far. So I wonder

What information does that chunk contain?
Is this in some way related to the pixel density at the time the screenshot was taken?
Where is the specification for this?
Are there any plans to register the chunk type?
How can that data be accessed programmatically?
What's the meaning of the often reported error message
ImageIO: PNG invalid PNG file: iDOT doesn't point to valid IDAT chunk