After looking at the screenshot, I took a MacBook Pro with a Retina display and launched OS X 10.11, I found that it contains these pieces:
IHDR, iCCP, pHYs, iTXt, iDOT, IDAT…, IEND
All of this is part of the 2003 specification , with the exception of iDOT , which is a small (28 byte) fragment. In accordance with the block naming conventions , the fact that his second letter is capital should indicate that this is a piece with a public specification. However, I could not find its specification. It is not listed in the register of public PNG fragments and keywords, version 1.4.6 , although it looks like the latest version .
There are many sites on the Internet that mention this snippet, including many of them. Most of them describe error messages in strings.
ImageIO: PNG invalid PNG file: iDOT doesn't point to valid IDAT chunk
and those that were resolved discovered some kind of image distortion, which is not necessarily due to this fragment, or applied some kind of transformation that supposedly also deleted this fragment.
Many pages also mention Retina displays. I think and hope that this fragment somehow indicates the density of the display when the picture is taken. This would be massively useful for automatically scaling screenshots .
Edit: after taking a few more screenshots, I found that, apparently, the role of the pixel plays the role of: launching the display in its own resolution. I do not get such a fragment and image size, as shown in the screenshot. Only at a higher density do I get a piece and the size of a PNG image, which is an integer multiple of the one displayed. It seems that 28 bytes of data are 7 low-character 32-bit integers. For me it was (2, 0, h, 40, h, h, x) , where 2 supposed to indicate the pixel density, h is the apparent height of the image (i.e., half actually saved), and x is some number, which I don’t understand at all. I do not know how fractional pixel density will enter this game.
Where can I find the details and possibly even the specification of this piece? Should I contact Apple or the registry, or is there someone here who can provide more details?