EncodeToPng() with custom ppi

Well, I'm not sure if Unity actually includes any "pHYs" chunk in the png data it generates, however i guess it doesn't. So it's possible to manually sneak in that chunk yourself. I haven't really played around with the PNG format yet (only with GIF and BMP) but it seems to be straight forward.

What your PNG file would need is a "pHYs" chunk which contains the physical properties of the image. That includes the "pixels per unit" for each axis and a unit specifier. Though currently the only actual unit supported is "metre". So if you want to specify "DPI" you need to convert it into DPM. 1 DPI is about 39.3701 DPM.

The only tricky part is that each chunk has a CRC at the end. So when creating / adding or modifying a chunk you need to calculate the CRC of that chunk. Though there's a reference implementation in C which should make things easier. The pHYs chunk could be placed right after the IHDR chunk

Maybe if i find the time i try to implements such a "modifier" for a PNG file.

edit
I did it ^^. I quickly hacked those PNGTools together. You can simply do

 byte[] data = someImage.EncodeToPNG();
 data = B83.Image.PNGTools.ChangePPI(data, 96, 96);

The actual functionality is rather small at the moment. Though i created the basic framework to work with PNG files and its chunktypes for future extension.

PNG uses BigEndian byte ordering everywhere. That makes the use of the BinaryReader / Writer a pain but i created some helpers (at the end) to read 32 and 64 bit integers in big endian format. It turns out i don't really need much more for this.

I can't really test this on systems which might already be big endian. If someone has found any issues, feel free to leave a comment (with details!! Posting "doesn't work" it pointless). Windows 10 file details doesn't really show any difference between an image with pHYs chunk and one without. However i opened the two image versions in Paint.NET (I'm not an artist, I don't really use Photoshop ^^) and in the "change size" menu it shows the PPM i've setup.