I cry every day I'm on windows and need to do those simple things which Preview made so easy while being light.
Thank you for this!
I don't think it was very common at Apple for an individual engineer to conceive, implement, and ship a feature like this. There was a general sentiment on the team of "let's do something with signatures," but we knew that very few people had scanners. We thought about touchpad input, but decided against it at the time. (That came much later, in either 10.11 or 10.12)
I was thinking, "well, almost no one has a scanner, but practically everyone using this application has a camera in their Mac." I built the initial prototype in OpenCV and then ported it to Apple's vDSP/Accelerate frameworks.
My favorite detail, which doesn't seem to be present in 10.12, is that you could just click on a horizontal line in a PDF; since I recorded the signature's offset relative to the baseline superimposed on the camera image, it would place the signature exactly on the line, with descenders nicely descending.
I've since moved on from macOS/iOS development, but I had a really positive experience at Apple. Met so many amazingly talented people there.
I used to hate PDFs before getting a Mac, but the first-class support in Preview, with features like that (along with the ability to tear out, reorder and attach pages) made me change my mind.
It must have saved many many millions of sheets of paper.
Quartz is like QuickTime plus something like OpenGL shaders plus something like NeXTStep "display PDF". I have no idea if this encouraged PDF integration into the display model.
Rather, I would say that NeXT, and then Apple, had some great IP and cross-licensing of PostScript and PDF display tech, and so they could ship the OS distribution with PDF as the printing model.
That is, one reason Windows might today re-render PDF → XPS → PDF is that they had needed to create display tech like PDF anyway, and so they did, and this was after humans had been playing with HTML for a while... Silverlight was pretty.
That's not what's happening unless you have a printer which directly supports PDF. Most likely what is happening on Windows is either:
PDF → XPS → PCL
PDF → XPS → PostScript
Depending on the printer. Which makes far more sense and is analogous to what OS X does: PDF → Quartz → PCL
PDF → Quartz → PostScript
There's really no notable difference between these two approaches as both Quartz and XPS are based on the same printing model as PDF. Don't confuse Quartz with PDF just because it offers import/export to PDF - Quartz has its own internal imaging model.