Where does the 1/72 PDFBox value come from?

Question

I'm extracting images from the PDF page using the PDFBox. In the example I used as a basis ( PrintImageLocations ), the value of 72 dpi is used for calculation. My question is, where does this value 72 come from?

// position in user space units. 1 unit = 1/72 inch at 72 dpi
System.out.println("position in PDF = " + ctmNew.getTranslateX() + ", " + ctmNew.getTranslateY() + " in user space units");

// raw size in pixels
System.out.println("raw image size  = " + imageWidth + ", " + imageHeight + " in pixels");

// displayed size in user space units
System.out.println("displayed size  = " + imageXScale + ", " + imageYScale + " in user space units");

// displayed size in inches at 72 dpi rendering
imageXScale /= 72;
imageYScale /= 72;
 System.out.println("displayed size  = " + imageXScale + ", " + imageYScale + " in inches at 72 dpi rendering");

// displayed size in millimeters at 72 dpi rendering
imageXScale *= 25.4;
imageYScale *= 25.4;
System.out.println("displayed size  = " + imageXScale + ", " + imageYScale + " in millimeters at 72 dpi rendering");

Answer 1

Not the most technical of answers... but its been a "standard" for some time... one that is arbitrary and rather silly... Here's a random article that talks about its silliness.

https://petapixel.com/2020/02/13/why-wont-the-72dpi-myth-die/

PDF is closer to being a collection of pixels like a bitmap, than it is to being a token based document like a text file. So for sizing elements on the screen/page it has to assume certain resolution... Because 72dpi was so prevalent for images for so long it makes sense that pdf followed suit.