How to Calculate Leading in PDF Document

Question

How do I calculate leading in a PDF document?

For example:

48 0 0 48 72 677.28 Tm
(Hello World) Tj
0 -1.1075 TD

This renders the text Hello World at 48pt/57.6pt (120% line height) in Times-Roman.

According to the PDF Reference manual, "the leading parameter is measured in unscaled text space units. It specifies the vertical distance between the baselines of adjacent lines of text... The number is expressed in thousandths of a unit of text space."

Can someone please explain how 1.1075 and 57.6 are related?

Answer 1

The leading is usually set in the PDF by the command TL, just like this:

12 TL
(El ingenioso hidalgo don Quijote de la Mancha)'

That 12 indicates a leading of 12 points until another TL is found.

I hope it helps you. I think this is the easiest way to do it :)

Answer 2

You pdf commands is incorrect. I suppose you mean:

48 0 0 48 72 677.28 Tm
0 -1.1075 TD    
(Hello World) Tj

This code set text coordinate system to (Tm command):

Scale x48 on x and x48 on y 
Start position (72, 677.28)

Then it's move position to next line. Next line in 1.1075 "text" pixels. And then move start position by -1.1075 "text" pixels on y coordinate. Text pixel in this example it's pdf pixel multiplyed by 48. It's set by Tm command.

I may simplify you PDF code. It's the same:

48 0 0 48 72 570.096 Tm
(Hello World) Tj

Explanation: 677.28 - (1.1075*48) - (1.1075*48)

YOU should always remember that PDF it's a language. To calculate the real coordinates you shoud parse all previous commands. There may be something like this before you commands:

10 0 0 10 0 0 cm