Question

I’m rendering some HTML in a QT QLabel. The HTML looks like this:

<pre>foo\tbar</pre> 

(note that I’ve put ‘\t’ where there is a tab chracter in the code).

This renders fine, but the tab character appears to be rendered as eight spaces, whereas I want it to be redered as 4. How can I change this without having to change the source HTML?

Answer 1

According to W3 (HTML4):

The horizontal tab character (decimal 9 in [ISO10646] and [ISO88591]) is usually interpreted by visual user agents as the smallest non-zero number of spaces necessary to line characters up along tab stops that are every 8 characters. We strongly discourage using horizontal tabs in preformatted text since it is common practice, when editing, to set the tab-spacing to other values, leading to misaligned documents.

It’s implementation-defined, essencially. Most, if not all, browsers/renderers use eight spaces for tabs. This cannot be configured in Qt.

It is, however somewhat trivial to go through your HTML and replace tabs with however many spaces you wish. Write a simple parser for that. Pseudocode:

for each <pre> block {     for each line in block {         position_in_line = 0          for each character in line {             if character is a tab {                 remove tab character                  do {                     add a space character                      ++position_in_line                 } while position_in_line % 8 != 0             } else {                 ++position_in_line             }         }     } } 

In case you’re curious, HTML3 specifies the use of eight-character tabs:

Within <PRE>, the tab should be interpreted to shift the horizontal column position to the next position which is a multiple of 8 on the same line; that is, col := (col+8) mod 8.