Open Event Server Ticket PDF: Where and Where-not to use static frame in xhtml2pdf

One among the very important features of Open Event Server project is the tickets sales feature, where a user can buy a number of different tickets for a number of people after which he is given a link to download the ticket pdf. However, an issue concerning our Open Event Server project was that if a buyer bought different tickets at a time with different individual ticket holders, all tickets contained the same name, type and QR-Code, which in no way was acceptable since tickets and holders were different.

We use the xhtml2pdf facility in order to convert an html to pdf. An xhtml2pdf facility helps us in converting HTML contents into PDF without the use of browser ‘print’ facility. In order to do this, we use the help of pages and frames, pages being the page of the PDF document, while a frame being that part of area within the page where the contents get stored.

What is a PDF and HTML?

The basic understanding of a PDF and an HTML is that, a PDF or Portable Document Format has layout such that it is measured in terms of specific width and height. However, for an HTML or Hyper Text Markup Language, we do no not have those specific widths and heights. Rather an HTML’s width depends on a person’s device of view and height can be infinitely long as desired.

In terms of xhtml2pdf relation with pages and frames layout, we can identify with this diagram:

+-page———————+
|                                    |
|  +-content_frame-+  |
|  |                          |    |
|  |                          |    |
|  |                          |    |
|  |                          |    |
|  +———————+   |
|                                    |
+—————————-+


The stated issue was due to static frame in xhtml2pdf.

What is that you ask?

Static frames vs Content frames
xhtml2pdf uses the concept of Static Frames to define content that remains the same across different pages (like headers and footers), and uses Content Frames to position the to-be-converted HTML content.

Static Frames are defined through use of the @frame property -pdf-frame-content. Regular HTML content will not flow through Static Frames.

Content Frames are @frame objects without this property defined. Regular HTML content will flow through Content Frames.(xhtml2pdf documentation)

So, the basic idea of the use of  -pdf-frame-content  was to make the contents of the pdf static i.e. without having to continuously change alignments of the page. It made the whole pdf stay static.

This actually caused the whole pdf content to stay constant, event while the loop was going over different name, QR-code and ticket-name values. That means the first loop content stayed over even with changes in values.

Just a simple fix of not making the frame static was the solution.

Here is a few insight of what we had before that was causing the issue:

 <style>
        @page {
            size: a4 portrait;
            background-image: url('{{ base_dir }}/static/data/ticket-trans-
                                  notext.png');
            margin: 0;

            @frame col1_frame {             /* Content frame 1 */
                left: 80pt;  top: 30pt;
                height: 250pt; width: 300pt;
                -pdf-frame-content: main_content;
            }

            @frame qrcode {
                left: 455pt;  top: 50pt;
                width: 100pt; height: 120pt;
                -pdf-frame-content: qr_code;
            }
            @frame number_frame {
                 left: 455pt;  top: 25pt;
                width: 100pt; height: 30pt;
                -pdf-frame-content: number_content;
            }


             @frame col2_frame {
                left: 440pt; top: 170pt;
                -pdf-frame-content: personal_content;
            }
        }


Here,
main_content, qrcode, personal_content and number_content? are simply the class you want to make static.
The following was done to fix it:

 <style>
        @page {
            size: a4 portrait;
            background-image: url('{{ base_dir }}/static/data/ticket-trans-
                                  notext.png');
            margin: 0;

            @frame col1_frame {             /* Content frame 1 */
                left: 80pt;  top: 30pt;
                height: 250pt; width: 300pt;
            }

            @frame qrcode {
                left: 455pt;  top: 50pt;
                width: 100pt; height: 120pt;
            }
            @frame number_frame {
                 left: 455pt;  top: 25pt;
                width: 100pt; height: 30pt;
                -pdf-frame-content: number_content;
            }


             @frame col2_frame {
                left: 440pt; top: 170pt;
            }
        }



But the main question was, ‘where and where-not to use static frame’.
So the simple answer to that is, when you want the contents to stay exactly the same over the different pages of the pdf (for example the company logo or the administrator signature, etc), then you can happily get on with -pdf-frame-content. i.e:

          -pdf-frame-content: <class_name>


If not, or you need to loop over different values, then use Content frames.

Also, for more detailed understanding of PDF and HTML pages and frames, we have this good documentation written about it here: https://github.com/xhtml2pdf/xhtml2pdf/blob/master/doc/source/format_html.rst#static-frames-vs-content-frames