The PDF preview is all good, but it’s hard, finickeity and feels dirty
to embed a PDF in a web page. It’s a more natural thing to embed an
image in a web page.
So this commit adds another endpoint to return an image of a letter
template. It generates this image from the PDF preview, so the stack
looks like:
1. `template.png` (generated in admin)
2. `template.pdf` (generated in admin)
3. HTML preview (generated by a `Renderer` in utils)
4. `Template` instance
5. serialised template from API
6. Template stored in database
The library used to convert the PDF to an image is Wand[1], which binds
to ImageMagick underneath. So in order to get this working locally on a
Mac you will probably need to do:
`brew install imagemagick ghostscript cairo pango`.
To get it working on Ubuntu/EC2 is an exercise left to the reader…
1. http://docs.wand-py.org/en/0.4.4/
Previewing a letter is different to previewing an email or text message
because:
- a letter has a layout
- the layout is fixed, ie it doesn’t depend on the user’s device
- the ‘send yourself a test’ feature won’t be as useful because it has
a lead time, so the feedback loop will be much longer
For these reasons a HTML-only preview of the letter won’t be enough (we
don’t think). A PDF is more appropriate because:
- it can replicate the layout of the letter exactly
- it is a print format, so the user could even print themselves a copy
locally to get a feel for how it will look
This commit makes use of Flask WeasyPrint [1] to take a HTML
representation of the letter, convert it to a PDF and serve it back to
the user.
The actual work to generate the HTML and specify the layout is done in
utils, same as we do for rendering other messages.
1. https://pythonhosted.org/Flask-WeasyPrint/
