What limits does the matplotlib backend place on rendering formats?

Question

I'm confused about the role played by the backend used by matplotlib in determining what formats can be rendered.

For example, the documentation says that the 'agg' backend generates PNG, "raster graphics" but if I

import matplotlib
matplotlib.use(‘agg’)

import matplotlib.pyplot
fig, ax = matplotlib.pyplot.subplots()
#...

I can use

fig.savefig(“thefig.pdf”)

to generate a PDF, or

fig.savefig(“thefig.svg”)

to produce vector graphics.

What role does the backend play in limiting what formats and kinds of rendering (vector vs. raster) matplotlib can produce?

Answer 1

Good question! The Agg backend itself only produces raster graphics.

What's happening when you call fig.savefig('name.pdf') (or .svg) is that the backend is temporarily changed to generate vector output.

Each backend can choose how it handles this, but for Agg, when you save vector output, it basically does:

pdf = self.switch_backends(FigureCanvasPdf)
return pdf.print_pdf(*args, **kwargs)

Similarly, for the PDF backend, it temporarily switches to Agg if asked to save a raster image.

Switching backends can be done on the fly for non-interactive backends, so this technique is used extensively "behind-the-scenes" to allow figures to be saved in multiple formats.

---

A bit more detail, in case you're ever writing a matplotlib backend: Any matplotlib Canvas instance has several print_<format> methods:

In [24]: backend_bases.FigureCanvasBase.print_<tab>
backend_bases.FigureCanvasBase.print_bmp           
backend_bases.FigureCanvasBase.print_eps           
backend_bases.FigureCanvasBase.print_figure        
backend_bases.FigureCanvasBase.print_jpeg          
backend_bases.FigureCanvasBase.print_jpg           
backend_bases.FigureCanvasBase.print_pdf           
backend_bases.FigureCanvasBase.print_pgf           
backend_bases.FigureCanvasBase.print_png           
backend_bases.FigureCanvasBase.print_ps            
backend_bases.FigureCanvasBase.print_raw           
backend_bases.FigureCanvasBase.print_rgba          
backend_bases.FigureCanvasBase.print_svg           
backend_bases.FigureCanvasBase.print_svgz          
backend_bases.FigureCanvasBase.print_tif           
backend_bases.FigureCanvasBase.print_tiff 

The print_figure method controls saving in general. Saving to a specific format is handled by looking up the approriate print_<formatname> method (See FigureCanvasBase._get_print_method for exact details).

Each backend specifies which formats it can save to by overriding those methods. The default for each is to temporarily switch back to the relevant "base" backend (e.g. Agg for raster formats, PDF for pdf, SVG for svg, etc). This allows figures to be easily saved to all formats even if the backend you're using only supports a single format.