Distinguishing between CairoSVGSurface and CairoPDFSurface

[MaximeBouton]

When trying to write a cairo surface to a file, it is not possible to distinguish whether the surface object is a pdf or svg surface.

MWE:

using Cairo 

function Base.write(filename::String, surface::Cairo.CairoSurfaceIOStream)
    finish(surface)
    seek(surface.stream, 0)
    open(filename, "w") do io
        write(io, read(surface.stream, String))
    end
    return
end

c = CairoSVGSurface(IOBuffer(), 1000, 600);
write("output.svg", c) # this works fine 
write("output.pdf", c) # this would also have worked but the output file would not really be a pdf

Is there a convenient way to enforce that the file extension matches the type of Cairo stream?
Right now both CairoSVGSurface and CairoPDFSurface outputs a CairoSurfaceIOStream which makes it not possible to dispatch between the two.

[lobingera]

What are you trying to do? The logic of libcairo is, that you decide the type of surface before starting to draw and to render. If you are looking for a lazy/late decision, you might look at recordingSurfaces.
btw: the non-stream (with filename) surface constructors still exist (although libcairo documentation discourages the use).

[MaximeBouton]

We have a visualization package that relies on Cairo: https://github.com/sisl/AutoViz.jl
The main usage is through a render function, that accepts a CairoSurface as keyword argument (default to CairoSVGSurface).

Now we would like users to be able to save the rendered scenes in different format using write.
If they want a PDF they would have to pass in a CairoPDFSurface to the function which is fine, but we were wondering if there was a way to warn user when they try to do something like this:

c = render([roadway, veh1, veh2], camera=CarFollowCamera(...))  # default to SVGSurface 
write("scene.pdf", c) 

This code will run fine but the user might think they have a pdf and instead they have an svg.

The correct way would have been to do:

c = render([roadway, veh1, veh2], camera=CarFollowCamera(...), surface=CairoPDFSurface(...))
write("scene.pdf", c) 

[lobingera]

I see.
a) One could get the 'type' of a surface by looking via the surface type ptr into the c-struct of a cairo surface i.e.

struct _cairo_surface {
    const cairo_surface_backend_t *backend;
    cairo_device_t *device;

    /* We allow surfaces to override the backend->type by shoving something
     * else into surface->type. This is for "wrapper" surfaces that want to
     * hide their internal type from the user-level API. */
    cairo_surface_type_t type;

    cairo_content_t content;

    cairo_reference_count_t ref_count;
    cairo_status_t status;

b) workaround could be to look at the actual stream, as both PDF and SVG have headers (files need to start with a sequence of know bytes)

c) take the information along your 'c' output (type) of the render function.
d) ... as someone not believing in file extensions -> you cannot avoid users to do wrong.

[MaximeBouton]

Thanks for the tips, I will give a shot to a)!

[MaximeBouton]

this worked:

typ = ccall((:cairo_surface_get_type, Cairo.libcairo), Cint, (Ptr{Nothing},), c.ptr)
if typ == Cairo.CAIRO_SURFACE_TYPE_PDF 
    # do stuff 
end 

Thanks!