Choose drawing size after placing contents?

[evanfields]

Toy example: let's say I wanted to draw a bunch of circles in a row, but I don't know a priori how many.

@drawsvg begin
    background("antiquewhite")
    n_circles = rand(1:20)
    for _ in 1:n_circles
        circle(O, :10, :stroke)
        translate(30, 0)
    end
end 200 50

If n_circles is too big, some of the circles won't fit in the drawing. Is there a way to draw things on some abstract canvas and then choose the drawing size?

Of course in this example n_circles could be precomputed outside the @drawsvg macro and from there a width and height computed. But in practice it may not be so easy to know the width and height needed before hand, for example if objects are placed using Luxor's drawing-specific geometric tools like relative translations.

Also, even in this toy example, precomputing width and height offloads to the user some manual geometric calculation (how far does the right side of the drawing need to be? Well, the first circle at centered-origin is centered at x = width/2 from the left side ... then each successive circle ... plus the stroke width ... plus some buffer ...).

So in summary, I'm hoping for a Luxor-idiomatic way to do two related but separate things:

1. Find the bounding box of all objects drawn
2. [more importantly] resize the drawing surface before rendering/serialization to cover a specified bounding box, whether that box comes from a magic function or is simply manually calculated by the user

Thanks in advance for any help!

[cormullion]

Hi! I don't think there's anything simple that will do this.

Interactive drawing applications probably store all graphics 'in the abstract' and then render them on any size drawing at scale/position, etc. And image editing applications can probably copy a buffer quickly onto another surface of a different size. With Cairo/Luxor, it's more like when you start to draw on a sheet of paper or whiteboard and then realize too late that you started out in the wrong place... :)

The nearest you're going to get at present is by using a recording surface and snapshots. I don't know anything about these (the feature was coded by @hustf :)), but the idea is that you specify a bounding box when you output the file. Of course, you still have to be able to keep track of and calculate the bounding box somehow (since graphics are inaccessible once they've been drawn).

Drawing(NaN, NaN, :rec) # no bounds 
background("antiquewhite")
n_circles = 5
margin = 10
@layer begin # non-permanent transformations 
    for _ in 1:n_circles
        randomhue()
        circle(O, 10, :fill)
        translate(30, 0)
    end
end
snapshot(fname="/tmp/temp.svg", 
    cb=BoundingBox(Point(-margin, -100), Point(150, 100)))
finish()

[cormullion]

Talking of recording surfaces, there appears to be a Cairo function called cairo_recording_surface_ink_extents() (https://cairographics.org/manual/cairo-Recording-Surfaces.html#cairo-recording-surface-ink-extents). This suggests that it could calculate the bounding box required to enclose the recording surface. However, my abilities in calling Cairo functions - and my understanding of the recording surface feature - are limited, and my attempt below doesn't work.

function recording_surface_ink_extents()
    x0 = Cdouble[0]
    y0 = Cdouble[0]
    w = Cdouble[0]
    h = Cdouble[0]
    d = Luxor._get_current_drawing_save()
    if d.surfacetype === :rec
        ccall((:cairo_recording_surface_ink_extents, Luxor.Cairo.libcairo),
            Ptr{Nothing},
            (Ptr{Nothing}, Ptr{Cdouble}, Ptr{Cdouble}, Ptr{Cdouble}, Ptr{Cdouble}),
            d.surface.ptr, x0, y0, w, h)
        return BoundingBox(Point(x0[1], y0[1]), Point(x0[1] + w[1], y0[1] + h[1]))
    else
        throw(error("not a recording surface"))
    end
end

See JuliaGraphics/Cairo.jl#356 for upstream solution.

[evanfields]

Hi! I don't think there's anything simple that will do this.

Thanks for the speedy response! I think the recording surface + snapshot looks like a great solution, and luckily in my case manually tracking the bounding box isn't too hard. From the docs on snapshot referencing drawing it seems like the snapshots can also be in memory with fname = :svg? Seems excellent.

[cormullion]

If you encounter problems, it night be worth running on the current master. There’s been some recent work on this which hasn’t yet been released. (Here’s a place to start: #150 )

[hustf]

Yes, this complicates things fast, which is why it's not in Luxor currently. Also, a surprising number of Luxor functions uses the limits defined in Drawing. In #258, I linked to a proposal, which makes alternative functions for limitless drawings.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.