Athens - apply blur on arbitrary shape

Not possible. Not with Athens and not with cairo. (There was a discussion on cairos ml about drop shadows (and filter functions?, I am not sure) but

I don't think there is something like that currently).

Inkscape a vector/svg editor and viewer uses cairo but implements the svg filter rendering on its own.

You can pre-render the cairo image and apply a filter function and use the result as a form paint.

And you can cheat, and just compose multiple shape drawings with different alpha values, but of course, this is slow:

    | view star kernel s r root2pi |
    s := 3.
    r := 7.
    root2pi := (2 * Float pi) sqrt.
    kernel := (-1 * r to: r) collect: [ :i | 1 / (root2pi * s) * (-0.5 * (i / s) squared) exp ].
    view := AthensSceneView new.
    star := {32.0 @ 44.04868495317364.
    13.458980337503156 @ 57.519524250561204.
    20.541019662496844 @ 35.72324841040037.
    1.999999999999993 @ 22.252409113012813.
    24.917960675006306 @ 22.25240911301281.
    31.999999999999993 @ 0.4561332728519787.
    39.08203932499369 @ 22.252409113012803.
    62.0 @ 22.252409113012803.
    43.458980337503164 @ 35.72324841040037.
    50.54101966249685 @ 57.5195242505612}.
    view
        scene: [ :canvas |
            | path |
            path := canvas
                createPath: [ :builder |
                    builder
                        absolute;
                        moveTo: star first.
                    star allButFirstDo: [ :p | builder lineTo: p ].
                    builder close ].
            canvas surface clear: Color white.
            canvas paintMode over.
            r negated to: r do: [ :i |
                | a |
                a := kernel at: r + i + 1.
                r negated to: r do: [ :j |
                    | b |
                    b := a * kernel at: r + j + 1.
                    canvas setPaint: (Color black alpha: b).
                    canvas pathTransform
                        restoreAfter: [
                            | sx sy |
                            sx := canvas pathTransform sx.
                            sy := canvas pathTransform sy.
                            canvas pathTransform translateBy: (i / sx) @ (j / sy).
                            canvas drawShape: path ] ] ].
            canvas pathTransform
                restoreAfter: [
                    canvas pathTransform translateBy: (5 @ 5) negated.
                    canvas setPaint: Color red.
                    canvas drawShape: path ] ].
    view openInWindow