Transparent stroke/fill breaks SVG context

[kimikage]

At least since 2014, when I learned about Gadfly.jl, using transparent (i.e. alpha != 1.0) strokes and fills breaks the SVG context (i.e. stroke-opacity and fill-opacity). Is this a known issue or desired behavior?

Compose.jl/src/svg.jl

Lines 608 to 615 in e08b5ca

	function print_property(img::SVG, property::StrokePrimitive)
	if property.color.alpha != 1.0
	@printf(img.out, " stroke=\"%s\" stroke-opacity=\"%0.3f\"",
	svg_fmt_color(color(property.color)), property.color.alpha)
	else
	@printf(img.out, " stroke=\"%s\"", svg_fmt_color(color(property.color)))
	end
	end

Compose.jl/src/svg.jl

Lines 617 to 624 in e08b5ca

	function print_property(img::SVG, property::FillPrimitive)
	if property.color.alpha != 1.0
	@printf(img.out, " fill=\"%s\" fill-opacity=\"%0.3f\"",
	svg_fmt_color(color(property.color)), property.color.alpha)
	else
	@printf(img.out, " fill=\"%s\"", svg_fmt_color(color(property.color)))
	end
	end

Of course, adding stroke-opacity="1"/fill-opacity="1" for opaque colors also causes context breaking.

This problem can be avoided by specifying stroke-opacity and fill-opacity manually.
If it is an official solution, we need to fix the code of Gadfly. The stroke(nothing) sets stroke-opacity to zero.

While it is not a complete solution, at least, it is better to distinguish between the strokes/fills which actually exist but fully-transparent, and no-stroke/no-fill.

Compose.jl/src/property.jl

Line 54 in 2b561b0

stroke(c::Nothing) = Stroke([StrokePrimitive(RGBA{Float64}(0, 0, 0, 0))])

Compose.jl/src/property.jl

Line 71 in 2b561b0

fill(c::Nothing) = Fill([FillPrimitive(RGBA{Float64}(0.0, 0.0, 0.0, 0.0))])

[Mattriks]

While work has been done to fix fillopacity (#314, #318, #319, #322, #332), strokeopacity needs work. Note fillopacity is documented, but not strokeopacity.

[kimikage]

The following is the case with "explicit" fillopacity(a).

Compose.jl/src/property.jl

Lines 212 to 214 in e08b5ca

	fillopacity(value)
	Define a fill opacity, where 0≤value≤1. For svg, nested contexts will inherit from parent contexts e.g. `(context(), fillopacity(a), (context(), fill(c::String), circle()))`.

For users who understand the inheritance in SVG, like us, this explanation may be sufficient. What I want to emphasize in this issue is that transparent colors implicitly change stroke-opacity/fill-opacity and opaque colors not.

I think at least additional docs are needed. In particular, the latter nothing is reminiscent of none for users familiar with SVG.

[Mattriks]

The way I think fill(RGBA(r,g,b,a)) and fillopacity(a) should currently work is explained in GiovineItalia/Gadfly.jl#1290 (comment). Yes the design/inheritance rules could be better documented. Because Compose uses multiple backends, Compose inheritance may not work in precisely the same way as a particular backend. Color inheritance (different from opacity inheritance) would also work better if svg2's rgba spec was implemented (#319), but users don't like that rgba is not supported by Inkscape.

Perhaps you could implement the rgba spec in Inkscape?

[kimikage]

Perhaps you could implement the rgba spec in Inkscape?

Inkscape places importance on conforming to the standard, so I think it is difficult to implement it within SVG1.

BTW, SVG 1.2 Full was canceled, so version="1.2" is just for SVG Tiny (i.e. with baseProfile="tiny").

Compose.jl/src/svg.jl

Line 342 in e08b5ca

version="1.2"

I believe this problem can be mitigated by being able to determine whether fill-opacity was set by FillOpacityPrimitive or FillPrimitive.