ngon and xgon are transformed incorrectly in UnitBox?

[tkf]

ngon and xgon are transformed incorrectly in UnitBox?

[Mattriks]

Currently, Compose is based on 3 type of units:

1. Context (position) units: cx, cy
2. Relative units: w, h
3. Absolute units: mm, inch

The issue that occurs above is because Compose needs a new unit that describes "size relative to context units".

4. Size wrt Context: sx, sy

Design example:

module t

using Compose

import Compose: Length, AbsoluteBox, Transform, width, resolve
import Base.(*)

size_x_measure(a::Measure) = a
size_x_measure(a::T) where T = Length{:sx, T}(a)

cmpgrid2(args...) = (context(), 
    line([[(x, 0), (x, 1)] for x in 0:0.1:1]), line([[(0, y), (1, y)] for y in 0:0.1:1]), args..., linewidth(0.1mm))

resolve(box::AbsoluteBox, units::UnitBox, t::Transform, a::Length{:sx}) = 
    a.value / width(units) * box.a[1]

end

Here I use a different red grid to better illustrate the current issues (green polygon). The new (blue) polygon is correct:

s = t.size_x_measure(0.3)
newpoly = polygon([(0.5cx, 0.5cy-s), (0.5cx+s, 0.5cy), (0.5cx, 0.5cy+s), (0.5cx-s, 0.5cy), (0.5cx, 0.5cy-s)])

set_default_graphic_size(10cm, 10cm)

img = compose(context(), t.cmpgrid2(stroke("gray")), fill("transparent"),
    (context(0.5, 0.5, 0.4, 0.4; units = UnitBox(-0.1, 0., 1.1, 1.)), t.cmpgrid2(stroke("red")),
        (context(), ngon(0.5, 0.5, 0.3, 4), stroke("green")),
        (context(), newpoly, stroke("blue"))
))

I'll open a PR sometime soon.