Context CoordView not scaling path coordinates correctly with `renderers/rasterizer`

[DeedleFake]

The following program, assuming that I'm understanding things correctly, should create a 100x200 image and draw a triangle in it with vertices at (10,10), (10,190), and (90,190). However, it actually draws at (10,10), (10,11), and (11,11).

package main

import (
	"image/png"
	"os"

	"github.com/tdewolff/canvas"
	"github.com/tdewolff/canvas/renderers/rasterizer"
)

func main() {
	r := rasterizer.New(100, 200, 1, nil)

	c := canvas.NewContext(r)
	c.SetCoordSystem(canvas.CartesianIV)
	c.SetCoordRect(canvas.Rect{10, 10, 90, 190}, 1, 1)

	c.MoveTo(0, 0)
	c.LineTo(0, 1)
	c.LineTo(1, 1)
	c.Close()
	c.Fill()

	err := png.Encode(os.Stdout, r)
	if err != nil {
		panic(err)
	}
}

Changing the calls to

c.LineTo(0, 180)
c.LineTo(80, 180)

draws the expected triangle.

[DeedleFake]

I tried changing the vertex calls to

c.MoveTo(1, 1)
c.LineTo(0, 1)
c.LineTo(0, 0)

to see if maybe only the MoveTo() was getting transformed correctly, but it made no difference. It still puts that coordinate at (11,11). The scaling seems to just not apply at all, but the translation does.

[DeedleFake]

Also of note, transforming the coordinates manually as in the following code works perfectly:

package main

import (
	"image/png"
	"os"

	"github.com/tdewolff/canvas"
	"github.com/tdewolff/canvas/renderers/rasterizer"
)

func t(x, y float64, m canvas.Matrix) (float64, float64) {
	p := m.Dot(canvas.Point{X: x, Y: y})
	return p.X, p.Y
}

func main() {
	r := rasterizer.New(100, 200, 1, nil)

	c := canvas.NewContext(r)
	c.SetCoordSystem(canvas.CartesianIV)

	m := canvas.Identity.Translate(10, 10).Scale(80, 180)
	c.MoveTo(t(1, 1, m))
	c.LineTo(t(0, 1, m))
	c.LineTo(t(0, 0, m))
	c.Close()
	c.Fill()

	err := png.Encode(os.Stdout, r)
	if err != nil {
		panic(err)
	}
}

[tdewolff]

Interesting project! There are two types of transformations: those of coordinates (positions where objects are located) and those of the view. When drawing an object with DrawPath, DrawText, or DrawImage you specify the coordinate, which is transformed by the coordinate system or CoordView. The actual path/text/image is transformed by the View.

Since the MoveTo/LineTo/Close/... commands build an internal path and draw it at coordinate 0,0, it doesn't work the way you think it should. Perhaps those direct commands would need to be transformed by the coordinate view, and thus behave differently than building the path independently and calling DrawPath. What do you think?

[DeedleFake]

It may just need to be documented what exactly it does. I assumed that CoordView would transform any coordinates that were passed, and nothing in the documentation that I saw contradicted that. Transforming the path manually before passing it isn't complicated. It's just non-obvious.