Fix single pixel lines

internal/painter/draw.go

@@ -47,6 +47,9 @@ func DrawLine(line *canvas.Line, vectorPad float32, scale func(float32) float32)
 	width := int(scale(line.Size().Width + vectorPad*2))
 	height := int(scale(line.Size().Height + vectorPad*2))
 	stroke := scale(line.StrokeWidth)
+	if stroke < 1 { // software painter doesn't fade lines to compensate
+		stroke = 1
+	}
 
 	raw := image.NewRGBA(image.Rect(0, 0, width, height))
 	scanner := rasterx.NewScannerGV(int(line.Size().Width), int(line.Size().Height), raw, raw.Bounds())
@@ -56,6 +59,17 @@ func DrawLine(line *canvas.Line, vectorPad float32, scale func(float32) float32)
 	p1x, p1y := scale(line.Position1.X-line.Position().X+vectorPad), scale(line.Position1.Y-line.Position().Y+vectorPad)
 	p2x, p2y := scale(line.Position2.X-line.Position().X+vectorPad), scale(line.Position2.Y-line.Position().Y+vectorPad)
 
+	if stroke <= 1.5 { // adjust to support 1px
+		if p1x == p2x {
+			p1x -= 0.5
+			p2x -= 0.5
+		}
+		if p1y == p2y {
+			p1y -= 0.5
+			p2y -= 0.5
+		}
+	}
+
 	dasher.Start(rasterx.ToFixedP(float64(p1x), float64(p1y)))
 	dasher.Line(rasterx.ToFixedP(float64(p2x), float64(p2y)))
 	dasher.Stop(true)

internal/painter/gl/draw.go

@@ -113,6 +113,17 @@ func (p *glPainter) lineCoords(pos, pos1, pos2 fyne.Position, lineWidth, feather
 	pos2.X = roundToPixel(pos2.X+xPosDiff, p.pixScale)
 	pos2.Y = roundToPixel(pos2.Y+yPosDiff, p.pixScale)
 
+	if lineWidth <= 0.5 && p.pixScale > 1 { // adjust to support 1px drawing on HiDPI (width 0.5)
+		if pos1.X == pos2.X {
+			pos1.X -= 0.25
+			pos2.X -= 0.25
+		}
+		if pos1.Y == pos2.Y {
+			pos1.Y -= 0.25
+			pos2.Y -= 0.25
+		}
+	}
+
 	x1Pos := pos1.X / frame.Width
 	x1 := -1 + x1Pos*2
 	y1Pos := pos1.Y / frame.Height
@@ -131,7 +142,7 @@ func (p *glPainter) lineCoords(pos, pos1, pos2 fyne.Position, lineWidth, feather
 	normalObjX := normalX * 0.5 * frame.Width
 	normalObjY := normalY * 0.5 * frame.Height
 	widthMultiplier := float32(math.Sqrt(float64(normalObjX*normalObjX + normalObjY*normalObjY)))
-	halfWidth := (lineWidth*0.5 + feather) / widthMultiplier
+	halfWidth := (roundToPixel(lineWidth+feather, p.pixScale) * 0.5) / widthMultiplier
 	featherWidth := feather / widthMultiplier
 
 	return []float32{

internal/painter/software/painter_test.go

@@ -222,6 +222,24 @@ func TestPainter_paintLine(t *testing.T) {
 	test.AssertImageMatches(t, "draw_line.png", p.Paint(c))
 }
 
+func TestPainter_paintLine_thin(t *testing.T) {
+	c := test.NewCanvas()
+	lines := [5]*canvas.Line{}
+	sws := []float32{4, 2, 1, 0.5, 0.3}
+	for i, sw := range sws {
+		lines[i] = canvas.NewLine(color.RGBA{255, 0, 0, 255})
+		lines[i].StrokeWidth = sw
+		x := float32(i * 20)
+		lines[i].Position1 = fyne.NewPos(x, 10)
+		lines[i].Position2 = fyne.NewPos(x+15, 10)
+	}
+	c.SetContent(container.NewWithoutLayout(lines[0], lines[1], lines[2], lines[3], lines[4]))
+	c.Resize(fyne.NewSize(109, 28))
+
+	p := software.NewPainter()
+	test.AssertImageMatches(t, "draw_line_thin.png", p.Paint(c))
+}
+
 func TestPainter_paintRaster(t *testing.T) {
 	img := canvas.NewRasterWithPixels(func(x, y, w, h int) color.Color {
 		x = x / 5