It displays things on a screen

canvas/canvas.go

@@ -14,18 +14,19 @@ type Canvas struct {
 }
 
 // New creates a new canvas from a bounding rectangle.
-func New (x *xgbutil.XUtil, bounds image.Rectangle) Canvas {
-	return Canvas { xgraphics.New(x, bounds) }
+func New (x *xgbutil.XUtil, bounds image.Rectangle) *Canvas {
+	return NewFrom(xgraphics.New(x, bounds))
 }
 
 // NewFrom creates a new canvas from an existing xgraphics.Image.
-func NewFrom (image *xgraphics.Image) Canvas {
-	return Canvas { image }
+func NewFrom (image *xgraphics.Image) *Canvas {
+	if image == nil { return nil }
+	return &Canvas { image }
 }
 
 // Pen returns a new drawing context.
-func (this Canvas) Pen () canvas.Pen {
-	return pen {
+func (this *Canvas) Pen () canvas.Pen {
+	return &pen {
 		image: this.Image,
 		gfx:   ggfx.Image[uint8] {
 			Pix:    this.Image.Pix,
@@ -37,16 +38,25 @@ func (this Canvas) Pen () canvas.Pen {
 }
 
 // Clip returns a sub-canvas of this canvas.
-func (this Canvas) Clip (bounds image.Rectangle) canvas.Canvas {
-	return Canvas { this.Image.SubImage(bounds).(*xgraphics.Image) }
+func (this *Canvas) Clip (bounds image.Rectangle) canvas.Canvas {
+	this.assert()
+	subImage := this.Image.SubImage(bounds)
+	if subImage == nil { return nil }
+	xImage := subImage.(*xgraphics.Image)
+	return &Canvas { xImage }
 }
 
 // Push pushes this canvas to the screen.
-func (this Canvas) Push (window xproto.Window) {
+func (this *Canvas) Push (window xproto.Window) {
+	this.assert()
 	this.XDraw()
 	this.XExpPaint(window, this.Bounds().Min.X, this.Bounds().Min.Y)
 }
 
+func (this *Canvas) assert () {
+	if this == nil { panic("nil canvas") }
+}
+
 // TODO: we need to implement:
 //  - cap
 //  - joint
@@ -65,7 +75,7 @@ type pen struct {
 	fill   [4]uint8
 }
 
-func (this pen) Rectangle (bounds image.Rectangle) {
+func (this *pen) Rectangle (bounds image.Rectangle) {
 	if this.weight == 0 {
 		this.gfx.FillRectangle(this.fill[:], bounds)
 	} else {
@@ -73,7 +83,7 @@ func (this pen) Rectangle (bounds image.Rectangle) {
 	}
 }
 
-func (this pen) Path (points ...image.Point) {
+func (this *pen) Path (points ...image.Point) {
 	if this.weight == 0 {
 		this.gfx.FillPolygon(this.fill[:], points...)
 	} else if this.closed {
@@ -83,21 +93,21 @@ func (this pen) Path (points ...image.Point) {
 	}
 }
  
-func (this pen) Closed       (closed bool)              { this.closed = closed }
-func (this pen) Cap          (endCap canvas.Cap)        { this.endCap = endCap }
-func (this pen) Joint        (joint  canvas.Joint)      { this.joint  = joint  }
-func (this pen) StrokeWeight (weight int)               { this.weight = weight }
-func (this pen) StrokeAlign  (align canvas.StrokeAlign) { this.align  = align  }
+func (this *pen) Closed       (closed bool)              { this.closed = closed }
+func (this *pen) Cap          (endCap canvas.Cap)        { this.endCap = endCap }
+func (this *pen) Joint        (joint  canvas.Joint)      { this.joint  = joint  }
+func (this *pen) StrokeWeight (weight int)               { this.weight = weight }
+func (this *pen) StrokeAlign  (align canvas.StrokeAlign) { this.align  = align  }
 
-func (this pen) Stroke (stroke color.Color) { this.stroke = convertColor(stroke) }
-func (this pen) Fill (fill color.Color)     { this.fill   = convertColor(fill)   }
+func (this *pen) Stroke (stroke color.Color) { this.stroke = convertColor(stroke) }
+func (this *pen) Fill (fill color.Color)     { this.fill   = convertColor(fill)   }
 
 func convertColor (c color.Color) [4]uint8 {
 	r, g, b, a := c.RGBA()
 	return [4]uint8 {
 		uint8(b >> 8),
 		uint8(g >> 8),
-		uint8(a >> 8),
 		uint8(r >> 8),
+		uint8(a >> 8),
 	}
 }