line: optional bounds check

This has significant overhead so is made optional.

line.go

@@ -3,6 +3,7 @@ package caption
 import (
 	"image"
 	"image/color"
+	"syscall"
 
 	"github.com/golang/freetype/truetype"
 	"golang.org/x/image/draw"
@@ -20,11 +21,16 @@ type Line struct {
 
 	// contains reference to parsed font, populated once per instance
 	face font.Face
+	// whether to fail if pixels are drawn out-of-bounds
+	bounds bool
 }
 
 // SetFont clears line font cache and sets the new font.
 func (l *Line) SetFont(v []byte, opts *truetype.Options) { l.face = nil; l.Font = v; l.Options = opts }
 
+// SetBoundsCheck changes whether the frame's bounding box is enforced.
+func (l *Line) SetBoundsCheck(v bool) { l.bounds = v }
+
 type Segment struct {
 	Value string      `json:"value"`
 	Color color.Color `json:"color"`
@@ -59,12 +65,26 @@ func (l *Line) Render(ctx *Context, frame draw.Image, x, y int) error {
 		face = ctx.defaultFont
 	}
 
+	outOfBounds := false
+	bounds := frame.Bounds()
+	boundsFixed := fixed.R(bounds.Min.X, bounds.Min.Y, bounds.Max.X, bounds.Max.Y)
+
 	drawer := &font.Drawer{Dst: frame, Src: new(image.Uniform), Face: face, Dot: fixed.Point26_6{X: fixed.I(x), Y: fixed.I(y)}}
 	for _, seg := range l.Data {
 		drawer.Src.(*image.Uniform).C = seg.Color
+
+		// optional bounds check to reduce overhead
+		if l.bounds && !outOfBounds {
+			b, _ := drawer.BoundString(seg.Value)
+			outOfBounds = !b.In(boundsFixed)
+		}
+
 		drawer.DrawString(seg.Value)
 	}
 
+	if outOfBounds {
+		return syscall.EDOM
+	}
 	return nil
 }
 

line_test.go

@@ -7,6 +7,8 @@ import (
 	"image"
 	"image/color"
 	"image/png"
+	"runtime"
+	"syscall"
 	"testing"
 
 	"git.gensokyo.uk/yonah/caption"
@@ -33,6 +35,16 @@ func TestRender(t *testing.T) {
 		{"bad fast path font", &caption.Line{}, caption.NewContext([]byte{0xfd}, nil),
 			0, 0, "", truetype.FormatError("TTF data is too short")},
 
+		{"out of bounds top left", &caption.Line{
+			Data: []caption.Segment{{"Beispieltext", color.Black}},
+		}, nil, 0, 0, "", syscall.EDOM},
+		{"out of bounds right", &caption.Line{
+			Data: []caption.Segment{{"Beispieltext", color.Black}},
+		}, nil, 1<<10 - 0x3e, 1 << 8, "", syscall.EDOM},
+		{"right edge", &caption.Line{
+			Data: []caption.Segment{{"Beispieltext", color.Black}},
+		}, nil, 1<<10 - 0x3f, 1 << 8, "8037459938bcaa6c427412be2ec1e8e77f8ff0e4", nil},
+
 		{"custom line options", &caption.Line{
 			Data:    []caption.Segment{{"Beispieltext", color.Black}},
 			Options: &truetype.Options{Size: 24},
@@ -74,12 +86,14 @@ func TestRender(t *testing.T) {
 				ctx = sharedCtx
 			}
 
+			tc.line.SetBoundsCheck(true)
+
 			redraw := ""
 		redraw:
 			got := image.NewRGBA64(image.Rectangle{Max: image.Point{X: 1 << 10, Y: 1 << 9}})
 			err := tc.line.Render(ctx, got, tc.x, tc.y)
 			for _, f := range beforeCheck {
-				f(t, tc.name, redraw == "", got)
+				f(t, tc.name, redraw != "", got)
 			}
 
 			if !errors.Is(err, tc.wantErr) {
@@ -113,20 +127,56 @@ func TestRender(t *testing.T) {
 		})
 	}
 
+	scratchpad := image.NewRGBA64(image.Rectangle{Max: image.Point{X: 1 << 10, Y: 1 << 9}})
+
 	t.Run("bad global default", func(t *testing.T) {
 		wantErr := truetype.FormatError("TTF data is too short")
 		caption.OverrideGlobalDefault(t, []byte{0xfd})
 		err := (&caption.Line{Options: new(truetype.Options)}).
-			Render(
-				new(caption.Context),
-				image.NewRGBA64(image.Rectangle{Max: image.Point{X: 1 << 10, Y: 1 << 9}}),
-				0, 1<<8,
-			)
+			Render(new(caption.Context), scratchpad, 0, 1<<8)
 		if !errors.Is(err, wantErr) {
-			t.Errorf("Render: error = %q, want %q",
-				err, wantErr)
+			t.Errorf("Render: error = %q, want %q", err, wantErr)
+		}
+	})
+
+	t.Run("bounds check skip", func(t *testing.T) {
+		err := (&caption.Line{Data: []caption.Segment{{"Beispieltext", color.Black}}}).
+			Render(new(caption.Context), scratchpad, 0, 0)
+		if err != nil {
+			t.Errorf("Render: error = %q", err)
 		}
 	})
 }
 
 func c(r, g, b uint8) color.Color { return color.NRGBA{R: r, G: g, B: b, A: 0xff} }
+
+type stubImage struct {
+	image.Uniform
+}
+
+func (s *stubImage) Set(x, y int, c color.Color) {
+	runtime.KeepAlive(x)
+	runtime.KeepAlive(y)
+	runtime.KeepAlive(c)
+}
+
+func (s *stubImage) Bounds() image.Rectangle {
+	return image.Rectangle{Max: image.Point{X: 1 << 10, Y: 1 << 9}}
+}
+
+func benchmarkRender(b *testing.B, bounds bool) {
+	stub := new(stubImage)
+	stub.C = color.Black
+	ctx := new(caption.Context)
+	line := &caption.Line{Data: []caption.Segment{{"Beispieltext", color.Black}}}
+	line.SetBoundsCheck(bounds)
+	for i := 0; i < b.N; i++ {
+		err := line.Render(ctx, stub, 0, 1<<8)
+		if err != nil {
+			b.Fatalf("Render: error = %q", err)
+		}
+	}
+}
+
+func BenchmarkRender(b *testing.B)       { benchmarkRender(b, false) }
+func BenchmarkRenderBounds(b *testing.B) { benchmarkRender(b, true) }