caption: basic line rendering

.gitignore

@@ -0,0 +1,25 @@
+# Binaries for programs and plugins
+*.exe
+*.exe~
+*.dll
+*.so
+*.dylib
+*.pkg
+
+# Test binary, built with `go test -c`
+*.test
+
+# Output of the go coverage tool, specifically when used with LiteIDE
+*.out
+
+# Dependency directories (remove the comment below to include it)
+# vendor/
+
+# Go workspace file
+go.work
+go.work.sum
+
+# env file
+.env
+.idea
+.vscode

caption.go

@@ -0,0 +1,10 @@
+// Package caption provides a caption rendering interface that prioritises ease of use.
+package caption
+
+import (
+	"golang.org/x/image/font/gofont/goregular"
+)
+
+var (
+	defaultFont = goregular.TTF
+)

caption_sample_test.go

@@ -0,0 +1,30 @@
+//go:build export_samples
+
+package caption_test
+
+import (
+	"image"
+	"image/png"
+	"os"
+	"path"
+	"strings"
+	"testing"
+)
+
+func init() {
+	beforeCheck = append(beforeCheck, func(t *testing.T, name string, got image.Image) {
+		t.Run("export", func(t *testing.T) {
+			outPath := strings.TrimSpace(os.Getenv("TEST_RENDER_OUT_PATH"))
+			if len(outPath) == 0 {
+				outPath = os.TempDir()
+			}
+			outPath = path.Join(outPath, "overlay-"+name+".png")
+			if re, err := os.Create(outPath); err != nil {
+				t.Fatalf("cannot create: %v", err)
+			} else if err = (&png.Encoder{CompressionLevel: png.NoCompression}).Encode(re, got); err != nil {
+				t.Fatalf("cannot encode to file: %v", err)
+			}
+			t.Logf("test case exported to %s", outPath)
+		})
+	})
+}

caption_test.go

@@ -0,0 +1,75 @@
+package caption_test
+
+import (
+	"crypto/sha1"
+	"encoding/hex"
+	"errors"
+	"image"
+	"image/color"
+	"image/png"
+	"testing"
+
+	"git.gensokyo.uk/yonah/caption"
+	"github.com/golang/freetype/truetype"
+)
+
+var (
+	beforeCheck []func(t *testing.T, name string, got image.Image)
+)
+
+func TestRender(t *testing.T) {
+	testCases := []struct {
+		name    string
+		line    *caption.Line
+		x, y    int
+		want    string
+		wantErr error
+	}{
+		{"bad font", &caption.Line{Font: []byte{0xfd}},
+			0, 0, "", truetype.FormatError("TTF data is too short")},
+		{"colors", &caption.Line{Data: []caption.Segment{
+			{"Blue", c(0x55, 0xcd, 0xfc)},
+			{"Pink", c(0xf7, 0xa8, 0xb8)},
+			{"White", color.White},
+			{"Pink", c(0xf7, 0xa8, 0xb8)},
+			{"Blue", c(0x55, 0xcd, 0xfc)},
+		}}, 7 << 6, 1 << 8, "2af4b09361aeb3a9befc79cc0b28dd3874657fa0", nil},
+	}
+
+	var digest []byte
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			got := image.NewRGBA64(image.Rectangle{Max: image.Point{X: 1 << 10, Y: 1 << 9}})
+			err := tc.line.Render(got, tc.x, tc.y)
+			if err != nil {
+				if !errors.Is(err, tc.wantErr) {
+					t.Errorf("Render: error = %q, want %q",
+						err, tc.wantErr)
+				}
+				return
+			}
+
+			for _, f := range beforeCheck {
+				f(t, tc.name, got)
+			}
+
+			t.Run("hash", func(t *testing.T) {
+				var wantDigest []byte
+				if wantDigest, err = hex.DecodeString(tc.want); err != nil {
+					t.Errorf("cannot parse expected hash: %v", err)
+				}
+
+				h := sha1.New()
+				if err = (&png.Encoder{CompressionLevel: png.NoCompression}).Encode(h, got); err != nil {
+					t.Fatalf("cannot encode png for hashing: %v", err)
+				}
+				digest := h.Sum(digest[:0])
+				if string(digest) != string(wantDigest) {
+					t.Errorf("Render: %x, want %x", digest, wantDigest)
+				}
+			})
+		})
+	}
+}
+
+func c(r, g, b uint8) color.Color { return color.NRGBA{R: r, G: g, B: b, A: 0xff} }

go.mod

@@ -0,0 +1,8 @@
+module git.gensokyo.uk/yonah/caption
+
+go 1.24.4
+
+require (
+	github.com/golang/freetype v0.0.0-20170609003504-e2365dfdc4a0
+	golang.org/x/image v0.29.0
+)

go.sum

@@ -0,0 +1,4 @@
+github.com/golang/freetype v0.0.0-20170609003504-e2365dfdc4a0 h1:DACJavvAHhabrF08vX0COfcOBJRhZ8lUbR+ZWIs0Y5g=
+github.com/golang/freetype v0.0.0-20170609003504-e2365dfdc4a0/go.mod h1:E/TSTwGwJL78qG/PmXZO1EjYhfJinVAhrmmHX6Z8B9k=
+golang.org/x/image v0.29.0 h1:HcdsyR4Gsuys/Axh0rDEmlBmB68rW1U9BUdB3UVHsas=
+golang.org/x/image v0.29.0/go.mod h1:RVJROnf3SLK8d26OW91j4FrIHGbsJ8QnbEocVTOWQDA=

line.go

@@ -0,0 +1,52 @@
+package caption
+
+import (
+	"image"
+	"image/color"
+
+	"github.com/golang/freetype/truetype"
+	"golang.org/x/image/draw"
+	"golang.org/x/image/font"
+	"golang.org/x/image/math/fixed"
+)
+
+type Line struct {
+	// collection of all text segments
+	Data []Segment `json:"data"`
+	// applies to all segments; avoid direct assignment and use SetFont instead
+	Font []byte `json:"font"`
+	// applies to all segments; avoid direct assignment and use SetFont instead
+	Options *truetype.Options `json:"options"`
+
+	// contains reference to parsed font, populated once per instance
+	face font.Face
+}
+
+// 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 }
+
+type Segment struct {
+	Value string      `json:"value"`
+	Color color.Color `json:"color"`
+}
+
+func (l *Line) Render(frame draw.Image, x, y int) error {
+	if l.face == nil {
+		if len(l.Font) == 0 {
+			l.SetFont(defaultFont, l.Options)
+		}
+		if f, err := truetype.Parse(l.Font); err != nil {
+			return err
+		} else {
+			l.face = truetype.NewFace(f, l.Options)
+		}
+	}
+
+	drawer := &font.Drawer{Dst: frame, Face: l.face, Dot: fixed.Point26_6{X: fixed.I(x), Y: fixed.I(y)}}
+	for _, seg := range l.Data {
+		drawer.Src = image.NewUniform(seg.Color)
+		drawer.DrawString(seg.Value)
+	}
+
+	return nil
+}