feat(imageutil): add upscale and numbered grid-overlay preprocessing

internal/imageutil/imageutil_test.go

@@ -1,12 +1,34 @@
 package imageutil
 
 import (
+	"bytes"
 	"encoding/base64"
+	"image"
+	_ "image/png" // register PNG decoder for test helpers
+	"image/color"
+	"image/png"
 	"os"
 	"path/filepath"
 	"testing"
 )
 
+// makePNG creates an in-memory PNG of the given size filled with a solid color.
+func makePNG(t *testing.T, w, h int) []byte {
+	t.Helper()
+	img := image.NewRGBA(image.Rect(0, 0, w, h))
+	c := color.RGBA{R: 100, G: 149, B: 237, A: 255}
+	for y := 0; y < h; y++ {
+		for x := 0; x < w; x++ {
+			img.SetRGBA(x, y, c)
+		}
+	}
+	var buf bytes.Buffer
+	if err := png.Encode(&buf, img); err != nil {
+		t.Fatalf("makePNG encode: %v", err)
+	}
+	return buf.Bytes()
+}
+
 func TestDecode(t *testing.T) {
 	raw := []byte{0x89, 0x50, 0x4e, 0x47} // PNG magic prefix
 	b64 := base64.StdEncoding.EncodeToString(raw)
@@ -36,6 +58,100 @@ func TestDecode(t *testing.T) {
 	})
 }
 
+func TestUpscale(t *testing.T) {
+	const srcW, srcH = 30, 30
+
+	t.Run("factor 3 doubles dims", func(t *testing.T) {
+		src := makePNG(t, srcW, srcH)
+		out, mt, err := Upscale(src, 3)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if mt != "image/png" {
+			t.Fatalf("want image/png, got %q", mt)
+		}
+		img, _, err := image.Decode(bytes.NewReader(out))
+		if err != nil {
+			t.Fatalf("result not decodable: %v", err)
+		}
+		b := img.Bounds()
+		if b.Dx() != srcW*3 || b.Dy() != srcH*3 {
+			t.Fatalf("want %dx%d, got %dx%d", srcW*3, srcH*3, b.Dx(), b.Dy())
+		}
+	})
+
+	t.Run("factor 10 clamps to 6", func(t *testing.T) {
+		src := makePNG(t, srcW, srcH)
+		out, _, err := Upscale(src, 10)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		img, _, err := image.Decode(bytes.NewReader(out))
+		if err != nil {
+			t.Fatalf("result not decodable: %v", err)
+		}
+		b := img.Bounds()
+		if b.Dx() != srcW*6 || b.Dy() != srcH*6 {
+			t.Fatalf("want %dx%d (clamped to 6), got %dx%d", srcW*6, srcH*6, b.Dx(), b.Dy())
+		}
+	})
+
+	t.Run("factor 1 unchanged dims", func(t *testing.T) {
+		src := makePNG(t, srcW, srcH)
+		out, mt, err := Upscale(src, 1)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if mt != "image/png" {
+			t.Fatalf("want image/png, got %q", mt)
+		}
+		img, _, err := image.Decode(bytes.NewReader(out))
+		if err != nil {
+			t.Fatalf("result not decodable: %v", err)
+		}
+		b := img.Bounds()
+		if b.Dx() != srcW || b.Dy() != srcH {
+			t.Fatalf("want %dx%d, got %dx%d", srcW, srcH, b.Dx(), b.Dy())
+		}
+	})
+}
+
+func TestAnnotateGrid(t *testing.T) {
+	const size = 90
+
+	t.Run("3x3 grid same dims", func(t *testing.T) {
+		src := makePNG(t, size, size)
+		out, err := AnnotateGrid(src, 3, 3)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		img, _, err := image.Decode(bytes.NewReader(out))
+		if err != nil {
+			t.Fatalf("result not decodable: %v", err)
+		}
+		b := img.Bounds()
+		if b.Dx() != size || b.Dy() != size {
+			t.Fatalf("want %dx%d, got %dx%d", size, size, b.Dx(), b.Dy())
+		}
+	})
+
+	t.Run("rows=0 returns valid image", func(t *testing.T) {
+		src := makePNG(t, size, size)
+		out, err := AnnotateGrid(src, 0, 3)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		img, _, err := image.Decode(bytes.NewReader(out))
+		if err != nil {
+			t.Fatalf("result not decodable: %v", err)
+		}
+		b := img.Bounds()
+		if b.Dx() != size || b.Dy() != size {
+			t.Fatalf("want %dx%d, got %dx%d", size, size, b.Dx(), b.Dy())
+		}
+	})
+}
+
 func TestGridCentroids(t *testing.T) {
 	pts := GridCentroids(300, 300, 3, 3)
 	if len(pts) != 9 {