go-libwebp

commit c39ff56bebedaa274ffee3e7237a470701a77a91
parent 2b3a104db86fd2d93cbf1e09d0ea819d479398e2
Author: Jack Mordaunt <jackmordaunt.dev@gmail.com>
Date:   Sun, 20 Sep 2026 15:31:26 -0300

webp: cover lossy decode in the backend tests

Every existing test went through VP8L: the golden is lossless and the fuzz
targets only decode when lossless is set. Nothing decoded a lossy image
through a pure-Go backend, so a transpiled build whose VP8 bit reader aborted
on the first call passed the whole suite.

Diffstat:
M
webp/backend_transpiled_test.go
 | 
4
++++
M
webp/backend_wazero_test.go
 | 
4
++++
M
webp/webp_test.go
 | 
25
+++++++++++++++++++++++++

3 files changed, 33 insertions(+), 0 deletions(-)
diff --git a/webp/backend_transpiled_test.go b/webp/backend_transpiled_test.go
@@ -41,6 +41,10 @@ func TestTranspiledDecodesCurrentGolden(t *testing.T) {
 	}
 }
 
+func TestTranspiledDecodesLossy(t *testing.T) {
+	assertLossyDecode(t, transpiled.EncodeImpl, transpiled.DecodeImpl)
+}
+
 func FuzzEncodeTranspiled(f *testing.F) {
 	addEncodeSeeds(f)
 	f.Fuzz(func(t *testing.T, x1, y1 uint16, seed int64, quality float32, lossless bool) {
diff --git a/webp/backend_wazero_test.go b/webp/backend_wazero_test.go
@@ -38,6 +38,10 @@ func TestWasmRoundTrip(t *testing.T) {
 	assertOutput(t, m, &buf, decodeReaderWith(wasm.DecodeImpl))
 }
 
+func TestWasmDecodesLossy(t *testing.T) {
+	assertLossyDecode(t, wasm.EncodeImpl, wasm.DecodeImpl)
+}
+
 func FuzzEncodeWasm(f *testing.F) {
 	addEncodeSeeds(f)
 	f.Fuzz(func(t *testing.T, x1, y1 uint16, seed int64, quality float32, lossless bool) {
diff --git a/webp/webp_test.go b/webp/webp_test.go
@@ -122,6 +122,31 @@ func decodeReaderWith(fn func([]byte) (image.Image, error)) func(io.Reader) (ima
 	}
 }
 
+// assertLossyDecode encodes lossily and decodes the result through the same
+// backend, checking dimensions rather than pixels.
+//
+// This is the VP8 decode path, which is entirely separate from the VP8L path
+// the golden and the fuzz seeds exercise. A transpiled build that fell back
+// to inline-asm byte swapping passed every other test and aborted here.
+func assertLossyDecode(t *testing.T,
+	encode func(io.Writer, *image.NRGBA, float32) error,
+	decode func([]byte) (image.Image, error),
+) {
+	t.Helper()
+	m := goldenNRGBA(t)
+	var buf bytes.Buffer
+	if err := encode(&buf, m, 0.75); err != nil {
+		t.Fatalf("lossy encode: %v", err)
+	}
+	got, err := decode(buf.Bytes())
+	if err != nil {
+		t.Fatalf("lossy decode: %v", err)
+	}
+	if got.Bounds() != m.Bounds() {
+		t.Fatalf("bounds: got %v want %v", got.Bounds(), m.Bounds())
+	}
+}
+
 func save(t *testing.T, name string, m image.Image) {
 	t.Helper()