Added alpha_quality argument when saving WebP images

Tests/test_file_webp_alpha.py

@@ -151,3 +151,15 @@ def test_write_unsupported_mode_PA(tmp_path: Path) -> None:
             target = im.convert("RGBA")
 
         assert_image_similar(image, target, 25.0)
+
+
+def test_alpha_quality(tmp_path: Path) -> None:
+    with Image.open("Tests/images/transparent.png") as im:
+        out = str(tmp_path / "temp.webp")
+        im.save(out)
+
+        out_quality = str(tmp_path / "quality.webp")
+        im.save(out_quality, alpha_quality=50)
+        with Image.open(out) as reloaded:
+            with Image.open(out_quality) as reloaded_quality:
+                assert reloaded.tobytes() != reloaded_quality.tobytes()

Tests/test_file_webp_animated.py

@@ -188,3 +188,21 @@ def test_seek_errors() -> None:
 
         with pytest.raises(EOFError):
             im.seek(42)
+
+
+def test_alpha_quality(tmp_path: Path) -> None:
+    with Image.open("Tests/images/transparent.png") as im:
+        first_frame = Image.new("L", im.size)
+
+        out = str(tmp_path / "temp.webp")
+        first_frame.save(out, save_all=True, append_images=[im])
+
+        out_quality = str(tmp_path / "quality.webp")
+        first_frame.save(
+            out_quality, save_all=True, append_images=[im], alpha_quality=50
+        )
+        with Image.open(out) as reloaded:
+            reloaded.seek(1)
+            with Image.open(out_quality) as reloaded_quality:
+                reloaded_quality.seek(1)
+                assert reloaded.tobytes() != reloaded_quality.tobytes()

docs/handbook/image-file-formats.rst

@@ -1234,11 +1234,15 @@ The :py:meth:`~PIL.Image.Image.save` method supports the following options:
     If present and true, instructs the WebP writer to use lossless compression.
 
 **quality**
-    Integer, 0-100, Defaults to 80. For lossy, 0 gives the smallest
+    Integer, 0-100, defaults to 80. For lossy, 0 gives the smallest
     size and 100 the largest. For lossless, this parameter is the amount
     of effort put into the compression: 0 is the fastest, but gives larger
     files compared to the slowest, but best, 100.
 
+**alpha_quality**
+    Integer, 0-100, defaults to 100. For lossy compression only. 0 gives the
+    smallest size and 100 is lossless.
+
 **method**
     Quality/speed trade-off (0=fast, 6=slower-better). Defaults to 4.
 

src/PIL/WebPImagePlugin.py

@@ -217,6 +217,7 @@ def _save_all(im, fp, filename):
     verbose = False
     lossless = im.encoderinfo.get("lossless", False)
     quality = im.encoderinfo.get("quality", 80)
+    alpha_quality = im.encoderinfo.get("alpha_quality", 100)
     method = im.encoderinfo.get("method", 0)
     icc_profile = im.encoderinfo.get("icc_profile") or ""
     exif = im.encoderinfo.get("exif", "")
@@ -296,6 +297,7 @@ def _save_all(im, fp, filename):
                     rawmode,
                     lossless,
                     quality,
+                    alpha_quality,
                     method,
                 )
 
@@ -310,7 +312,7 @@ def _save_all(im, fp, filename):
         im.seek(cur_idx)
 
     # Force encoder to flush frames
-    enc.add(None, round(timestamp), 0, 0, "", lossless, quality, 0)
+    enc.add(None, round(timestamp), 0, 0, "", lossless, quality, alpha_quality, 0)
 
     # Get the final output from the encoder
     data = enc.assemble(icc_profile, exif, xmp)
@@ -324,6 +326,7 @@ def _save_all(im, fp, filename):
 def _save(im, fp, filename):
     lossless = im.encoderinfo.get("lossless", False)
     quality = im.encoderinfo.get("quality", 80)
+    alpha_quality = im.encoderinfo.get("alpha_quality", 100)
     icc_profile = im.encoderinfo.get("icc_profile") or ""
     exif = im.encoderinfo.get("exif", b"")
     if isinstance(exif, Image.Exif):
@@ -343,6 +346,7 @@ def _save(im, fp, filename):
         im.size[1],
         lossless,
         float(quality),
+        float(alpha_quality),
         im.mode,
         icc_profile,
         method,

src/_webp.c

@@ -195,6 +195,7 @@ _anim_encoder_add(PyObject *self, PyObject *args) {
     char *mode;
     int lossless;
     float quality_factor;
+    float alpha_quality_factor;
     int method;
     WebPConfig config;
     WebPAnimEncoderObject *encp = (WebPAnimEncoderObject *)self;
@@ -203,7 +204,7 @@ _anim_encoder_add(PyObject *self, PyObject *args) {
 
     if (!PyArg_ParseTuple(
             args,
-            "z#iiisifi",
+            "z#iiisiffi",
             (char **)&rgb,
             &size,
             &timestamp,
@@ -212,6 +213,7 @@ _anim_encoder_add(PyObject *self, PyObject *args) {
             &mode,
             &lossless,
             &quality_factor,
+            &alpha_quality_factor,
             &method)) {
         return NULL;
     }
@@ -229,6 +231,7 @@ _anim_encoder_add(PyObject *self, PyObject *args) {
     }
     config.lossless = lossless;
     config.quality = quality_factor;
+    config.alpha_quality = alpha_quality_factor;
     config.method = method;
 
     // Validate the config
@@ -578,6 +581,7 @@ WebPEncode_wrapper(PyObject *self, PyObject *args) {
     int height;
     int lossless;
     float quality_factor;
+    float alpha_quality_factor;
     int method;
     int exact;
     uint8_t *rgb;
@@ -601,13 +605,14 @@ WebPEncode_wrapper(PyObject *self, PyObject *args) {
 
     if (!PyArg_ParseTuple(
             args,
-            "y#iiifss#iis#s#",
+            "y#iiiffss#iis#s#",
             (char **)&rgb,
             &size,
             &width,
             &height,
             &lossless,
             &quality_factor,
+            &alpha_quality_factor,
             &mode,
             &icc_bytes,
             &icc_size,
@@ -637,6 +642,7 @@ WebPEncode_wrapper(PyObject *self, PyObject *args) {
     }
     config.lossless = lossless;
     config.quality = quality_factor;
+    config.alpha_quality = alpha_quality_factor;
     config.method = method;
 #if WEBP_ENCODER_ABI_VERSION >= 0x0209
     // the "exact" flag is only available in libwebp 0.5.0 and later