Added "exact" option when saving WebP

Tests/test_file_webp_alpha.py

@@ -97,6 +97,35 @@ def test_write_rgba(tmp_path):
             assert_image_similar(image, pil_image, 1.0)
 
 
+def test_keep_rgb_values_when_transparent(tmp_path):
+    """
+    Saving transparent pixels should retain their original RGB values
+    when using the "exact" parameter.
+    """
+
+    image = hopper("RGB")
+
+    # create a copy of the image
+    # with the left half transparent
+    half_transparent_image = image.copy()
+    new_alpha = Image.new("L", (128, 128), 255)
+    new_alpha.paste(0, (0, 0, 64, 128))
+    half_transparent_image.putalpha(new_alpha)
+
+    # save with transparent area preserved
+    temp_file = str(tmp_path / "temp.webp")
+    half_transparent_image.save(temp_file, exact=True, lossless=True)
+
+    with Image.open(temp_file) as reloaded:
+        assert reloaded.mode == "RGBA"
+        assert reloaded.format == "WEBP"
+
+        # even though it is lossless, if we don't use exact=True
+        # in libwebp >= 0.5, the transparent area will be filled with black
+        # (or something more conducive to compression)
+        assert_image_equal(reloaded.convert("RGB"), image)
+
+
 def test_write_unsupported_mode_PA(tmp_path):
     """
     Saving a palette-based file with transparency to WebP format

docs/handbook/image-file-formats.rst

@@ -1124,6 +1124,11 @@ The :py:meth:`~PIL.Image.Image.save` method supports the following options:
 **method**
     Quality/speed trade-off (0=fast, 6=slower-better). Defaults to 4.
 
+**exact**
+    If true, preserve the transparent RGB values. Otherwise, discard
+    invisible RGB values for better compression. Defaults to false.
+    Requires libwebp 0.5.0 or later.
+
 **icc_profile**
     The ICC Profile to include in the saved file. Only supported if
     the system WebP library was built with webpmux support.

docs/releasenotes/9.4.0.rst

@@ -37,6 +37,14 @@ support a ``start`` argument. This tuple of horizontal and vertical offset
 will be used internally by :py:meth:`.ImageDraw.text` to more accurately place
 text at the ``xy`` coordinates.
 
+Added the ``exact`` encoding option for WebP
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The ``exact`` encoding option for WebP is now supported. The WebP encoder
+removes the hidden RGB values for better compression by default in libwebp 0.5
+or later. By setting this option to ``True``, the encoder will keep the hidden
+RGB values.
+
 Security
 ========
 

src/PIL/WebPImagePlugin.py

@@ -318,6 +318,7 @@ def _save(im, fp, filename):
         exif = exif[6:]
     xmp = im.encoderinfo.get("xmp", "")
     method = im.encoderinfo.get("method", 4)
+    exact = 1 if im.encoderinfo.get("exact") else 0
 
     if im.mode not in _VALID_WEBP_LEGACY_MODES:
         alpha = (
@@ -336,6 +337,7 @@ def _save(im, fp, filename):
         im.mode,
         icc_profile,
         method,
+        exact,
         exif,
         xmp,
     )

src/_webp.c

@@ -576,6 +576,7 @@ WebPEncode_wrapper(PyObject *self, PyObject *args) {
     int lossless;
     float quality_factor;
     int method;
+    int exact;
     uint8_t *rgb;
     uint8_t *icc_bytes;
     uint8_t *exif_bytes;
@@ -597,7 +598,7 @@ WebPEncode_wrapper(PyObject *self, PyObject *args) {
 
     if (!PyArg_ParseTuple(
             args,
-            "y#iiifss#is#s#",
+            "y#iiifss#iis#s#",
             (char **)&rgb,
             &size,
             &width,
@@ -608,6 +609,7 @@ WebPEncode_wrapper(PyObject *self, PyObject *args) {
             &icc_bytes,
             &icc_size,
             &method,
+            &exact,
             &exif_bytes,
             &exif_size,
             &xmp_bytes,
@@ -633,6 +635,10 @@ WebPEncode_wrapper(PyObject *self, PyObject *args) {
     config.lossless = lossless;
     config.quality = quality_factor;
     config.method = method;
+#if WEBP_ENCODER_ABI_VERSION >= 0x0209
+    // the "exact" flag is only available in libwebp 0.5.0 and later
+    config.exact = exact;
+#endif
 
     // Validate the config
     if (!WebPValidateConfig(&config)) {