From 9f619b814f65a99c2107852027ecf7db4ddec7ec Mon Sep 17 00:00:00 2001
From: Andrew Murray <radarhere@users.noreply.github.com>
Date: Sat, 15 Mar 2025 00:21:34 +1100
Subject: [PATCH] Added BC3 loading and saving

---
 Tests/test_file_dds.py     | 14 ++++++++++++++
 src/PIL/DdsImagePlugin.py  | 17 +++++++++++++++--
 src/libImaging/BcnEncode.c |  2 +-
 3 files changed, 30 insertions(+), 3 deletions(-)

diff --git a/Tests/test_file_dds.py b/Tests/test_file_dds.py
index 17d88451f..3c7c8e604 100644
--- a/Tests/test_file_dds.py
+++ b/Tests/test_file_dds.py
@@ -12,6 +12,7 @@ from PIL import DdsImagePlugin, Image
 from .helper import (
     assert_image_equal,
     assert_image_equal_tofile,
+    assert_image_similar,
     assert_image_similar_tofile,
     hopper,
 )
@@ -114,6 +115,19 @@ def test_sanity_ati1_bc4u(image_path: str) -> None:
         assert_image_equal_tofile(im, TEST_FILE_ATI1.replace(".dds", ".png"))
 
 
+def test_dx10_bc3(tmp_path: Path) -> None:
+    out = str(tmp_path / "temp.dds")
+    with Image.open(TEST_FILE_DXT5) as im:
+        im.save(out, pixel_format="BC3")
+
+    with Image.open(out) as reloaded:
+        assert reloaded.format == "DDS"
+        assert reloaded.mode == "RGBA"
+        assert reloaded.size == (256, 256)
+
+        assert_image_similar(im, reloaded, 3.69)
+
+
 @pytest.mark.parametrize(
     "image_path",
     (
diff --git a/src/PIL/DdsImagePlugin.py b/src/PIL/DdsImagePlugin.py
index 2d097fd16..a5e0a712b 100644
--- a/src/PIL/DdsImagePlugin.py
+++ b/src/PIL/DdsImagePlugin.py
@@ -419,6 +419,10 @@ class DdsImageFile(ImageFile.ImageFile):
                     self._mode = "RGBA"
                     self.pixel_format = "BC1"
                     n = 1
+                elif dxgi_format in (DXGI_FORMAT.BC3_TYPELESS, DXGI_FORMAT.BC3_UNORM):
+                    self._mode = "RGBA"
+                    self.pixel_format = "BC3"
+                    n = 3
                 elif dxgi_format in (DXGI_FORMAT.BC4_TYPELESS, DXGI_FORMAT.BC4_UNORM):
                     self._mode = "L"
                     self.pixel_format = "BC4"
@@ -521,14 +525,18 @@ def _save(im: Image.Image, fp: IO[bytes], filename: str | bytes) -> None:
     flags = DDSD.CAPS | DDSD.HEIGHT | DDSD.WIDTH | DDSD.PIXELFORMAT
     bitcount = len(im.getbands()) * 8
     pixel_format = im.encoderinfo.get("pixel_format")
-    if pixel_format in ("DXT1", "DXT5"):
+    if pixel_format in ("DXT1", "BC3", "DXT5"):
         codec_name = "bcn"
         flags |= DDSD.LINEARSIZE
         pitch = (im.width + 3) * 4
         args = pixel_format
         rgba_mask = [0, 0, 0, 0]
         pixel_flags = DDPF.FOURCC
-        fourcc = D3DFMT.DXT1 if pixel_format == "DXT1" else D3DFMT.DXT5
+        fourcc = {"DXT1": D3DFMT.DXT1, "BC3": D3DFMT.DX10, "DXT5": D3DFMT.DXT5}[
+            pixel_format
+        ]
+        if fourcc == D3DFMT.DX10:
+            dxgi_format = DXGI_FORMAT.BC3_TYPELESS
     else:
         codec_name = "raw"
         flags |= DDSD.PITCH
@@ -573,6 +581,11 @@ def _save(im: Image.Image, fp: IO[bytes], filename: str | bytes) -> None:
         + struct.pack("<4I", *rgba_mask)  # dwRGBABitMask
         + struct.pack("<5I", DDSCAPS.TEXTURE, 0, 0, 0, 0)
     )
+    if fourcc == D3DFMT.DX10:
+        fp.write(
+            # dxgi_format, 2D resource, misc, array size, straight alpha
+            struct.pack("<5I", dxgi_format, 3, 0, 0, 1)
+        )
     ImageFile._save(im, fp, [ImageFile._Tile(codec_name, (0, 0) + im.size, 0, args)])
 
 
diff --git a/src/libImaging/BcnEncode.c b/src/libImaging/BcnEncode.c
index 57353246a..66f9f39b1 100644
--- a/src/libImaging/BcnEncode.c
+++ b/src/libImaging/BcnEncode.c
@@ -218,7 +218,7 @@ encode_bc3_alpha(Imaging im, ImagingCodecState state, UINT8 *dst) {
 int
 ImagingBcnEncode(Imaging im, ImagingCodecState state, UINT8 *buf, int bytes) {
     char *pixel_format = ((BCNSTATE *)state->context)->pixel_format;
-    int n = strcmp(pixel_format, "DXT5") == 0 ? 3 : 1;
+    int n = strcmp(pixel_format, "DXT1") == 0 ? 1 : 3;
     int has_alpha_channel =
         strcmp(im->mode, "RGBA") == 0 || strcmp(im->mode, "LA") == 0;