Added DXT3 saving

Tests/test_file_dds.py

@@ -436,6 +436,29 @@ def test_save_dxt1(tmp_path: Path) -> None:
         assert reloaded.getpixel((0, 0)) == (0, 0, 0, 0)
 
 
+def test_save_dxt3(tmp_path: Path) -> None:
+    # RGB
+    out = str(tmp_path / "temp.dds")
+    with Image.open(TEST_FILE_DXT3) as im:
+        im_rgb = im.convert("RGB")
+    im_rgb.save(out, pixel_format="DXT3")
+    assert_image_similar_tofile(im_rgb.convert("RGBA"), out, 1.26)
+
+    # RGBA
+    im.save(out, pixel_format="DXT3")
+    assert_image_similar_tofile(im, out, 3.81)
+
+    # L
+    im_l = im.convert("L")
+    im_l.save(out, pixel_format="DXT3")
+    assert_image_similar_tofile(im_l.convert("RGBA"), out, 5.89)
+
+    # LA
+    im_la = im.convert("LA")
+    im_la.save(out, pixel_format="DXT3")
+    assert_image_similar_tofile(im_la.convert("RGBA"), out, 8.44)
+
+
 def test_save_dxt5(tmp_path: Path) -> None:
     # RGB
     out = str(tmp_path / "temp.dds")

src/PIL/DdsImagePlugin.py

@@ -525,18 +525,24 @@ 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", "BC3", "DXT5"):
+    args: tuple[int] | str
+    if pixel_format in ("DXT1", "DXT3", "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 = {"DXT1": D3DFMT.DXT1, "BC3": D3DFMT.DX10, "DXT5": D3DFMT.DXT5}[
+        if pixel_format == "DXT1":
-            pixel_format
+            fourcc = D3DFMT.DXT1
-        ]
+            args = (1,)
-        if fourcc == D3DFMT.DX10:
+        elif pixel_format == "DXT3":
-            dxgi_format = DXGI_FORMAT.BC3_TYPELESS
+            fourcc = D3DFMT.DXT3
+            args = (2,)
+        else:
+            fourcc = D3DFMT.DXT5 if pixel_format == "DXT5" else D3DFMT.DX10
+            args = (3,)
+            if fourcc == D3DFMT.DX10:
+                dxgi_format = DXGI_FORMAT.BC3_TYPELESS
     else:
         codec_name = "raw"
         flags |= DDSD.PITCH

src/encode.c

@@ -360,19 +360,18 @@ PyImaging_BcnEncoderNew(PyObject *self, PyObject *args) {
     ImagingEncoderObject *encoder;
 
     char *mode;
-    char *pixel_format;
+    int n;
-    if (!PyArg_ParseTuple(args, "ss", &mode, &pixel_format)) {
+    if (!PyArg_ParseTuple(args, "si", &mode, &n)) {
         return NULL;
     }
 
-    encoder = PyImaging_EncoderNew(sizeof(BCNSTATE));
+    encoder = PyImaging_EncoderNew(0);
     if (encoder == NULL) {
         return NULL;
     }
 
     encoder->encode = ImagingBcnEncode;
-
+    encoder->state.state = n;
-    ((BCNSTATE *)encoder->state.context)->pixel_format = pixel_format;
 
     return (PyObject *)encoder;
 }

src/libImaging/BcnEncode.c

@@ -10,8 +10,6 @@
 
 #include "Imaging.h"
 
-#include "Bcn.h"
-
 typedef struct {
     UINT8 color[3];
 } rgb;
@@ -57,14 +55,18 @@ encode_bc1_color(Imaging im, ImagingCodecState state, UINT8 *dst, int separate_a
     int transparency = 0;
     for (i = 0; i < 4; i++) {
         for (j = 0; j < 4; j++) {
+            current_rgba = &block[i + j * 4];
+
             int x = state->x + i * im->pixelsize;
             int y = state->y + j;
             if (x >= state->xsize * im->pixelsize || y >= state->ysize) {
                 // The 4x4 block extends past the edge of the image
+                for (k = 0; k < 3; k++) {
+                    current_rgba->color[k] = 0;
+                }
                 continue;
             }
 
-            current_rgba = &block[i + j * 4];
             for (k = 0; k < 3; k++) {
                 current_rgba->color[k] =
                     (UINT8)im->image[y][x + (im->pixelsize == 1 ? 0 : k)];
@@ -152,6 +154,36 @@ encode_bc1_color(Imaging im, ImagingCodecState state, UINT8 *dst, int separate_a
     }
 }
 
+static void
+encode_bc2_block(Imaging im, ImagingCodecState state, UINT8 *dst) {
+    int i, j;
+    UINT8 block[16], current_alpha;
+    for (i = 0; i < 4; i++) {
+        for (j = 0; j < 4; j++) {
+            int x = state->x + i * im->pixelsize;
+            int y = state->y + j;
+            if (x >= state->xsize * im->pixelsize || y >= state->ysize) {
+                // The 4x4 block extends past the edge of the image
+                block[i + j * 4] = 0;
+                continue;
+            }
+
+            current_alpha = (UINT8)im->image[y][x + 3];
+            block[i + j * 4] = current_alpha;
+        }
+    }
+
+    for (i = 0; i < 4; i++) {
+        UINT16 l = 0;
+        for (j = 3; j > -1; j--) {
+            current_alpha = block[i * 4 + j];
+            l |= current_alpha << (j * 4);
+        }
+        *dst++ = l;
+        *dst++ = l >> 8;
+    }
+}
+
 static void
 encode_bc3_alpha(Imaging im, ImagingCodecState state, UINT8 *dst) {
     int i, j;
@@ -166,6 +198,7 @@ encode_bc3_alpha(Imaging im, ImagingCodecState state, UINT8 *dst) {
             int y = state->y + j;
             if (x >= state->xsize * im->pixelsize || y >= state->ysize) {
                 // The 4x4 block extends past the edge of the image
+                block[i + j * 4] = 0;
                 continue;
             }
 
@@ -217,17 +250,20 @@ 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 = state->state;
-    int n = strcmp(pixel_format, "DXT1") == 0 ? 1 : 3;
     int has_alpha_channel =
         strcmp(im->mode, "RGBA") == 0 || strcmp(im->mode, "LA") == 0;
 
     UINT8 *dst = buf;
 
     for (;;) {
-        if (n == 3) {
+        if (n == 2 || n == 3) {
             if (has_alpha_channel) {
-                encode_bc3_alpha(im, state, dst);
+                if (n == 2) {
+                    encode_bc2_block(im, state, dst);
+                } else {
+                    encode_bc3_alpha(im, state, dst);
+                }
                 dst += 8;
             } else {
                 for (int i = 0; i < 8; i++) {