Add test and test files for ATI1 pixel format and fix image mode for ATI1

Tests/test_file_dds.py

@@ -10,6 +10,7 @@ from .helper import assert_image_equal, assert_image_equal_tofile, hopper
 TEST_FILE_DXT1 = "Tests/images/dxt1-rgb-4bbp-noalpha_MipMaps-1.dds"
 TEST_FILE_DXT3 = "Tests/images/dxt3-argb-8bbp-explicitalpha_MipMaps-1.dds"
 TEST_FILE_DXT5 = "Tests/images/dxt5-argb-8bbp-interpolatedalpha_MipMaps-1.dds"
+TEST_FILE_ATI1 = "Tests/images/ati1.dds"
 TEST_FILE_ATI2 = "Tests/images/ati2.dds"
 TEST_FILE_DX10_BC5_TYPELESS = "Tests/images/bc5_typeless.dds"
 TEST_FILE_DX10_BC5_UNORM = "Tests/images/bc5_unorm.dds"
@@ -63,6 +64,18 @@ def test_sanity_dxt5():
     assert_image_equal_tofile(im, TEST_FILE_DXT5.replace(".dds", ".png"))
 
 
+def test_sanity_ati1():
+    """Check ATI1 images can be opened"""
+
+    with Image.open(TEST_FILE_ATI1) as im:
+        im.load()
+
+        assert im.format == "DDS"
+        assert im.mode == "L"
+        assert im.size == (128, 128)
+
+        assert_image_equal_tofile(im, TEST_FILE_ATI1.replace(".dds", ".png"))
+
 def test_sanity_ati2():
     """Check ATI2 images can be opened"""
 

src/PIL/DdsImagePlugin.py

@@ -195,7 +195,7 @@ class DdsImageFile(ImageFile.ImageFile):
             elif fourcc == b"ATI1":
                 self.pixel_format = "BC4"
                 n = 4
-                self.mode = "RGB"
+                self.mode = "L"
             elif fourcc == b"ATI2":
                 self.pixel_format = "BC5"
                 n = 5