WebP: Add test for vertical ALPH chunk filtering_method

Author: https://github.com/nico
Commit: 52d17afd7e
Pull-request: https://github.com/SerenityOS/serenity/pull/19330

Tests/LibGfx/TestImageDecoder.cpp

@@ -404,6 +404,30 @@ TEST_CASE(test_webp_extended_lossy_alpha_horizontal_filter)
     EXPECT_EQ(frame.image->get_pixel(131, 131), Gfx::Color(0x8f, 0x51, 0x2f, 0x4b));
 }
 
+TEST_CASE(test_webp_extended_lossy_alpha_vertical_filter)
+{
+    // Also lossy rgb + lossless alpha, but with a vertical alpha filtering method.
+    // The image should look like smolkling.webp, but with a vertical alpha gradient, and with a fully transparent first column.
+    auto file = MUST(Core::MappedFile::map(TEST_INPUT("smolkling-vertical-alpha.webp"sv)));
+    EXPECT(Gfx::WebPImageDecoderPlugin::sniff(file->bytes()));
+    auto plugin_decoder = MUST(Gfx::WebPImageDecoderPlugin::create(file->bytes()));
+    MUST(plugin_decoder->initialize());
+
+    EXPECT_EQ(plugin_decoder->frame_count(), 1u);
+    EXPECT(!plugin_decoder->is_animated());
+    EXPECT(!plugin_decoder->loop_count());
+
+    EXPECT_EQ(plugin_decoder->size(), Gfx::IntSize(264, 264));
+
+    auto frame = MUST(plugin_decoder->frame(0));
+    EXPECT_EQ(frame.image->size(), Gfx::IntSize(264, 264));
+
+    // While VP8 YUV contents are defined bit-exact, the YUV->RGB conversion isn't.
+    // So pixels changing by 1 or so below is fine if you change code.
+    // The important component in this test is alpha, and that shouldn't change even by 1 as it's losslessly compressed and doesn't use YUV.
+    EXPECT_EQ(frame.image->get_pixel(131, 131), Gfx::Color(0x94, 0x50, 0x32, 0x4c));
+}
+
 TEST_CASE(test_webp_extended_lossy_alpha_gradient_filter)
 {
     // Also lossy rgb + lossless alpha, but with a gradient alpha filtering method.

Userland/Libraries/LibGfx/ImageFormats/WebPLoader.cpp

@@ -290,7 +290,6 @@ static ErrorOr<void> decode_webp_chunk_ALPH(Chunk const& alph_chunk, Bitmap& bit
         // "Method 2: predictor = B"
         // "The top-left value at location (0, 0) uses 0 as predictor value. Otherwise,
         //  For vertical or gradient filtering methods, the top-most pixels at location (x, 0) are predicted using the location (x-1, 0) on the left."
-        // FIXME: This branch is untested.
         for (int x = 1; x < bitmap.width(); ++x)
             alpha[x] += alpha[x - 1];
         for (int y = 1; y < bitmap.height(); ++y) {