From 816674de365ee4ce460d94201346aea46c7dd9e7 Mon Sep 17 00:00:00 2001
From: Nico Weber <thakis@chromium.org>
Date: Thu, 8 Jun 2023 17:28:59 -0400
Subject: [PATCH] WebP: Add test for horizontal ALPH chunk filtering_method

---
 Tests/LibGfx/TestImageDecoder.cpp             | 25 +++++++++++++++++++
 .../LibGfx/ImageFormats/WebPLoader.cpp        |  1 -
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/Tests/LibGfx/TestImageDecoder.cpp b/Tests/LibGfx/TestImageDecoder.cpp
index 1ccca42af0..092d7c3fa7 100644
--- a/Tests/LibGfx/TestImageDecoder.cpp
+++ b/Tests/LibGfx/TestImageDecoder.cpp
@@ -348,6 +348,7 @@ TEST_CASE(test_webp_simple_lossless)
 
 TEST_CASE(test_webp_extended_lossy)
 {
+    // This extended lossy image has an ALPH chunk for (losslessly compressed) alpha data.
     auto file = MUST(Core::MappedFile::map(TEST_INPUT("extended-lossy.webp"sv)));
     EXPECT(Gfx::WebPImageDecoderPlugin::sniff(file->bytes()));
     auto plugin_decoder = MUST(Gfx::WebPImageDecoderPlugin::create(file->bytes()));
@@ -379,6 +380,30 @@ TEST_CASE(test_webp_extended_lossy)
     EXPECT_EQ(frame.image->get_pixel(341, 75), Gfx::Color(0, 0, 0, 128));
 }
 
+TEST_CASE(test_webp_extended_lossy_alpha_horizontal_filter)
+{
+    // Also lossy rgb + lossless alpha, but with a horizontal alpha filtering method.
+    // The image should look like smolkling.webp, but with a horizontal alpha gradient.
+    auto file = MUST(Core::MappedFile::map(TEST_INPUT("smolkling-horizontal-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(0x8f, 0x51, 0x2f, 0x4b));
+}
+
 TEST_CASE(test_webp_extended_lossy_uncompressed_alpha)
 {
     auto file = MUST(Core::MappedFile::map(TEST_INPUT("extended-lossy-uncompressed-alpha.webp"sv)));
diff --git a/Userland/Libraries/LibGfx/ImageFormats/WebPLoader.cpp b/Userland/Libraries/LibGfx/ImageFormats/WebPLoader.cpp
index 56b9c3854c..5e2417c1b4 100644
--- a/Userland/Libraries/LibGfx/ImageFormats/WebPLoader.cpp
+++ b/Userland/Libraries/LibGfx/ImageFormats/WebPLoader.cpp
@@ -276,7 +276,6 @@ static ErrorOr<void> decode_webp_chunk_ALPH(Chunk const& alph_chunk, Bitmap& bit
         // "Method 1: predictor = A"
         // "The top-left value at location (0, 0) uses 0 as predictor value. Otherwise,
         //  For horizontal or gradient filtering methods, the left-most pixels at location (0, y) are predicted using the location (0, y-1) just above."
-        // FIXME: This branch is untested.
         for (int y = 1; y < bitmap.height(); ++y)
             alpha[y * bitmap.width()] += alpha[(y - 1) * bitmap.width()];
         for (int y = 0; y < bitmap.height(); ++y) {