Tests: Add TestImageWriter

For now, it tests that webps roundtrip, but it's easy to add basic
roundtrip testing for other image formats.

Tests/LibGfx/CMakeLists.txt

@@ -6,6 +6,7 @@ set(TEST_SOURCES
     TestGfxBitmap.cpp
     TestICCProfile.cpp
     TestImageDecoder.cpp
+    TestImageWriter.cpp
     TestPainter.cpp
     TestParseISOBMFF.cpp
     TestRect.cpp

Tests/LibGfx/TestImageWriter.cpp

@@ -0,0 +1,81 @@
+/*
+ * Copyright (c) 2024, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/MemoryStream.h>
+#include <LibGfx/Bitmap.h>
+#include <LibGfx/ImageFormats/WebPLoader.h>
+#include <LibGfx/ImageFormats/WebPWriter.h>
+#include <LibTest/TestCase.h>
+
+static ErrorOr<NonnullRefPtr<Gfx::Bitmap>> expect_single_frame(Gfx::ImageDecoderPlugin& plugin_decoder)
+{
+    EXPECT_EQ(plugin_decoder.frame_count(), 1u);
+    EXPECT(!plugin_decoder.is_animated());
+    EXPECT(!plugin_decoder.loop_count());
+
+    auto frame_descriptor = TRY(plugin_decoder.frame(0));
+    EXPECT_EQ(frame_descriptor.duration, 0);
+    return *frame_descriptor.image;
+}
+
+static ErrorOr<NonnullRefPtr<Gfx::Bitmap>> expect_single_frame_of_size(Gfx::ImageDecoderPlugin& plugin_decoder, Gfx::IntSize size)
+{
+    EXPECT_EQ(plugin_decoder.size(), size);
+    auto frame = TRY(expect_single_frame(plugin_decoder));
+    EXPECT_EQ(frame->size(), size);
+    return frame;
+}
+
+template<class Writer, class Loader>
+static ErrorOr<void> test_roundtrip(Gfx::Bitmap const& bitmap)
+{
+    AllocatingMemoryStream stream;
+    TRY(Writer::encode(stream, bitmap));
+    auto encoded_data = TRY(stream.read_until_eof());
+
+    auto plugin = TRY(Loader::create(encoded_data));
+    auto decoded = TRY(expect_single_frame_of_size(*plugin, bitmap.size()));
+
+    for (int y = 0; y < bitmap.height(); ++y)
+        for (int x = 0; x < bitmap.width(); ++x)
+            EXPECT_EQ(decoded->get_pixel(x, y), bitmap.get_pixel(x, y));
+
+    return {};
+}
+
+static ErrorOr<AK::NonnullRefPtr<Gfx::Bitmap>> create_test_rgb_bitmap()
+{
+    auto bitmap = TRY(Gfx::Bitmap::create(Gfx::BitmapFormat::BGRA8888, { 47, 33 }));
+
+    for (int y = 0; y < bitmap->height(); ++y)
+        for (int x = 0; x < bitmap->width(); ++x)
+            bitmap->set_pixel(x, y, Gfx::Color((x * 255) / bitmap->width(), (y * 255) / bitmap->height(), x + y));
+
+    return bitmap;
+}
+
+static ErrorOr<AK::NonnullRefPtr<Gfx::Bitmap>> create_test_rgba_bitmap()
+{
+    auto bitmap = TRY(create_test_rgb_bitmap());
+
+    for (int y = 0; y < bitmap->height(); ++y) {
+        for (int x = 0; x < bitmap->width(); ++x) {
+            Color pixel = bitmap->get_pixel(x, y);
+            pixel.set_alpha(255 - x);
+            bitmap->set_pixel(x, y, pixel);
+        }
+    }
+
+    return bitmap;
+}
+
+TEST_CASE(test_webp)
+{
+    auto err = test_roundtrip<Gfx::WebPWriter, Gfx::WebPImageDecoderPlugin>(TRY_OR_FAIL(create_test_rgb_bitmap()));
+    TRY_OR_FAIL(err);
+    err = test_roundtrip<Gfx::WebPWriter, Gfx::WebPImageDecoderPlugin>(TRY_OR_FAIL(create_test_rgba_bitmap()));
+    TRY_OR_FAIL(err);
+}