LibGfx: Make Bayer dithering more luminosity-conservative

Tests/LibGfx/TestBilevelImage.cpp

@@ -4,6 +4,9 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <LibGfx/Bitmap.h>
+#include <LibGfx/ICC/TagTypes.h>
+#include <LibGfx/ICC/WellKnownProfiles.h>
 #include <LibGfx/ImageFormats/BilevelImage.h>
 #include <LibTest/TestCase.h>
 
@@ -60,3 +63,45 @@ TEST_CASE(get_bits_over_8bits)
     EXPECT_EQ(bilevel->get_bits(8, 0, 8), 0xFE);
     EXPECT_EQ(bilevel->get_bits(12, 0, 4), 0xE);
 }
+
+static ErrorOr<void> test_bayer_dither(Gfx::DitheringAlgorithm algorithm, u32 size)
+{
+    auto srgb_curve = TRY(Gfx::ICC::sRGB_curve());
+    auto bitmap = TRY(Gfx::Bitmap::create(Gfx::BitmapFormat::BGRA8888, { size, size }));
+    auto number_of_states = size * size + 1;
+
+    auto test_luminosity = [&](f32 input_luminosity, f32 output_luminosity) -> ErrorOr<void> {
+        auto uncompressed = round_to<u8>(srgb_curve->evaluate_inverse(input_luminosity) * 255);
+        bitmap->fill(Gfx::Color(uncompressed, uncompressed, uncompressed));
+        auto bilevel = TRY(Gfx::BilevelImage::create_from_bitmap(bitmap, algorithm));
+        double average = 0;
+        for (u32 y = 0; y < bilevel->height(); ++y) {
+            for (u32 x = 0; x < bilevel->width(); ++x)
+                average += bilevel->get_bit(x, y) ? 0 : 1;
+        }
+
+        EXPECT_APPROXIMATE(average / (size * size), output_luminosity);
+        return {};
+    };
+
+    // Test full black and full white.
+    TRY(test_luminosity(0, 0));
+    TRY(test_luminosity(1, 1));
+
+    // Test all states at half the range.
+    for (u32 s = 0; s < number_of_states; ++s) {
+        // We test all states in the middle of there range.
+        auto value = (static_cast<f32>(s) + 0.5f) / number_of_states;
+        // There are only (number_of_states - 1) states of luminosity.
+        TRY(test_luminosity(value, static_cast<f32>(s) / (number_of_states - 1)));
+    }
+    return {};
+}
+
+TEST_CASE(bayer_dither)
+{
+
+    TRY_OR_FAIL(test_bayer_dither(Gfx::DitheringAlgorithm::Bayer2x2, 2));
+    TRY_OR_FAIL(test_bayer_dither(Gfx::DitheringAlgorithm::Bayer4x4, 4));
+    TRY_OR_FAIL(test_bayer_dither(Gfx::DitheringAlgorithm::Bayer8x8, 8));
+}

Userland/Libraries/LibGfx/ImageFormats/BilevelImage.cpp

@@ -312,9 +312,13 @@ ErrorOr<NonnullRefPtr<BilevelImage>> BilevelImage::create_from_bitmap(Gfx::Bitma
         VERIFY(is_power_of_two(n));
         auto mask = n - 1;
 
+        // A bayer matrix of dimension N has N x N +1 different states. First one
+        // is an all black matrix, and then one more for each element turning white.
+        u32 number_of_states = n * n + 1;
+
         for (int y = 0, i = 0; y < bitmap.height(); ++y) {
             for (int x = 0; x < bitmap.width(); ++x, ++i) {
-                u8 threshold = (bayer_matrix[(y & mask) * n + (x & mask)] * 255) / ((n * n) - 1);
+                u8 threshold = round_to<u8>((1 + bayer_matrix[(y & mask) * n + (x & mask)]) * 255.f / number_of_states);
                 bilevel_image->set_bit(x, y, gray_bitmap[i] > threshold ? 0 : 1);
             }
         }