From f1aba025261270b69f0dbd97ac23c5da51993e93 Mon Sep 17 00:00:00 2001
From: Lucas CHOLLET <lucas.chollet@free.fr>
Date: Thu, 23 Oct 2025 12:15:45 +0200
Subject: [PATCH] LibGfx/JPEG: Add a deringing pass to the encoder

The idea is described here https://kornel.ski/deringing/. And allows
reducing the noise around sharp white edges. This is visible when
encoding an image like `buggie.png`. One nice aspect of the optimization
is that it only affects macroblocks where it can help.
---
 .../LibGfx/ImageFormats/JPEGWriter.cpp        | 71 ++++++++++++++++++-
 .../LibGfx/ImageFormats/JPEGWriter.h          |  6 ++
 2 files changed, 74 insertions(+), 3 deletions(-)

diff --git a/Userland/Libraries/LibGfx/ImageFormats/JPEGWriter.cpp b/Userland/Libraries/LibGfx/ImageFormats/JPEGWriter.cpp
index aa3147fe5e8335..4ab90cb841ada0 100644
--- a/Userland/Libraries/LibGfx/ImageFormats/JPEGWriter.cpp
+++ b/Userland/Libraries/LibGfx/ImageFormats/JPEGWriter.cpp
@@ -98,6 +98,40 @@ class JPEGBigEndianOutputBitStream : public Stream {
     size_t m_bit_offset { 0 };
 };
 
+void interpolate(f32* component, f32 max_value, i8 start, i8 stop)
+{
+    // We're creating a uniform (ɑ = 1) Catmull–Rom curve for the missing points.
+    // That means that tᵢ₊₁ = tᵢ + 1.
+    // Note that component[start] should be interpolated but component[stop] should not.
+
+    // p1 and p2 are set to the ceil value.
+    // p0 is set to the last non-max value if possible, otherwise the value of p3 for symmetry.
+    // The same logic is applied to p3.
+    f32 const p0 = start == 0 ? component[zigzag_map[stop]] : component[zigzag_map[start - 1]];
+    f32 const p1 = max_value;
+    f32 const p2 = max_value;
+    f32 const p3 = stop > 63 ? p0 : component[zigzag_map[stop]];
+
+    f32 const t0 = 0.0f;
+    f32 const t1 = 1;
+    f32 const t2 = 2;
+    f32 const t3 = 3;
+
+    f32 const step = 1. / (stop - start + 1);
+    f32 t = t1;
+    for (i8 i = start; i < stop; ++i) {
+        t += step;
+        f32 const A1 = p0 * (t1 - t) / (t1 - t0) + p1 * (t - t0) / (t1 - t0);
+        f32 const A2 = p1 * (t2 - t) / (t2 - t1) + p2 * (t - t1) / (t2 - t1);
+        f32 const A3 = p2 * (t3 - t) / (t3 - t2) + p3 * (t - t2) / (t3 - t2);
+        f32 const B1 = A1 * (t2 - t) / (t2 - t0) + A2 * (t - t0) / (t2 - t0);
+        f32 const B2 = A2 * (t3 - t) / (t3 - t1) + A3 * (t - t1) / (t3 - t1);
+        f32 const C = B1 * (t2 - t) / (t2 - t1) + B2 * (t - t1) / (t2 - t1);
+
+        component[zigzag_map[i]] = C;
+    }
+}
+
 class JPEGEncodingContext {
 public:
     JPEGEncodingContext(JPEGBigEndianOutputBitStream output_stream)
@@ -256,6 +290,35 @@ class JPEGEncodingContext {
         }
     }
 
+    void apply_deringing()
+    {
+        // The method used here is described at: https://kornel.ski/deringing/.
+
+        for (auto& macroblock : m_macroblocks) {
+            for (auto component : { macroblock.r, macroblock.g, macroblock.b }) {
+                static constexpr auto maximum_value = NumericLimits<u8>::max();
+                Optional<u8> start;
+                u8 i = 0;
+                for (; i < 64; ++i) {
+                    if (component[zigzag_map[i]] == maximum_value) {
+                        if (!start.has_value())
+                            start = i;
+                        else
+                            continue;
+                    } else {
+                        if (start.has_value() && i - *start > 2) {
+                            interpolate(component, maximum_value, *start, i);
+                        }
+                        start.clear();
+                    }
+                }
+
+                if (start != 0 && component[zigzag_map[63]] == maximum_value)
+                    interpolate(component, maximum_value, *start, 64);
+            }
+        }
+    }
+
     ErrorOr<void> write_huffman_stream(Mode mode)
     {
         for (auto& float_macroblock : m_macroblocks) {
@@ -633,8 +696,10 @@ ErrorOr<void> add_headers(Stream& stream, JPEGEncodingContext& context, JPEGWrit
     return {};
 }
 
-ErrorOr<void> add_image(Stream& stream, JPEGEncodingContext& context, Mode mode)
+ErrorOr<void> add_image(Stream& stream, JPEGEncodingContext& context, JPEGWriter::Options const& options, Mode mode)
 {
+    if (options.use_deringing == JPEGEncoderOptions::UseDeringing::Yes)
+        context.apply_deringing();
     context.convert_to_ycbcr(mode);
     context.fdct_and_quantization(mode);
     TRY(context.write_huffman_stream(mode));
@@ -649,7 +714,7 @@ ErrorOr<void> JPEGWriter::encode(Stream& stream, Bitmap const& bitmap, Options c
     JPEGEncodingContext context { JPEGBigEndianOutputBitStream { stream } };
     TRY(add_headers(stream, context, options, bitmap.size(), Mode::RGB));
     TRY(context.initialize_mcu(bitmap));
-    TRY(add_image(stream, context, Mode::RGB));
+    TRY(add_image(stream, context, options, Mode::RGB));
     return {};
 }
 
@@ -658,7 +723,7 @@ ErrorOr<void> JPEGWriter::encode(Stream& stream, CMYKBitmap const& bitmap, Optio
     JPEGEncodingContext context { JPEGBigEndianOutputBitStream { stream } };
     TRY(add_headers(stream, context, options, bitmap.size(), Mode::CMYK));
     TRY(context.initialize_mcu(bitmap));
-    TRY(add_image(stream, context, Mode::CMYK));
+    TRY(add_image(stream, context, options, Mode::CMYK));
     return {};
 }
 
diff --git a/Userland/Libraries/LibGfx/ImageFormats/JPEGWriter.h b/Userland/Libraries/LibGfx/ImageFormats/JPEGWriter.h
index 4608623dda8074..5cad56aaa89108 100644
--- a/Userland/Libraries/LibGfx/ImageFormats/JPEGWriter.h
+++ b/Userland/Libraries/LibGfx/ImageFormats/JPEGWriter.h
@@ -12,8 +12,14 @@
 namespace Gfx {
 
 struct JPEGEncoderOptions {
+    enum class UseDeringing : u8 {
+        Yes,
+        No,
+    };
+
     Optional<ReadonlyBytes> icc_data;
     u8 quality { 75 };
+    UseDeringing use_deringing { UseDeringing::Yes };
 };
 
 class JPEGWriter {