From df6b7fff905ed8a71e70f099bc317bf6cc4671ce Mon Sep 17 00:00:00 2001
From: MacDue <macdue@dueutil.tech>
Date: Sat, 1 Oct 2022 22:42:31 +0100
Subject: [PATCH] LibGfx: Implement HueRotateFilter

This implements a simple and fairly efficient hue rotation filter.
It is based off the SVG feColorMatrix "hueRotate" matrix operation.
https://drafts.fxtf.org/filter-effects-1/#elementdef-fecolormatrix
---
 .../LibGfx/Filters/HueRotateFilter.h          | 84 +++++++++++++++++++
 1 file changed, 84 insertions(+)
 create mode 100644 Userland/Libraries/LibGfx/Filters/HueRotateFilter.h

diff --git a/Userland/Libraries/LibGfx/Filters/HueRotateFilter.h b/Userland/Libraries/LibGfx/Filters/HueRotateFilter.h
new file mode 100644
index 0000000000..a4350ded35
--- /dev/null
+++ b/Userland/Libraries/LibGfx/Filters/HueRotateFilter.h
@@ -0,0 +1,84 @@
+/*
+ * Copyright (c) 2022, MacDue <macdue@dueutil.tech>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Math.h>
+#include <LibGfx/Filters/ColorFilter.h>
+#include <LibGfx/Matrix3x3.h>
+
+namespace Gfx {
+
+class HueRotateFilter : public ColorFilter {
+public:
+    HueRotateFilter(float angle_degrees)
+        : ColorFilter(angle_degrees)
+        , m_operation(calculate_operation_matrix(angle_degrees))
+    {
+    }
+
+    virtual bool amount_handled_in_filter() const override
+    {
+        return true;
+    }
+
+    float angle_degrees() const
+    {
+        return m_amount;
+    }
+
+    virtual StringView class_name() const override { return "HueRotateFilter"sv; }
+
+protected:
+    Color convert_color(Color original) override
+    {
+        FloatVector3 rgb = {
+            original.red() / 256.0f,
+            original.green() / 256.0f,
+            original.blue() / 256.0f,
+        };
+        rgb = m_operation * rgb;
+        auto safe_float_to_u8 = [](float value) -> u8 {
+            return static_cast<u8>(AK::clamp(value, 0.0f, 1.0f) * 256);
+        };
+        return Color {
+            safe_float_to_u8(rgb[0]),
+            safe_float_to_u8(rgb[1]),
+            safe_float_to_u8(rgb[2]),
+            original.alpha()
+        };
+    }
+
+private:
+    static FloatMatrix3x3 calculate_operation_matrix(float angle)
+    {
+        float angle_rads = angle * (AK::Pi<float> / 180);
+        float cos_angle = 0;
+        float sin_angle = 0;
+        AK::sincos(angle_rads, sin_angle, cos_angle);
+        // The matrices here are taken directly from the SVG filter specification:
+        // https://drafts.fxtf.org/filter-effects-1/#feColorMatrixElement
+        // clang-format off
+        return FloatMatrix3x3 {
+            +0.213, +0.715, +0.072,
+            +0.213, +0.715, +0.072,
+            +0.213, +0.715, +0.072
+        } + cos_angle * FloatMatrix3x3 {
+            +0.787, -0.715, -0.072,
+            -0.213, +0.285, -0.072,
+            -0.213, -0.715, +0.928
+        } + sin_angle * FloatMatrix3x3 {
+            -0.213, -0.715, +0.928,
+            +0.143, +0.140, -0.283,
+            -0.787, +0.715, +0.072
+        };
+        // clang-format on
+    }
+
+    FloatMatrix3x3 m_operation;
+};
+
+}