From 0c27e8164a3e7347c293012db2b4fb032ee21eae Mon Sep 17 00:00:00 2001
From: Dane Wagner <dane.wagner@gmail.com>
Date: Tue, 13 Feb 2024 15:22:57 -0600
Subject: [PATCH] Mimic Milkdrop code when handling RGBA values outside [0.0,
 1.0]

---
 .../MilkdropPreset/CustomShape.cpp            | 23 ++++++----
 .../MilkdropPreset/CustomWaveform.cpp         |  8 ++--
 src/libprojectM/Renderer/RenderItem.hpp       | 42 +++++++++++++++++++
 3 files changed, 61 insertions(+), 12 deletions(-)

diff --git a/src/libprojectM/MilkdropPreset/CustomShape.cpp b/src/libprojectM/MilkdropPreset/CustomShape.cpp
index 0b5649064..d88a1e946 100644
--- a/src/libprojectM/MilkdropPreset/CustomShape.cpp
+++ b/src/libprojectM/MilkdropPreset/CustomShape.cpp
@@ -3,6 +3,7 @@
 #include "PresetFileParser.hpp"
 
 #include <Renderer/TextureManager.hpp>
+#include <Renderer/RenderItem.hpp>
 
 #include <vector>
 
@@ -155,15 +156,21 @@ void CustomShape::Draw()
         vertexData[0].u = 0.5f;
         vertexData[0].v = 0.5f;
 
-        vertexData[0].r = static_cast<float>(*m_perFrameContext.r);
-        vertexData[0].g = static_cast<float>(*m_perFrameContext.g);
-        vertexData[0].b = static_cast<float>(*m_perFrameContext.b);
-        vertexData[0].a = static_cast<float>(*m_perFrameContext.a);
+        // x = f*255.0 & 0xFF = (f*255.0) % 256
+        // f' = x/255.0 = f % (256/255)
+        // 1.0 -> 255 (0xFF)
+        // 2.0 -> 254 (0xFE)
+        // -1.0 -> 0x01
 
-        vertexData[1].r = static_cast<float>(*m_perFrameContext.r2);
-        vertexData[1].g = static_cast<float>(*m_perFrameContext.g2);
-        vertexData[1].b = static_cast<float>(*m_perFrameContext.b2);
-        vertexData[1].a = static_cast<float>(*m_perFrameContext.a2);
+        vertexData[0].r = Renderer::color_modulo(*m_perFrameContext.r);
+        vertexData[0].g = Renderer::color_modulo(*m_perFrameContext.g);
+        vertexData[0].b = Renderer::color_modulo(*m_perFrameContext.b);
+        vertexData[0].a = Renderer::color_modulo(*m_perFrameContext.a);
+
+        vertexData[1].r = Renderer::color_modulo(*m_perFrameContext.r2);
+        vertexData[1].g = Renderer::color_modulo(*m_perFrameContext.g2);
+        vertexData[1].b = Renderer::color_modulo(*m_perFrameContext.b2);
+        vertexData[1].a = Renderer::color_modulo(*m_perFrameContext.a2);
 
         for (int i = 1; i < sides + 1; i++)
         {
diff --git a/src/libprojectM/MilkdropPreset/CustomWaveform.cpp b/src/libprojectM/MilkdropPreset/CustomWaveform.cpp
index e63e8710a..d9d0937f6 100644
--- a/src/libprojectM/MilkdropPreset/CustomWaveform.cpp
+++ b/src/libprojectM/MilkdropPreset/CustomWaveform.cpp
@@ -157,10 +157,10 @@ void CustomWaveform::Draw(const PerFrameContext& presetPerFrameContext)
         pointsTransformed[sample].x = static_cast<float>((*m_perPointContext.x * 2.0 - 1.0) * m_presetState.renderContext.invAspectX);
         pointsTransformed[sample].y = static_cast<float>((*m_perPointContext.y * -2.0 + 1.0) * m_presetState.renderContext.invAspectY);
 
-        pointsTransformed[sample].r = static_cast<float>(*m_perPointContext.r);
-        pointsTransformed[sample].g = static_cast<float>(*m_perPointContext.g);
-        pointsTransformed[sample].b = static_cast<float>(*m_perPointContext.b);
-        pointsTransformed[sample].a = static_cast<float>(*m_perPointContext.a);
+        pointsTransformed[sample].r = Renderer::color_modulo(*m_perPointContext.r);
+        pointsTransformed[sample].g = Renderer::color_modulo(*m_perPointContext.g);
+        pointsTransformed[sample].b = Renderer::color_modulo(*m_perPointContext.b);
+        pointsTransformed[sample].a = Renderer::color_modulo(*m_perPointContext.a);
     }
 
     std::vector<ColoredPoint> pointsSmoothed(sampleCount * 2);
diff --git a/src/libprojectM/Renderer/RenderItem.hpp b/src/libprojectM/Renderer/RenderItem.hpp
index 3da337eda..7ebf9928f 100644
--- a/src/libprojectM/Renderer/RenderItem.hpp
+++ b/src/libprojectM/Renderer/RenderItem.hpp
@@ -1,10 +1,52 @@
 #pragma once
 
 #include <projectM-opengl.h>
+#include <cmath>
 
 namespace libprojectM {
 namespace Renderer {
 
+/**
+ * @brief Computes the modulus to wrap float values into the range of [0.0, 1.0].
+ * 
+ * This code mimics the following equations used by the original Milkdrop code:
+ * 
+ * v[0].Diffuse =
+ *     ((((int)(*pState->m_shape[i].var_pf_a * 255 * alpha_mult)) & 0xFF) << 24) |
+ *     ((((int)(*pState->m_shape[i].var_pf_r * 255)) & 0xFF) << 16) |
+ *     ((((int)(*pState->m_shape[i].var_pf_g * 255)) & 0xFF) <<  8) |
+ *     ((((int)(*pState->m_shape[i].var_pf_b * 255)) & 0xFF)      );
+ * 
+ * In projectM, we use float values when drawing primitives or configuring vertices.
+ * Converting the above back to a float looks like this:
+ * 
+ * d = (f * 255.0) & 0xFF = int((f * 255.0) % 256.0); *
+ * f' = float(d)/255.0;
+ * 
+ * * Here % represents the Euclidean modulus, not the traditional (signed) fractional
+ * remainder.
+ * 
+ * To avoid limiting ourselves to 8 bits, we combine the above equations into one that
+ * does not discard any information:
+ * 
+ * f' = ((f * 255.0) % 256.0) / 255.0;
+ *    = f % (256.0/255.0);
+ * 
+ * Since we're using the Euclidean modulus we need to generate it from the fractional
+ * remainder using a standard equation.
+ */
+inline float color_modulo(float x)
+{
+    const float m = 256.0f / 255.0f;
+    return std::fmod(std::fmod(x, m) + m, m);
+}
+
+inline float color_modulo(double x)
+{
+    // Convert the input to float before performing the computation.
+    return color_modulo(static_cast<float>(x));
+}
+
 /**
  * @brief Base class for render meshes.
  * Also defines a few standard vertex attribute structures for use with the shaders.