From de82e8477abf31111bcc617db4526dfc4ca7872e Mon Sep 17 00:00:00 2001
From: Anders Jenbo <anders@jenbo.dk>
Date: Mon, 9 Jun 2025 05:06:19 +0200
Subject: [PATCH] Use dedicated path for flat shading (#248)

---
 .../src/d3drm/backends/software/renderer.cpp  | 73 +++++++++++++------
 miniwin/src/internal/d3drmrenderer_software.h |  2 +-
 2 files changed, 53 insertions(+), 22 deletions(-)

diff --git a/miniwin/src/d3drm/backends/software/renderer.cpp b/miniwin/src/d3drm/backends/software/renderer.cpp
index a19f6a47..0fc243c1 100644
--- a/miniwin/src/d3drm/backends/software/renderer.cpp
+++ b/miniwin/src/d3drm/backends/software/renderer.cpp
@@ -146,15 +146,15 @@ void Direct3DRMSoftwareRenderer::BlendPixel(Uint8* pixelAddr, Uint8 r, Uint8 g,
 	memcpy(pixelAddr, &blended, m_bytesPerPixel);
 }
 
-SDL_Color Direct3DRMSoftwareRenderer::ApplyLighting(const D3DRMVERTEX& vertex, const Appearance& appearance)
+SDL_Color Direct3DRMSoftwareRenderer::ApplyLighting(
+	const D3DVECTOR& position,
+	const D3DVECTOR& normal,
+	const Appearance& appearance
+)
 {
 	FColor specular = {0, 0, 0, 0};
 	FColor diffuse = {0, 0, 0, 0};
 
-	// Position and normal
-	D3DVECTOR position = vertex.position;
-	D3DVECTOR normal = Normalize(vertex.normal);
-
 	for (const auto& light : m_lights) {
 		FColor lightColor = light.color;
 
@@ -201,6 +201,16 @@ SDL_Color Direct3DRMSoftwareRenderer::ApplyLighting(const D3DRMVERTEX& vertex, c
 	};
 }
 
+inline float EdgeFloat(float x0, float y0, float x1, float y1, float x, float y)
+{
+	return (x - x0) * (y1 - y0) - (y - y0) * (x1 - x0);
+}
+
+inline float EdgeDouble(double x0, double y0, double x1, double y1, double x, double y)
+{
+	return (x - x0) * (y1 - y0) - (y - y0) * (x1 - x0);
+}
+
 void Direct3DRMSoftwareRenderer::DrawTriangleProjected(
 	const D3DRMVERTEX& v0,
 	const D3DRMVERTEX& v1,
@@ -235,19 +245,20 @@ void Direct3DRMSoftwareRenderer::DrawTriangleProjected(
 		return;
 	}
 
-	auto edge = [](double x0, double y0, double x1, double y1, double x, double y) {
-		return (x - x0) * (y1 - y0) - (y - y0) * (x1 - x0);
-	};
-	float area = edge(p0.x, p0.y, p1.x, p1.y, p2.x, p2.y);
+	// Cull backfaces
+	float area = EdgeFloat(p0.x, p0.y, p1.x, p1.y, p2.x, p2.y);
 	if (area >= 0) {
 		return;
 	}
 	float invArea = 1.0f / area;
 
-	// Per-vertex lighting using vertex normals
-	SDL_Color c0 = ApplyLighting(v0, appearance);
-	SDL_Color c1 = ApplyLighting(v1, appearance);
-	SDL_Color c2 = ApplyLighting(v2, appearance);
+	Uint8 r, g, b;
+	SDL_Color c0 = ApplyLighting(v0.position, v0.normal, appearance);
+	SDL_Color c1, c2;
+	if (!appearance.flat) {
+		c1 = ApplyLighting(v1.position, v1.normal, appearance);
+		c2 = ApplyLighting(v2.position, v2.normal, appearance);
+	}
 
 	Uint32 textureId = appearance.textureId;
 	int texturePitch;
@@ -271,12 +282,12 @@ void Direct3DRMSoftwareRenderer::DrawTriangleProjected(
 		for (int x = minX; x <= maxX; ++x) {
 			float px = x + 0.5f;
 			float py = y + 0.5f;
-			float w0 = edge(p1.x, p1.y, p2.x, p2.y, px, py) * invArea;
+			float w0 = EdgeDouble(p1.x, p1.y, p2.x, p2.y, px, py) * invArea;
 			if (w0 < 0.0f || w0 > 1.0f) {
 				continue;
 			}
 
-			float w1 = edge(p2.x, p2.y, p0.x, p0.y, px, py) * invArea;
+			float w1 = EdgeDouble(p2.x, p2.y, p0.x, p0.y, px, py) * invArea;
 			if (w1 < 0.0f || w1 > 1.0f - w0) {
 				continue;
 			}
@@ -290,10 +301,16 @@ void Direct3DRMSoftwareRenderer::DrawTriangleProjected(
 				continue;
 			}
 
-			// Interpolate color
-			Uint8 r = static_cast<Uint8>(w0 * c0.r + w1 * c1.r + w2 * c2.r);
-			Uint8 g = static_cast<Uint8>(w0 * c0.g + w1 * c1.g + w2 * c2.g);
-			Uint8 b = static_cast<Uint8>(w0 * c0.b + w1 * c1.b + w2 * c2.b);
+			if (appearance.flat) {
+				r = c0.r;
+				g = c0.g;
+				b = c0.b;
+			}
+			else {
+				r = static_cast<Uint8>(w0 * c0.r + w1 * c1.r + w2 * c2.r);
+				g = static_cast<Uint8>(w0 * c0.g + w1 * c1.g + w2 * c2.g);
+				b = static_cast<Uint8>(w0 * c0.b + w1 * c1.b + w2 * c2.b);
+			}
 			Uint8* pixelAddr = pixels + y * pitch + x * m_bytesPerPixel;
 
 			if (appearance.color.a == 255) {
@@ -322,8 +339,22 @@ void Direct3DRMSoftwareRenderer::DrawTriangleProjected(
 
 					Uint8* texelAddr = texels + texY * texturePitch + texX * m_bytesPerPixel;
 
-					Uint32 texelColor = 0;
-					memcpy(&texelColor, texelAddr, m_bytesPerPixel);
+					Uint32 texelColor;
+					switch (m_bytesPerPixel) {
+					case 1:
+						texelColor = *texelAddr;
+						break;
+					case 2:
+						texelColor = *(Uint16*) texelAddr;
+						break;
+					case 3:
+						// Manually build the 24-bit color (assuming byte order)
+						texelColor = texelAddr[0] | (texelAddr[1] << 8) | (texelAddr[2] << 16);
+						break;
+					case 4:
+						texelColor = *(Uint32*) texelAddr;
+						break;
+					}
 
 					Uint8 tr, tg, tb, ta;
 					SDL_GetRGBA(texelColor, m_format, m_palette, &tr, &tg, &tb, &ta);
diff --git a/miniwin/src/internal/d3drmrenderer_software.h b/miniwin/src/internal/d3drmrenderer_software.h
index d253cd52..4ceb343c 100644
--- a/miniwin/src/internal/d3drmrenderer_software.h
+++ b/miniwin/src/internal/d3drmrenderer_software.h
@@ -49,7 +49,7 @@ private:
 	void DrawTriangleClipped(const D3DRMVERTEX (&v)[3], const Appearance& appearance);
 	void ProjectVertex(const D3DRMVERTEX& v, D3DRMVECTOR4D& p) const;
 	void BlendPixel(Uint8* pixelAddr, Uint8 r, Uint8 g, Uint8 b, Uint8 a);
-	SDL_Color ApplyLighting(const D3DRMVERTEX& vertex, const Appearance& appearance);
+	SDL_Color ApplyLighting(const D3DVECTOR& position, const D3DVECTOR& normal, const Appearance& appearance);
 	void AddTextureDestroyCallback(Uint32 id, IDirect3DRMTexture* texture);
 
 	DWORD m_width;