GS/HW: Make all vertices the same color in certain flat shading cases.

Fixes issues with AA1 with flat shading.

bin/resources/shaders/dx11/tfx.fx

@@ -2014,11 +2014,6 @@ VS_OUTPUT vs_main_expand(uint vid : SV_VertexID)
 		vtx.inv_cov = is_near_corner ? 0.0f : 1.0f; // Full coverage at near corner, otherwise none.
 	
 		vtx.interior = 0;
-
-		#if !VS_IIP
-			// Get the provoking vertex color (first vertex in DX)
-			vtx.c = i0 == 0 ? vtx.c : (i1 == 0 ? other.c : opposite.c);
-		#endif
 	}
 
 	return vtx;

bin/resources/shaders/opengl/tfx_vgs.glsl

@@ -455,11 +455,6 @@ void main()
 		VSout.inv_cov = is_near_corner ? 0.0f : 1.0f; // Full coverage at near corner, otherwise none.
 	
 		VSout.interior = 0;
-
-		#if !VS_IIP
-			// Get the provoking vertex color (last vertex in GL)
-			vtx.c = i0 == 2 ? vtx.c : (i1 == 2 ? other.c : opposite.c);
-		#endif
 	}
 
 #endif

bin/resources/shaders/vulkan/tfx.glsl

@@ -450,11 +450,6 @@ void main()
 		vsOut.inv_cov = is_near_corner ? 0.0f : 1.0f; // Full coverage at near corner, otherwise none.
 	
 		vsOut.interior = 0;
-
-		#if !VS_IIP
-			// Get the provoking vertex color (last vertex in VK)
-			vtx.c = i0 == 2 ? vtx.c : (i1 == 2 ? other.c : opposite.c);
-		#endif
 	}
 
 #endif

pcsx2/GS/Renderers/HW/GSRendererHW.cpp

@@ -5331,31 +5331,36 @@ bool GSRendererHW::VerifyIndices()
 	return true;
 }
 
-// Fix the colors in vertices in case the API only supports "provoking first vertex"
-// (i.e., when using flat shading the color comes from the first vertex, unlike PS2
-// which is "provoking last vertex").
-void GSRendererHW::HandleProvokingVertexFirst()
+void GSRendererHW::HandleFlatShadedVertices()
 {
-	                                                     // Early exit conditions:
-	if (g_gs_device->Features().provoking_vertex_last || // device supports provoking last vertex
-	    m_conf.vs.iip ||                                 // we are doing Gouraud shading
-	    m_vt.m_primclass == GS_POINT_CLASS ||            // drawing points (one vertex per primitive; color is unambiguous)
-	    m_vt.m_primclass == GS_SPRITE_CLASS)             // drawing sprites (handled by the sprites -> triangles expand shader)
+	// These cases might need fixing.
+	const bool maybe_fix_vertices = !m_conf.vs.iip &&
+		(!g_gs_device->Features().provoking_vertex_last || IsCoverageAlphaSupported());
+
+	// These cases definitely don't need fixing.
+	const bool dont_fix_vertices = m_vt.m_primclass == GS_POINT_CLASS || m_vt.m_primclass == GS_SPRITE_CLASS;
+
+	if (!maybe_fix_vertices || dont_fix_vertices)
 		return;
 
 	const int n = GSUtil::GetClassVertexCount(m_vt.m_primclass);
 
-	// If all first/last vertices have the same color there is nothing to do.
-	bool first_eq_last = true;
+	// If all vertices of each prim have the same color there is nothing to do.
+	bool prims_flat = true;
 	for (u32 i = 0; i < m_index->tail; i += n)
 	{
-		if (m_vertex->buff[m_index->buff[i]].RGBAQ.U32[0] != m_vertex->buff[m_index->buff[i + n - 1]].RGBAQ.U32[0])
+		for (u32 j = 0; j < n - 1; j++)
 		{
-			first_eq_last = false;
-			break;
+			if (m_vertex->buff[m_index->buff[i + j]].RGBAQ.U32[0] != m_vertex->buff[m_index->buff[i + n - 1]].RGBAQ.U32[0])
+			{
+				prims_flat = false;
+				break;
+			}
 		}
+		if (!prims_flat)
+			break;
 	}
-	if (first_eq_last)
+	if (prims_flat)
 		return;
 
 	// De-index the vertices using the copy buffer
@@ -5369,11 +5374,11 @@ void GSRendererHW::HandleProvokingVertexFirst()
 	std::swap(m_vertex->buff, m_vertex->buff_copy);
 	m_vertex->head = m_vertex->next = m_vertex->tail = m_index->tail;
 
-	// Put correct color in the first vertex
+	// Make all vertices the same color to simplify handling in expand shaders.
 	for (u32 i = 0; i < m_index->tail; i += n)
 	{
-		m_vertex->buff[i].RGBAQ.U32[0] = m_vertex->buff[i + n - 1].RGBAQ.U32[0];
-		m_vertex->buff[i + n - 1].RGBAQ.U32[0] = 0xff; // Make last vertex red for debugging if used improperly
+		for (u32 j = 0; j < n - 1; j++)
+			m_vertex->buff[i + j].RGBAQ.U32[0] = m_vertex->buff[i + n - 1].RGBAQ.U32[0];
 	}
 }
 
@@ -9450,7 +9455,7 @@ __ri void GSRendererHW::DrawPrims(GSTextureCache::Target* rt, GSTextureCache::Ta
 	DetermineROVUsage(rt, ds);
 	ConvertTextureTypeROV(rt, ds);
 
-	// Barriers must be determined before indices are modified via HandleProvokingVertexFirst/SetupIA.
+	// Barriers must be determined before indices are modified via HandleFlatShadedVertices/SetupIA.
 	// This also computes the drawlist if needed.
 	DetermineBarriers(rt, tex);
 
@@ -9475,7 +9480,7 @@ __ri void GSRendererHW::DrawPrims(GSTextureCache::Target* rt, GSTextureCache::Ta
 
 	m_conf.scissor = (date_options.enabled && !date_options.barrier) ? m_conf.drawarea : scissor;
 
-	HandleProvokingVertexFirst();
+	HandleFlatShadedVertices();
 
 	SetupIA(rtscale, vs_scale_x, vs_scale_y, m_channel_shuffle_width != 0, no_rt);
 

pcsx2/GS/Renderers/HW/GSRendererHW.h

@@ -215,7 +215,7 @@ private:
 	void DrawPrims(GSTextureCache::Target* rt, GSTextureCache::Target* ds, GSTextureCache::Source* tex, const TextureMinMaxResult& tmm);
 
 	void ResetStates();
-	void HandleProvokingVertexFirst();
+	void HandleFlatShadedVertices();
 	void SetupIA(float target_scale, float sx, float sy, bool req_vert_backup, const bool no_rt);
 	void EmulateTextureShuffleAndFbmask(GSTextureCache::Target* rt, GSTextureCache::Source* tex);
 	u32 EmulateChannelShuffle(GSTextureCache::Target* src, bool test_only, GSTextureCache::Target* rt = nullptr);

pcsx2/GS/Renderers/Metal/tfx.metal

@@ -555,12 +555,6 @@ vertex MainVSOut vs_main_expand(
 				out.inv_cov = is_near_corner ? 0.0f : 1.0f; // Full coverage at near corner, otherwise none.
 			
 				out.interior = 0;
-
-				if (NOT_IIP)
-				{
-					// Get the provoking vertex color (first vertex in Metal)
-					out.fc = i0 == 0 ? out.fc : (i1 == 0 ? other.fc : opposite.fc);
-				}
 			}
 
 			return out;

pcsx2/ShaderCacheVersion.h

@@ -3,4 +3,4 @@
 
 /// Version number for GS and other shaders. Increment whenever any of the contents of the
 /// shaders change, to invalidate the cache.
-static constexpr u32 SHADER_CACHE_VERSION = 106; // Last changed in PR 14643
+static constexpr u32 SHADER_CACHE_VERSION = 107; // Last changed in PR 14634