From 58f195fc0481e158e1fb1052decb4a78e16b67eb Mon Sep 17 00:00:00 2001
From: lightningterror <18107717+lightningterror@users.noreply.github.com>
Date: Thu, 20 Mar 2025 17:32:35 +0100
Subject: [PATCH] GS/HW: Adjust AA1 draw behavior.

Make sure we use coverage alpha on aa1 draw when there's no blending.

Make sure we set coverage alpha to 128 in vertex trace alpha min max.

Some const and cast cleanup.
---
 pcsx2/GS/Renderers/Common/GSVertexTrace.cpp | 23 ++++++++++++++-------
 pcsx2/GS/Renderers/HW/GSRendererHW.cpp      |  1 +
 2 files changed, 16 insertions(+), 8 deletions(-)

diff --git a/pcsx2/GS/Renderers/Common/GSVertexTrace.cpp b/pcsx2/GS/Renderers/Common/GSVertexTrace.cpp
index c431c86d3f..469ec14207 100644
--- a/pcsx2/GS/Renderers/Common/GSVertexTrace.cpp
+++ b/pcsx2/GS/Renderers/Common/GSVertexTrace.cpp
@@ -20,10 +20,10 @@ void GSVertexTrace::Update(const void* vertex, const u16* index, int v_count, in
 
 	m_primclass = primclass;
 
-	u32 iip = m_state->PRIM->IIP;
-	u32 tme = m_state->PRIM->TME;
-	u32 fst = m_state->PRIM->FST;
-	u32 color = !(m_state->PRIM->TME && m_state->m_context->TEX0.TFX == TFX_DECAL && m_state->m_context->TEX0.TCC);
+	const u32 iip = m_state->PRIM->IIP;
+	const u32 tme = m_state->PRIM->TME;
+	const u32 fst = m_state->PRIM->FST;
+	const u32 color = !(m_state->PRIM->TME && m_state->m_context->TEX0.TFX == TFX_DECAL && m_state->m_context->TEX0.TCC);
 
 	m_fmm[color][fst][tme][iip][primclass](*this, vertex, index, i_count);
 
@@ -36,6 +36,13 @@ void GSVertexTrace::Update(const void* vertex, const u16* index, int v_count, in
 		m_accurate_stq = true;
 	}
 
+	// AA1: Set alpha min max to coverage 128 when there is no alpha blending.
+	if (!m_state->PRIM->ABE && m_state->PRIM->AA1 && (m_primclass == GS_LINE_CLASS || m_primclass == GS_TRIANGLE_CLASS))
+	{
+		m_min.c.a = 128;
+		m_max.c.a = 128;
+	}
+
 	m_eq.value = (m_min.c == m_max.c).mask() | ((m_min.p == m_max.p).mask() << 16) | ((m_min.t == m_max.t).mask() << 20);
 
 	m_alpha.valid = false;
@@ -46,7 +53,7 @@ void GSVertexTrace::Update(const void* vertex, const u16* index, int v_count, in
 		CorrectDepthTrace(vertex, v_count);
 	}
 
-	if (m_state->PRIM->TME)
+	if (tme)
 	{
 		const GIFRegTEX1& TEX1 = m_state->m_context->TEX1;
 
@@ -59,17 +66,17 @@ void GSVertexTrace::Update(const void* vertex, const u16* index, int v_count, in
 		}
 		else
 		{
-			float K = (float)TEX1.K / 16;
+			const float K = static_cast<float>(TEX1.K) / 16;
 
 			if (TEX1.LCM == 0 && m_state->PRIM->FST == 0) // FST == 1 => Q is not interpolated
 			{
 				// LOD = log2(1/|Q|) * (1 << L) + K
 
-				GSVector4::storel(&m_lod, m_max.t.uph(m_min.t).log2(3).neg() * (float)(1 << TEX1.L) + K);
+				GSVector4::storel(&m_lod, m_max.t.uph(m_min.t).log2(3).neg() * static_cast<float>(1 << TEX1.L) + K);
 
 				if (m_lod.x > m_lod.y)
 				{
-					float tmp = m_lod.x;
+					const float tmp = m_lod.x;
 					m_lod.x = m_lod.y;
 					m_lod.y = tmp;
 				}
diff --git a/pcsx2/GS/Renderers/HW/GSRendererHW.cpp b/pcsx2/GS/Renderers/HW/GSRendererHW.cpp
index 7d307f797a..981be4bff7 100644
--- a/pcsx2/GS/Renderers/HW/GSRendererHW.cpp
+++ b/pcsx2/GS/Renderers/HW/GSRendererHW.cpp
@@ -6177,6 +6177,7 @@ __ri void GSRendererHW::DrawPrims(GSTextureCache::Target* rt, GSTextureCache::Ta
 	{
 		m_conf.blend = {}; // No blending please
 		m_conf.ps.no_color1 = true;
+		m_conf.ps.fixed_one_a = IsCoverageAlpha();
 
 		if (can_scale_rt_alpha && !new_scale_rt_alpha && m_conf.colormask.wa)
 		{