From ceaa040750dc481c67e679e6469aa4c69464b4db Mon Sep 17 00:00:00 2001
From: Christoph Oelckers <coelckers@zdoom.fake>
Date: Sun, 18 Sep 2016 17:45:02 +0200
Subject: [PATCH] - added a workaround to avoid sprite splitting when it may
 cause glitches for sprites that get rotated in the draw pass.

---
 src/gl/scene/gl_drawinfo.cpp | 25 +++++++++++++++++++++++++
 src/gl/scene/gl_sprite.cpp   |  4 +---
 2 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/src/gl/scene/gl_drawinfo.cpp b/src/gl/scene/gl_drawinfo.cpp
index 0d612cf64..d1dd60c4f 100644
--- a/src/gl/scene/gl_drawinfo.cpp
+++ b/src/gl/scene/gl_drawinfo.cpp
@@ -497,6 +497,10 @@ void GLDrawList::SortWallIntoWall(SortNode * head,SortNode * sort)
 // 
 //
 //==========================================================================
+EXTERN_CVAR(Int, gl_billboard_mode)
+EXTERN_CVAR(Bool, gl_billboard_faces_camera)
+EXTERN_CVAR(Bool, gl_billboard_particles)
+
 void GLDrawList::SortSpriteIntoWall(SortNode * head,SortNode * sort)
 {
 	GLWall * wh=&walls[drawitems[head->itemindex].index];
@@ -527,6 +531,27 @@ void GLDrawList::SortSpriteIntoWall(SortNode * head,SortNode * sort)
 	}
 	else
 	{
+		const bool drawWithXYBillboard = ((ss->particle && gl_billboard_particles) || (!(ss->actor && ss->actor->renderflags & RF_FORCEYBILLBOARD)
+			&& (gl_billboard_mode == 1 || (ss->actor && ss->actor->renderflags & RF_FORCEXYBILLBOARD))));
+
+		const bool drawBillboardFacingCamera = gl_billboard_faces_camera;
+		// [Nash] has +ROLLSPRITE
+		const bool rotated = (ss->actor != nullptr && ss->actor->renderflags & RF_ROLLSPRITE | RF_WALLSPRITE | RF_FLATSPRITE);
+
+		// cannot sort them at the moment. This requires more complex splitting.
+		if (drawWithXYBillboard || drawBillboardFacingCamera || rotated)
+		{
+			float v1 = wh->PointOnSide(ss->x, ss->y);
+			if (v1 < 0)
+			{
+				head->AddToLeft(sort);
+			}
+			else
+			{
+				head->AddToRight(sort);
+			}
+			return;
+		}
 		double r=ss->CalcIntersectionVertex(wh);
 
 		float ix=(float)(ss->x1 + r * (ss->x2-ss->x1));
diff --git a/src/gl/scene/gl_sprite.cpp b/src/gl/scene/gl_sprite.cpp
index af76de826..720df3b2b 100644
--- a/src/gl/scene/gl_sprite.cpp
+++ b/src/gl/scene/gl_sprite.cpp
@@ -923,7 +923,7 @@ void GLSprite::Process(AActor* thing, sector_t * sector, int thruportal)
 		// This is a non-translucent sprite (i.e. STYLE_Normal or equivalent)
 		trans=1.f;
 		
-		if (!gl_sprite_blend || modelframe || (thing->renderflags & RF_SPRITETYPEMASK) == RF_WALLSPRITE)
+		if (!gl_sprite_blend || modelframe || (thing->renderflags & (RF_FLATSPRITE|RF_WALLSPRITE)) || gl_billboard_faces_camera)
 		{
 			RenderStyle.SrcAlpha = STYLEALPHA_One;
 			RenderStyle.DestAlpha = STYLEALPHA_Zero;
@@ -934,8 +934,6 @@ void GLSprite::Process(AActor* thing, sector_t * sector, int thruportal)
 			RenderStyle.SrcAlpha = STYLEALPHA_Src;
 			RenderStyle.DestAlpha = STYLEALPHA_InvSrc;
 		}
-
-
 	}
 	if ((gltexture && gltexture->GetTransparent()) || (RenderStyle.Flags & STYLEF_RedIsAlpha))
 	{