From b320787102ab0ef6502517f595a6fdeccf851366 Mon Sep 17 00:00:00 2001
From: Christopher Bruns <cmbruns@rotatingpenguin.com>
Date: Sat, 30 Apr 2016 15:52:39 -0400
Subject: [PATCH] Implement new CVAR gl_billboard_faces_camera, which orients
 sprites toward camera, rather than along view direction.

# Conflicts:
#	gz3doom/GZ3DoomRiftMonitor1.bat
#	src/gl/scene/gl_sprite.cpp
#	src/gl/scene/gl_stereo3d.cpp
---
 src/gl/scene/gl_sprite.cpp | 42 ++++++++++++++++++++++++++++++--------
 1 file changed, 33 insertions(+), 9 deletions(-)

diff --git a/src/gl/scene/gl_sprite.cpp b/src/gl/scene/gl_sprite.cpp
index 8e36c766e4..bc633e7419 100644
--- a/src/gl/scene/gl_sprite.cpp
+++ b/src/gl/scene/gl_sprite.cpp
@@ -73,6 +73,7 @@ CVAR(Float, gl_sclipthreshold, 10.0, CVAR_ARCHIVE)
 CVAR(Float, gl_sclipfactor, 1.8, CVAR_ARCHIVE)
 CVAR(Int, gl_particles_style, 2, CVAR_ARCHIVE | CVAR_GLOBALCONFIG) // 0 = square, 1 = round, 2 = smooth
 CVAR(Int, gl_billboard_mode, 0, CVAR_ARCHIVE | CVAR_GLOBALCONFIG)
+CVAR(Bool, gl_billboard_faces_camera, false, CVAR_ARCHIVE | CVAR_GLOBALCONFIG)
 CVAR(Bool, gl_billboard_particles, true, CVAR_ARCHIVE | CVAR_GLOBALCONFIG)
 CVAR(Int, gl_enhanced_nv_stealth, 3, CVAR_ARCHIVE | CVAR_GLOBALCONFIG)
 CUSTOM_CVAR(Int, gl_fuzztype, 0, CVAR_ARCHIVE)
@@ -262,6 +263,8 @@ void GLSprite::Draw(int pass)
 				//&& GLRenderer->mViewActor != NULL
 				&& (gl_billboard_mode == 1 || (actor && actor->renderflags & RF_FORCEXYBILLBOARD))));
 
+			const bool drawBillboardFacingCamera = gl_billboard_faces_camera;
+
 			gl_RenderState.Apply();
 
 			Vector v1;
@@ -269,11 +272,9 @@ void GLSprite::Draw(int pass)
 			Vector v3;
 			Vector v4;
 
-			if (drawWithXYBillboard)
+			if (drawWithXYBillboard || drawBillboardFacingCamera)
 			{
-				// Rotate the sprite about the vector starting at the center of the sprite
-				// triangle strip and with direction orthogonal to where the player is looking
-				// in the x/y plane.
+				// Compute center of sprite
 				float xcenter = (x1 + x2)*0.5;
 				float ycenter = (y1 + y2)*0.5;
 				float zcenter = (z1 + z2)*0.5;
@@ -281,17 +282,40 @@ void GLSprite::Draw(int pass)
 
 				Matrix3x4 mat;
 				mat.MakeIdentity();
-				mat.Translate(xcenter, zcenter, ycenter);
-				mat.Rotate(-sin(angleRad), 0, cos(angleRad), -GLRenderer->mAngles.Pitch.Degrees);
-				mat.Translate(-xcenter, -zcenter, -ycenter);
+
+				if (drawWithXYBillboard)
+				{
+					// Rotate the sprite about the vector starting at the center of the sprite
+					// triangle strip and with direction orthogonal to where the player is looking
+					// in the x/y plane.
+					mat.Translate(xcenter, zcenter, ycenter);
+					mat.Rotate(-sin(angleRad), 0, cos(angleRad), -GLRenderer->mAngles.Pitch.Degrees);
+					mat.Translate(-xcenter, -zcenter, -ycenter);
+				}
+
+				if (drawBillboardFacingCamera) {
+					// [CMB] Rotate relative to camera XY position, not just camera direction,
+					// which is nicer in VR
+					float xrel = xcenter - FIXED2FLOAT(GLRenderer->mViewActor->X());
+					float yrel = ycenter - FIXED2FLOAT(GLRenderer->mViewActor->Y());
+					float zrel = zcenter - FIXED2FLOAT(GLRenderer->mViewActor->Z());
+
+					float absAngleDeg = RAD2DEG(atan2(-yrel, xrel));
+					float counterRotationDeg = 270. - float(GLRenderer->mAngles.Yaw); // counteracts existing sprite rotation
+					float relAngleDeg = counterRotationDeg + absAngleDeg;
+
+					mat.Translate(xcenter, zcenter, ycenter);
+					mat.Rotate(0, 1, 0, relAngleDeg);
+					mat.Translate(-xcenter, -zcenter, -ycenter);
+				}
+
 				v1 = mat * Vector(x1, z1, y1);
 				v2 = mat * Vector(x2, z1, y2);
 				v3 = mat * Vector(x1, z2, y1);
 				v4 = mat * Vector(x2, z2, y2);
 			}
-			else
+			else 
 			{
-
 				v1 = Vector(x1, z1, y1);
 				v2 = Vector(x2, z1, y2);
 				v3 = Vector(x1, z2, y1);