Fix bad depth buffer reads with depth fade

On my machine the depth fade shader sometimes reads out bad values when a
polygonOffset shader is used (or if the material system is on). Fix it
by calling glTextureBarrier before using a depth fade shader.

Author: slipher

src/engine/renderer/tr_backend.cpp

@@ -2587,6 +2587,9 @@ static void RB_RenderView( bool depthPass )
 	// draw everything that is translucent
 	if ( glConfig2.usingMaterialSystem ) {
 		materialSystem.RenderMaterials( shaderSort_t::SS_ENVIRONMENT_NOFOG, shaderSort_t::SS_POST_PROCESS, backEnd.viewParms.viewID );
+
+		// HACK: assume surfaces with depth fade don't use the material system
+		backEnd.dirtyDepthBuffer = true;
 	}
 	RB_RenderDrawSurfaces( shaderSort_t::SS_ENVIRONMENT_NOFOG, shaderSort_t::SS_POST_PROCESS, DRAWSURFACES_ALL );
 
@@ -3256,6 +3259,8 @@ const RenderCommand *ClearBufferCommand::ExecuteSelf( ) const
 	// 2D images again
 	backEnd.projection2D = false;
 
+	backEnd.dirtyDepthBuffer = false;
+
 	// set the modelview matrix for the viewer
 	SetViewportAndScissor();
 

src/engine/renderer/tr_local.h

@@ -2286,6 +2286,7 @@ enum class ssaoMode {
 		bool          isHyperspace;
 		trRefEntity_t     *currentEntity;
 		bool          skyRenderedThisView; // flag for drawing sun
+		bool dirtyDepthBuffer;
 		bool postDepthLightTileRendered = false;
 
 		bool          projection2D; // if true, drawstretchpic doesn't need to change modes

src/engine/renderer/tr_public.h

@@ -135,6 +135,7 @@ struct glconfig2_t
 	bool uniformBufferObjectAvailable;
 	bool mapBufferRangeAvailable;
 	bool syncAvailable;
+	bool textureBarrierAvailable;
 	bool halfFloatVertexAvailable;
 
 	bool colorGrading;

src/engine/renderer/tr_shade.cpp

@@ -550,6 +550,11 @@ void Tess_DrawElements()
 		backEnd.pc.c_indexes += tess.numIndexes;
 		backEnd.pc.c_vertexes += tess.numVertexes;
 	}
+
+	if ( glState.glStateBits & GLS_DEPTHMASK_TRUE )
+	{
+		backEnd.dirtyDepthBuffer = true;
+	}
 }
 
 /*
@@ -863,6 +868,14 @@ void Render_generic3D( shaderStage_t *pStage )
 	bool hasDepthFade = pStage->hasDepthFade;
 	bool needDepthMap = pStage->hasDepthFade;
 
+	if ( needDepthMap && backEnd.dirtyDepthBuffer && glConfig2.textureBarrierAvailable )
+	{
+		// Flush depth buffer to make sure it is available for reading in the depth fade
+		// GLSL - prevents https://github.com/DaemonEngine/Daemon/issues/1676
+		glTextureBarrier();
+		backEnd.dirtyDepthBuffer = false;
+	}
+
 	// choose right shader program ----------------------------------
 	ProcessShaderGeneric3D( pStage );
 	gl_genericShader->BindProgram( pStage->deformIndex );

src/engine/sys/sdl_glimp.cpp

@@ -102,6 +102,8 @@ static Cvar::Cvar<bool> r_arb_shading_language_420pack( "r_arb_shading_language_
 	"Use GL_ARB_shading_language_420pack if available", Cvar::NONE, true );
 static Cvar::Cvar<bool> r_arb_sync( "r_arb_sync",
 	"Use GL_ARB_sync if available", Cvar::NONE, true );
+static Cvar::Cvar<bool> r_arb_texture_barrier( "r_arb_texture_barrier",
+	"Use GL_ARB_texture_barrier if available", Cvar::NONE, true );
 static Cvar::Cvar<bool> r_arb_texture_gather( "r_arb_texture_gather",
 	"Use GL_ARB_texture_gather if available", Cvar::NONE, true );
 static Cvar::Cvar<bool> r_arb_uniform_buffer_object( "r_arb_uniform_buffer_object",
@@ -2366,6 +2368,9 @@ static void GLimp_InitExtensions()
 	// made required in OpenGL 3.2
 	glConfig2.syncAvailable = LOAD_EXTENSION_WITH_TEST( ExtFlag_CORE, ARB_sync, r_arb_sync.Get() );
 
+	// made required in OpenGL 4.5
+	glConfig2.textureBarrierAvailable = LOAD_EXTENSION_WITH_TEST( ExtFlag_NONE, ARB_texture_barrier, r_arb_texture_barrier.Get() );
+
 	// made required in OpenGL 4.3
 	glConfig2.computeShaderAvailable = LOAD_EXTENSION_WITH_TEST( ExtFlag_NONE, ARB_compute_shader, r_arb_compute_shader.Get() );