Stop z clipping of 2d drawing without depth clamp

With RmlUi transforms and 3D CSS directives such as transform:
rotateX(...), nonzero z coordinates could be produced. Instead of
relying on GL_DEPTH_CLAMP which is not always available to prevent
clipping by near or far planes, just use a projection matrix that always
maps z to 0.

Author: slipher

src/engine/renderer/tr_backend.cpp

@@ -824,6 +824,9 @@ static void RB_SetGL2D()
 	GL_Scissor( 0, 0, glConfig.vidWidth, glConfig.vidHeight );
 
 	MatrixOrthogonalProjection( proj, 0, glConfig.vidWidth, glConfig.vidHeight, 0, 0, 1 );
+	// zero the z coordinate so it's never near/far clipped
+	proj[ 2 ] = proj[ 6 ] = proj[ 10 ] = proj[ 14 ] = 0;
+
 	GL_LoadProjectionMatrix( proj );
 	GL_LoadModelViewMatrix( matrixIdentity );
 

src/engine/renderer/tr_public.h

@@ -132,7 +132,6 @@ struct glconfig2_t
 	bool uniformBufferObjectAvailable;
 	bool mapBufferRangeAvailable;
 	bool syncAvailable;
-	bool depthClampAvailable;
 	bool halfFloatVertexAvailable;
 
 	bool colorGrading;

src/engine/renderer/tr_shade.cpp

@@ -851,21 +851,11 @@ static void Render_generic2D( shaderStage_t *pStage )
 	gl_generic2DShader->SetUniform_ColorMapBindless( BindAnimatedImage( 0, &pStage->bundle[TB_COLORMAP] ) );
 	gl_generic2DShader->SetUniform_TextureMatrix( tess.svars.texMatrices[ TB_COLORMAP ] );
 
-	if ( glConfig2.depthClampAvailable )
-	{
-		glEnable( GL_DEPTH_CLAMP );
-	}
-
 	gl_generic2DShader->SetRequiredVertexPointers();
 
 	Tess_DrawElements();
 	GL_CheckErrors();
 
-	if ( glConfig2.depthClampAvailable )
-	{
-		glDisable( GL_DEPTH_CLAMP );
-	}
-
 	GL_CheckErrors();
 }
 

src/engine/sys/sdl_glimp.cpp

@@ -70,8 +70,6 @@ static Cvar::Cvar<bool> r_arb_buffer_storage( "r_arb_buffer_storage",
 	"Use GL_ARB_buffer_storage if available", Cvar::NONE, true );
 static Cvar::Cvar<bool> r_arb_compute_shader( "r_arb_compute_shader",
 	"Use GL_ARB_compute_shader if available", Cvar::NONE, true );
-static Cvar::Cvar<bool> r_arb_depth_clamp( "r_arb_depth_clamp",
-	"Use GL_ARB_depth_clamp if available", Cvar::NONE, true );
 static Cvar::Cvar<bool> r_arb_framebuffer_object( "r_arb_framebuffer_object",
 	"Use GL_ARB_framebuffer_object if available", Cvar::NONE, true );
 static Cvar::Cvar<bool> r_arb_explicit_uniform_location( "r_arb_explicit_uniform_location",
@@ -1973,7 +1971,6 @@ static void GLimp_InitExtensions()
 	Cvar::Latch( r_arb_bindless_texture );
 	Cvar::Latch( r_arb_buffer_storage );
 	Cvar::Latch( r_arb_compute_shader );
-	Cvar::Latch( r_arb_depth_clamp );
 	Cvar::Latch( r_arb_explicit_uniform_location );
 	Cvar::Latch( r_arb_framebuffer_object );
 	Cvar::Latch( r_arb_gpu_shader5 );
@@ -2395,9 +2392,6 @@ 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 3.2
-	glConfig2.depthClampAvailable = LOAD_EXTENSION_WITH_TEST( ExtFlag_CORE, ARB_depth_clamp, r_arb_depth_clamp.Get() );
-
 	// made required in OpenGL 4.3
 	glConfig2.computeShaderAvailable = LOAD_EXTENSION_WITH_TEST( ExtFlag_NONE, ARB_compute_shader, r_arb_compute_shader.Get() );