diff --git a/src/gl/renderer/gl_renderstate.cpp b/src/gl/renderer/gl_renderstate.cpp
index 9a3ea12e6..1bfed5fc6 100644
--- a/src/gl/renderer/gl_renderstate.cpp
+++ b/src/gl/renderer/gl_renderstate.cpp
@@ -440,7 +440,8 @@ void FGLRenderState::SetStencil(int offs, int op, int flags)
 	glStencilOp(GL_KEEP, GL_KEEP, op2gl[op]);		// this stage doesn't modify the stencil
 
 	bool cmon = !(flags & SF_ColorMaskOff);
-	glColorMask(cmon, cmon, cmon, cmon);						// don't write to the graphics buffer
+	bool cmalpha = cmon || (flags & SF_ColorMaskAlpha);
+	glColorMask(cmon, cmon, cmon, cmalpha);						// don't write to the graphics buffer
 	glDepthMask(!(flags & SF_DepthMaskOff));
 }
 
diff --git a/src/hwrenderer/scene/hw_portal.cpp b/src/hwrenderer/scene/hw_portal.cpp
index dc2c8f4fb..6426e4213 100644
--- a/src/hwrenderer/scene/hw_portal.cpp
+++ b/src/hwrenderer/scene/hw_portal.cpp
@@ -301,7 +301,7 @@ void HWPortal::RemoveStencil(HWDrawInfo *di, FRenderState &state, bool usestenci
 		state.EnableTexture(false);
 		state.SetRenderStyle(STYLE_Source);
 
-		state.SetStencil(0, SOP_Keep, SF_ColorMaskOff);
+		state.SetStencil(0, SOP_Keep, SF_ColorMaskOff | SF_ColorMaskAlpha);	// SSAO needs the alpha channel as a marker.
 		if (needdepth) state.Clear(CT_Depth);
 		state.SetDepthRange(0, 1);
 		state.SetDepthFunc(DF_LEqual);
@@ -946,8 +946,8 @@ void HWEEHorizonPortal::DrawContents(HWDrawInfo *di, FRenderState &state)
 	{
 		GLSkyInfo skyinfo;
 		skyinfo.init(sector->sky, 0);
-		//GLSkyPortal sky(mState, &skyinfo, true);
-		//sky.DrawContents(di, state);
+		HWSkyPortal sky(screen->mSkyData, mState, &skyinfo, true);
+		sky.DrawContents(di, state);
 	}
 	if (sector->GetTexture(sector_t::ceiling) != skyflatnum)
 	{
diff --git a/src/hwrenderer/scene/hw_renderstate.h b/src/hwrenderer/scene/hw_renderstate.h
index 52a053d46..530041391 100644
--- a/src/hwrenderer/scene/hw_renderstate.h
+++ b/src/hwrenderer/scene/hw_renderstate.h
@@ -57,6 +57,7 @@ enum EStencilFlags
 	SF_AllOn = 0,
 	SF_ColorMaskOff = 1,
 	SF_DepthMaskOff = 2,
+	SF_ColorMaskAlpha = 4,	// hack value for SSAO
 };
 
 enum EStencilOp