From 9c03327701939be39012346293b1f9175a256a48 Mon Sep 17 00:00:00 2001
From: Geoffrey McRae <geoff@hostfission.com>
Date: Fri, 15 Dec 2017 16:21:38 +1100
Subject: [PATCH] [client] opengl: added back double buffering and vsync
 support

This adds back in double buffering and vsync support. This has been
carefully implemented so that the render function blocks until the video
card reports that it has advanced a frame, this ensures that the OpenGL
pipeline never buffers frames.
---
 client/renderers/opengl.c | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/client/renderers/opengl.c b/client/renderers/opengl.c
index 2edda7ac..490dafb4 100644
--- a/client/renderers/opengl.c
+++ b/client/renderers/opengl.c
@@ -47,6 +47,7 @@ struct LGR_OpenGL
 {
   LG_RendererParams params;
   bool              configured;
+  SDL_Window      * sdlWindow;
   SDL_GLContext     glContext;
   bool              doneInfo;
 
@@ -131,7 +132,7 @@ bool lgr_opengl_initialize(void ** opaque, const LG_RendererParams params, Uint3
   }
 
   *sdlFlags = SDL_WINDOW_OPENGL;
-  SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 0);
+  SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
   return true;
 }
 
@@ -147,6 +148,7 @@ bool lgr_opengl_configure(void * opaque, SDL_Window *window, const LG_RendererFo
     return false;
   }
 
+  this->sdlWindow = window;
   this->glContext = SDL_GL_CreateContext(window);
   if (!this->glContext)
   {
@@ -168,7 +170,7 @@ bool lgr_opengl_configure(void * opaque, SDL_Window *window, const LG_RendererFo
     return false;
   }
 
-  SDL_GL_SetSwapInterval(0);
+  SDL_GL_SetSwapInterval(this->params.vsync ? 1 : 0);
 
   // check if the GPU supports GL_ARB_buffer_storage first
   // there is no advantage to this renderer if it is not present.
@@ -706,10 +708,6 @@ bool lgr_opengl_render(void * opaque)
       return true;
   }
 
-  // wait for vsync
-  unsigned int count;
-  glXWaitVideoSyncSGI(1, 0, &count);
-
   glDisable(GL_SCISSOR_TEST);
   glClear(GL_COLOR_BUFFER_BIT);
   glEnable(GL_SCISSOR_TEST);
@@ -718,7 +716,15 @@ bool lgr_opengl_render(void * opaque)
   lgr_opengl_draw_mouse(this);
   if (this->fpsTexture)
     glCallList(this->fpsList);
-  glFlush();
+
+  unsigned int before, after;
+  glXGetVideoSyncSGI(&before);
+  SDL_GL_SwapWindow(this->sdlWindow);
+
+  // wait for the swap to happen to ensure we dont buffer frames
+  glXGetVideoSyncSGI(&after);
+  if (before == after)
+    glXWaitVideoSyncSGI(1, 0, &before);
 
   ++this->frameCount;
   const uint64_t t    = nanotime();