Preserve depth buffer between 3D layers + optimize render order (#9931)

Port of https://github.com/mapbox/mapbox-gl-js/pull/5101: adds a new render pass `Pass3D` before any other rendering wherein we render layers with 3D passes (fill-extrusion layers) to offscreen framebuffers, sharing a depth renderbuffer between those layers in order to render 3D space correctly. Those framebuffers are saved on the RenderLayers and copied back to the map during the translucent pass. Rendering to offscreen framebuffers before we do any clear + draw means we can avoid expensive framebuffer restores.

2 files changed, 23 insertions, 18 deletions

diff --git a/src/mbgl/util/offscreen_texture.cpp b/src/mbgl/util/offscreen_texture.cpp
index d8a68b9a4a..339e74b250 100644
--- a/src/mbgl/util/offscreen_texture.cpp
+++ b/src/mbgl/util/offscreen_texture.cpp
@@ -11,20 +11,22 @@ OffscreenTexture& OffscreenTexture::operator=(OffscreenTexture&&) = default;
 
 class OffscreenTexture::Impl {
 public:
-    Impl(gl::Context& context_, const Size size_, OffscreenTextureAttachment type_)
-        : context(context_), size(std::move(size_)), type(type_) {
+    Impl(gl::Context& context_, const Size size_)
+        : context(context_), size(std::move(size_)) {
+        assert(!size.isEmpty());
+    }
+    Impl(gl::Context& context_,
+         const Size size_,
+         gl::Renderbuffer<gl::RenderbufferType::DepthComponent>& depth_)
+        : context(context_), size(std::move(size_)), depth(&depth_) {
         assert(!size.isEmpty());
     }
 
     void bind() {
         if (!framebuffer) {
             texture = context.createTexture(size, gl::TextureFormat::RGBA);
-
-            if (type == OffscreenTextureAttachment::Depth) {
-                gl::Renderbuffer<gl::RenderbufferType::DepthComponent> depth =
-                    context.createRenderbuffer<gl::RenderbufferType::DepthComponent>(size);
-                framebuffer = context.createFramebuffer(*texture, depth);
-
+            if (depth) {
+                framebuffer = context.createFramebuffer(*texture, *depth);
             } else {
                 framebuffer = context.createFramebuffer(*texture);
             }
@@ -53,15 +55,21 @@ public:
 private:
     gl::Context& context;
     const Size size;
-    OffscreenTextureAttachment type;
     optional<gl::Framebuffer> framebuffer;
     optional<gl::Texture> texture;
+    gl::Renderbuffer<gl::RenderbufferType::DepthComponent>* depth = nullptr;
 };
 
 OffscreenTexture::OffscreenTexture(gl::Context& context,
+                                   const Size size)
+    : impl(std::make_unique<Impl>(context, std::move(size))) {
+    assert(!size.isEmpty());
+}
+
+OffscreenTexture::OffscreenTexture(gl::Context& context,
                                    const Size size,
-                                   OffscreenTextureAttachment type)
-    : impl(std::make_unique<Impl>(context, std::move(size), type)) {
+                                   gl::Renderbuffer<gl::RenderbufferType::DepthComponent>& renderbuffer)
+    : impl(std::make_unique<Impl>(context, std::move(size), renderbuffer)) {
     assert(!size.isEmpty());
 }
 
diff --git a/src/mbgl/util/offscreen_texture.hpp b/src/mbgl/util/offscreen_texture.hpp
index 0353f3f9c5..7f7e0f0338 100644
--- a/src/mbgl/util/offscreen_texture.hpp
+++ b/src/mbgl/util/offscreen_texture.hpp
@@ -9,16 +9,13 @@ class Context;
 class Texture;
 } // namespace gl
 
-enum class OffscreenTextureAttachment {
-    None,
-    Depth,
-};
-
 class OffscreenTexture {
 public:
     OffscreenTexture(gl::Context&,
-                     Size size = { 256, 256 },
-                     OffscreenTextureAttachment type = OffscreenTextureAttachment::None);
+                     Size size = { 256, 256 });
+    OffscreenTexture(gl::Context&,
+                     Size size,
+                     gl::Renderbuffer<gl::RenderbufferType::DepthComponent>&);
     ~OffscreenTexture();
     OffscreenTexture(OffscreenTexture&&);
     OffscreenTexture& operator=(OffscreenTexture&&);