Add billboard script and example (#25)

examples/assets/sprites.atlas

@@ -0,0 +1,7 @@
+images {
+  image: "/builtins/assets/images/logo/logo_256.png"
+  sprite_trim_mode: SPRITE_TRIM_MODE_OFF
+}
+margin: 4
+extrude_borders: 0
+inner_padding: 0

examples/scenes/playground/playground.collection

@@ -662,3 +662,78 @@ embedded_instances {
     z: 1.0
   }
 }
+embedded_instances {
+  id: "billboard_sprite"
+  data: "components {\n"
+  "  id: \"billboard\"\n"
+  "  component: \"/scene3d/scripts/sprites/billboard.script\"\n"
+  "  position {\n"
+  "    x: 0.0\n"
+  "    y: 0.0\n"
+  "    z: 0.0\n"
+  "  }\n"
+  "  rotation {\n"
+  "    x: 0.0\n"
+  "    y: 0.0\n"
+  "    z: 0.0\n"
+  "    w: 1.0\n"
+  "  }\n"
+  "  properties {\n"
+  "    id: \"look_at_object_id\"\n"
+  "    value: \"/main_camera\"\n"
+  "    type: PROPERTY_TYPE_HASH\n"
+  "  }\n"
+  "  properties {\n"
+  "    id: \"use_rotation\"\n"
+  "    value: \"false\"\n"
+  "    type: PROPERTY_TYPE_BOOLEAN\n"
+  "  }\n"
+  "  properties {\n"
+  "    id: \"late_update\"\n"
+  "    value: \"true\"\n"
+  "    type: PROPERTY_TYPE_BOOLEAN\n"
+  "  }\n"
+  "  properties {\n"
+  "    id: \"late_update_priority\"\n"
+  "    value: \"20.0\"\n"
+  "    type: PROPERTY_TYPE_NUMBER\n"
+  "  }\n"
+  "}\n"
+  "embedded_components {\n"
+  "  id: \"sprite\"\n"
+  "  type: \"sprite\"\n"
+  "  data: \"tile_set: \\\"/examples/assets/sprites.atlas\\\"\\n"
+  "default_animation: \\\"logo_256\\\"\\n"
+  "material: \\\"/builtins/materials/sprite.material\\\"\\n"
+  "blend_mode: BLEND_MODE_ALPHA\\n"
+  "\"\n"
+  "  position {\n"
+  "    x: 0.0\n"
+  "    y: 0.0\n"
+  "    z: 0.0\n"
+  "  }\n"
+  "  rotation {\n"
+  "    x: 0.0\n"
+  "    y: 0.0\n"
+  "    z: 0.0\n"
+  "    w: 1.0\n"
+  "  }\n"
+  "}\n"
+  ""
+  position {
+    x: -8.0
+    y: 5.0
+    z: 6.5
+  }
+  rotation {
+    x: 0.0
+    y: 0.0
+    z: 0.0
+    w: 1.0
+  }
+  scale3 {
+    x: 0.005
+    y: 0.005
+    z: 0.005
+  }
+}

scene3d/render/render3d.lua

@@ -21,6 +21,7 @@ M.fov = 0
 M.near = 0
 M.far = 0
 M.view_position = vmath.vector3()
+M.view_rotation = vmath.quat() -- Note: it can be nil, if camera doesn't use function `view_from_rotation`.
 M.view_world_up = vmath.vector3()
 M.view_front = vmath.vector3()
 M.view_right = vmath.vector3()
@@ -94,6 +95,7 @@ function M.view_from_rotation(quat, viewport)
     viewport = viewport or M
 
     viewport.view_world_up = M.UP
+    viewport.view_rotation = quat
     viewport.view_front = vmath.rotate(quat, M.FORWARD)
     viewport.view_right = vmath.rotate(quat, M.RIGHT)
     viewport.view_up = vmath.rotate(quat, M.UP)
@@ -103,6 +105,7 @@ function M.view_direction(direction, viewport)
     viewport = viewport or M
 
     viewport.view_world_up = M.UP
+    viewport.view_rotation = nil -- Check the note above.
     viewport.view_front = direction
     -- Re-calculate the Right and Up vector, plus normalize the vectors,
     -- because their length gets closer to 0 the more you look up or down

scene3d/scripts/sprites/billboard.script

@@ -0,0 +1,61 @@
+-- Sometimes we'd like to make labels or other things that always face the camera. 
+-- Add this script as a component to your gameobject to make this happen.
+
+go.property("enabled", true)
+go.property("look_at_object_id", hash(""))
+go.property("use_rotation", true)
+go.property("use_world_transform", false)
+
+go.property("late_update", false)
+go.property("late_update_priority", 1)
+
+local render3d = require("scene3d.render.render3d")
+local math3d = require("scene3d.helpers.math3d")
+
+local EMPTY_HASH = hash("")
+
+local function update_rotation(self)
+    if self.use_rotation then
+        local rotation = render3d.view_rotation
+        if self.look_at_object_id ~= EMPTY_HASH then
+            rotation = self.use_world_transform and go.get_world_rotation(self.look_at_object_id) or go.get_rotation(self.look_at_object_id)
+        end
+
+        if rotation then
+            go.set_rotation(rotation)
+            return
+        end
+    end
+
+    local position = self.use_world_transform and go.get_world_position() or go.get_position()
+    local look_at_pos = render3d.view_position
+    if self.look_at_object_id ~= EMPTY_HASH then
+        look_at_pos = self.use_world_transform and go.get_world_position(self.look_at_object_id) or go.get_position(self.look_at_object_id)
+    end
+
+    local rotation = math3d.quat_look_rotation(position - look_at_pos, render3d.UP)
+    go.set_rotation(rotation)
+end
+
+-- This update will happen after game objects have been moved by the physics engine
+local function late_update(self)
+    update_rotation(self)
+end
+
+function init(self)
+    if self.late_update then
+        self.late_update_id = scene3d.prerender_register(late_update, self.late_update_priority)
+    end
+end
+
+function final(self)
+    if self.late_update_id then
+        scene3d.prerender_unregister(self.late_update_id)
+    end
+end
+
+function update(self, dt)
+    if not self.late_update_id then
+        update_rotation(self)
+    end
+end