VR: Initial Virtual Reality support - Milestone 1, Scene Inspection

NOTE: While most of the milestone 1 goals are there, a few smaller features and
improvements are still to be done.

Big picture of this milestone: Initial, OpenXR-based virtual reality support
for users and foundation for advanced use cases.
Maniphest Task: https://developer.blender.org/T71347
The tasks contains more information about this milestone.

To be clear: This is not a feature rich VR implementation, it's focused on the
initial scene inspection use case. We intentionally focused on that, further
features like controller support are part of the next milestone.

- How to use?
Instructions on how to use this are here:
https://wiki.blender.org/wiki/User:Severin/GSoC-2019/How_to_Test
These will be updated and moved to a more official place (likely the manual) soon.

Currently Windows Mixed Reality and Oculus devices are usable. Valve/HTC
headsets don't support the OpenXR standard yet and hence, do not work with this
implementation.

---------------

This is the C-side implementation of the features added for initial VR
support as per milestone 1. A "VR Scene Inspection" Add-on will be
committed separately, to expose the VR functionality in the UI. It also
adds some further features for milestone 1, namely a landmarking system
(stored view locations in the VR space)

Main additions/features:
* Support for rendering viewports to an HMD, with good performance.
* Option to sync the VR view perspective with a fully interactive,
  regular 3D View (VR-Mirror).
* Option to disable positional tracking. Keeps the current position (calculated
  based on the VR eye center pose) when enabled while a VR session is running.
* Some regular viewport settings for the VR view
* RNA/Python-API to query and set VR session state information.
* WM-XR: Layer tying Ghost-XR to the Blender specific APIs/data
* wmSurface API: drawable, non-window container (manages Ghost-OpenGL and GPU
  context)
* DNA/RNA for management of VR session settings
* `--debug-xr` and `--debug-xr-time` commandline options
* Utility batch & config file for using the Oculus runtime on Windows.
* Most VR data is runtime only. The exception is user settings which are saved
  to files (`XrSessionSettings`).
* VR support can be disabled through the `WITH_XR_OPENXR` compiler flag.

For architecture and code documentation, see
https://wiki.blender.org/wiki/Source/Interface/XR.

---------------

A few thank you's:
* A huge shoutout to Ray Molenkamp for his help during the project - it would
  have not been that successful without him!
* Sebastian Koenig and Simeon Conzendorf for testing and feedback!
* The reviewers, especially Brecht Van Lommel!
* Dalai Felinto for pushing and managing me to get this done ;)
* The OpenXR working group for providing an open standard. I think we're the
  first bigger application to adopt OpenXR. Congratulations to them and
  ourselves :)

This project started as a Google Summer of Code 2019 project - "Core Support of
Virtual Reality Headsets through OpenXR" (see
https://wiki.blender.org/wiki/User:Severin/GSoC-2019/).
Some further information, including ideas for further improvements can be found
in the final GSoC report:
https://wiki.blender.org/wiki/User:Severin/GSoC-2019/Final_Report

Differential Revisions: D6193, D7098

Reviewed by: Brecht Van Lommel, Jeroen Bakker

1 files changed, 38 insertions, 8 deletions

diff --git a/source/blender/gpu/intern/gpu_viewport.c b/source/blender/gpu/intern/gpu_viewport.c
index 8ccfa3d9898..e4fd5d3f122 100644
--- a/source/blender/gpu/intern/gpu_viewport.c
+++ b/source/blender/gpu/intern/gpu_viewport.c
@@ -532,7 +532,13 @@ static void gpu_viewport_draw_colormanaged(GPUViewport *viewport,
   }
 }
 
-void GPU_viewport_draw_to_screen(GPUViewport *viewport, const rcti *rect)
+/**
+ * Version of #GPU_viewport_draw_to_screen() that lets caller decide if display colorspace
+ * transform should be performed.
+ */
+void GPU_viewport_draw_to_screen_ex(GPUViewport *viewport,
+                                    const rcti *rect,
+                                    bool display_colorspace)
 {
   DefaultFramebufferList *dfbl = viewport->fbl;
   DefaultTextureList *dtxl = viewport->txl;
@@ -545,18 +551,22 @@ void GPU_viewport_draw_to_screen(GPUViewport *viewport, const rcti *rect)
   const float w = (float)GPU_texture_width(color);
   const float h = (float)GPU_texture_height(color);
 
-  BLI_assert(w == BLI_rcti_size_x(rect) + 1);
-  BLI_assert(h == BLI_rcti_size_y(rect) + 1);
+  /* We allow rects with min/max swapped, but we also need coorectly assigned coordinates. */
+  rcti sanitized_rect = *rect;
+  BLI_rcti_sanitize(&sanitized_rect);
+
+  BLI_assert(w == BLI_rcti_size_x(&sanitized_rect) + 1);
+  BLI_assert(h == BLI_rcti_size_y(&sanitized_rect) + 1);
 
   /* wmOrtho for the screen has this same offset */
   const float halfx = GLA_PIXEL_OFS / w;
   const float halfy = GLA_PIXEL_OFS / h;
 
   rctf pos_rect = {
-      .xmin = rect->xmin,
-      .ymin = rect->ymin,
-      .xmax = rect->xmin + w,
-      .ymax = rect->ymin + h,
+      .xmin = sanitized_rect.xmin,
+      .ymin = sanitized_rect.ymin,
+      .xmax = sanitized_rect.xmin + w,
+      .ymax = sanitized_rect.ymin + h,
   };
 
   rctf uv_rect = {
@@ -565,8 +575,28 @@ void GPU_viewport_draw_to_screen(GPUViewport *viewport, const rcti *rect)
       .xmax = halfx + 1.0f,
       .ymax = halfy + 1.0f,
   };
+  /* Mirror the UV rect in case axis-swapped drawing is requested (by passing a rect with min and
+   * max values swapped). */
+  if (BLI_rcti_size_x(rect) < 0) {
+    SWAP(float, uv_rect.xmin, uv_rect.xmax);
+  }
+  if (BLI_rcti_size_y(rect) < 0) {
+    SWAP(float, uv_rect.ymin, uv_rect.ymax);
+  }
+
+  gpu_viewport_draw_colormanaged(viewport, &pos_rect, &uv_rect, display_colorspace);
+}
 
-  gpu_viewport_draw_colormanaged(viewport, &pos_rect, &uv_rect, true);
+/**
+ * Merge and draw the buffers of \a viewport into the currently active framebuffer, performing
+ * color transform to display space.
+ *
+ * \param rect: Coordinates to draw into. By swapping min and max values, drawing can be done with
+ *              inversed axis coordinates (upside down or sideways).
+ */
+void GPU_viewport_draw_to_screen(GPUViewport *viewport, const rcti *rect)
+{
+  GPU_viewport_draw_to_screen_ex(viewport, rect, true);
 }
 
 /**