DRW: View: Add ViewVecs calculation

This will remove some code duplication between draw engines.

source/blender/draw/intern/draw_manager.h

@@ -380,6 +380,7 @@ typedef struct DRWViewUboStorage {
   float wininv[4][4];
 
   float clipplanes[6][4];
+  float viewvecs[2][4];
   /* Should not be here. Not view dependent (only main view). */
   float viewcamtexcofac[4];
 } DRWViewUboStorage;

source/blender/draw/intern/draw_manager_data.c

@@ -1656,6 +1656,52 @@ static void draw_view_matrix_state_update(DRWViewUboStorage *storage,
 
   mul_m4_m4m4(storage->persmat, winmat, viewmat);
   invert_m4_m4(storage->persinv, storage->persmat);
+
+  const bool is_persp = (winmat[3][3] == 0.0f);
+
+  /* Near clip distance. */
+  storage->viewvecs[0][3] = (is_persp) ? -winmat[3][2] / (winmat[2][2] - 1.0f) :
+                                         -(winmat[3][2] + 1.0f) / winmat[2][2];
+
+  /* Far clip distance. */
+  storage->viewvecs[1][3] = (is_persp) ? -winmat[3][2] / (winmat[2][2] + 1.0f) :
+                                         -(winmat[3][2] - 1.0f) / winmat[2][2];
+
+  /* view vectors for the corners of the view frustum.
+   * Can be used to recreate the world space position easily */
+  float view_vecs[4][3] = {
+      {-1.0f, -1.0f, -1.0f},
+      {1.0f, -1.0f, -1.0f},
+      {-1.0f, 1.0f, -1.0f},
+      {-1.0f, -1.0f, 1.0f},
+  };
+
+  /* convert the view vectors to view space */
+  for (int i = 0; i < 4; i++) {
+    mul_project_m4_v3(storage->wininv, view_vecs[i]);
+    /* normalized trick see:
+     * http://www.derschmale.com/2014/01/26/reconstructing-positions-from-the-depth-buffer */
+    if (is_persp) {
+      /* Divide XY by Z. */
+      mul_v2_fl(view_vecs[i], 1.0f / view_vecs[i][2]);
+    }
+  }
+
+  /**
+   * If ortho : view_vecs[0] is the near-bottom-left corner of the frustum and
+   *            view_vecs[1] is the vector going from the near-bottom-left corner to
+   *            the far-top-right corner.
+   * If Persp : view_vecs[0].xy and view_vecs[1].xy are respectively the bottom-left corner
+   *            when Z = 1, and top-left corner if Z = 1.
+   *            view_vecs[0].z the near clip distance and view_vecs[1].z is the (signed)
+   *            distance from the near plane to the far clip plane.
+   */
+  copy_v3_v3(storage->viewvecs[0], view_vecs[0]);
+
+  /* we need to store the differences */
+  storage->viewvecs[1][0] = view_vecs[1][0] - view_vecs[0][0];
+  storage->viewvecs[1][1] = view_vecs[2][1] - view_vecs[0][1];
+  storage->viewvecs[1][2] = view_vecs[3][2] - view_vecs[0][2];
 }
 
 /* Create a view with culling. */

source/blender/draw/intern/shaders/common_view_lib.glsl

@@ -14,6 +14,10 @@ layout(std140) uniform viewBlock
 
   vec4 clipPlanes[6];
 
+  /* View frustum corners [NDC(-1.0, -1.0, -1.0) & NDC(1.0, 1.0, 1.0)].
+   * Fourth components are near and far values. */
+  vec4 ViewVecs[2];
+
   /* TODO move it elsewhere. */
   vec4 CameraTexCoFactors;
 };