Fix #131420: EEVEE: Temporal projection+Motion blur artifacts

Regression from #131134.

The shader still reads `vel.obj.ofs[STEP_NEXT]`  even when in the
viewport.

- Ensure that `vel.obj.ofs[STEP_NEXT]` points to a valid memory
  location even if the computed value is still wrong.
- Ensure that the viewport always uses a 2 channel  vector_tx and all
  texture reads are always swizzled.

This also seems to fix the motion vector pass for the realtime compositor.

Pull Request: https://projects.blender.org/blender/blender/pulls/131445

source/blender/draw/engines/eevee_next/eevee_motion_blur.cc

@@ -243,19 +243,8 @@ void MotionBlurModule::render(View &view, GPUTexture **input_tx, GPUTexture **ou
 
   tile_indirection_buf_.clear_to_zero();
 
-  const bool do_motion_vectors_swizzle = inst_.render_buffers.vector_tx_format() == GPU_RG16F;
-  if (do_motion_vectors_swizzle) {
-    /* Change texture swizzling to avoid complexity in gather pass shader. */
-    GPU_texture_swizzle_set(inst_.render_buffers.vector_tx, "rgrg");
-  }
-
   inst_.manager->submit(motion_blur_ps_, view);
 
-  if (do_motion_vectors_swizzle) {
-    /* Reset swizzle since this texture might be reused in other places. */
-    GPU_texture_swizzle_set(inst_.render_buffers.vector_tx, "rgba");
-  }
-
   tiles_tx_.release();
 
   DRW_stats_group_end();

source/blender/draw/engines/eevee_next/eevee_renderbuffers.cc

@@ -82,6 +82,12 @@ void RenderBuffers::acquire(int2 extent)
   /* TODO(fclem): Make vector pass allocation optional if no TAA or motion blur is needed. */
   vector_tx.acquire(extent, vector_tx_format(), usage_attachment_read_write);
 
+  const bool do_motion_vectors_swizzle = vector_tx_format() == GPU_RG16F;
+  if (do_motion_vectors_swizzle) {
+    /* Change texture swizzling to avoid complexity in shaders. */
+    GPU_texture_swizzle_set(vector_tx, "rgrg");
+  }
+
   int color_len = data.color_len + data.aovs.color_len;
   int value_len = data.value_len + data.aovs.value_len;
 
@@ -117,18 +123,25 @@ void RenderBuffers::release()
   // depth_tx.release();
   combined_tx.release();
 
+  const bool do_motion_vectors_swizzle = vector_tx_format() == GPU_RG16F;
+  if (do_motion_vectors_swizzle) {
+    /* Reset swizzle since this texture might be reused in other places. */
+    GPU_texture_swizzle_set(vector_tx, "rgba");
+  }
   vector_tx.release();
+
   cryptomatte_tx.release();
 }
 
 eGPUTextureFormat RenderBuffers::vector_tx_format()
 {
   const eViewLayerEEVEEPassType enabled_passes = inst_.film.enabled_passes_get();
-  bool do_vector_render_pass = (enabled_passes & EEVEE_RENDER_PASS_VECTOR) ||
-                               (inst_.motion_blur.postfx_enabled() && !inst_.is_viewport());
+  bool do_full_vector_render_pass = ((enabled_passes & EEVEE_RENDER_PASS_VECTOR) ||
+                                     inst_.motion_blur.postfx_enabled()) &&
+                                    !inst_.is_viewport();
 
-  /* Only RG16F when only doing only reprojection or motion blur. */
-  return do_vector_render_pass ? GPU_RGBA16F : GPU_RG16F;
+  /* Only RG16F (motion.prev) for the viewport. */
+  return do_full_vector_render_pass ? GPU_RGBA16F : GPU_RG16F;
 }
 
 }  // namespace blender::eevee

source/blender/draw/engines/eevee_next/eevee_velocity.cc

@@ -167,10 +167,15 @@ bool VelocityModule::step_object_sync(ObjectKey &object_key,
       object_steps[STEP_PREVIOUS]->get_or_resize(
           vel.obj.ofs[STEP_PREVIOUS]) = ob->object_to_world();
     }
-    /* STEP_NEXT is not used in viewport. */
-    if (vel.obj.ofs[STEP_NEXT] == -1 && !inst_.is_viewport()) {
-      vel.obj.ofs[STEP_NEXT] = object_steps_usage[STEP_NEXT]++;
-      object_steps[STEP_NEXT]->get_or_resize(vel.obj.ofs[STEP_NEXT]) = ob->object_to_world();
+    if (vel.obj.ofs[STEP_NEXT] == -1) {
+      if (inst_.is_viewport()) {
+        /* Just set it to 0. motion.next is not meant to be valid in the viewport. */
+        vel.obj.ofs[STEP_NEXT] = 0;
+      }
+      else {
+        vel.obj.ofs[STEP_NEXT] = object_steps_usage[STEP_NEXT]++;
+        object_steps[STEP_NEXT]->get_or_resize(vel.obj.ofs[STEP_NEXT]) = ob->object_to_world();
+      }
     }
   }
 

source/blender/draw/engines/eevee_next/shaders/eevee_velocity_lib.glsl

@@ -128,6 +128,7 @@ void velocity_local_pos_get(vec3 lP, int vert_id, out vec3 lP_prev, out vec3 lP_
 /**
  * Given a triple of position, compute the previous and next motion vectors.
  * Returns a tuple of world space motion deltas.
+ * WARNING: The returned motion_next is invalid when rendering the viewport.
  */
 void velocity_vertex(
     vec3 lP_prev, vec3 lP, vec3 lP_next, out vec3 motion_prev, out vec3 motion_next)

source/blender/draw/engines/eevee_next/shaders/infos/eevee_velocity_info.hh

@@ -24,7 +24,8 @@
  * \{ */
 
 /* Pass world space deltas to the fragment shader.
- * This is to make sure that the resulting motion vectors are valid even with displacement. */
+ * This is to make sure that the resulting motion vectors are valid even with displacement.
+ * WARNING: The next value is invalid when rendering the viewport. */
 GPU_SHADER_NAMED_INTERFACE_INFO(eevee_velocity_surface_iface, motion)
 SMOOTH(VEC3, prev)
 SMOOTH(VEC3, next)