UV: Support Edge and Vert Slide

This commit adapts the existing Edge Slide and Vert Slide to work with
UVs as well.

It addresses [0] for the addition of an edge slide tool to the UV
Editor.

This feature allows users to slide UVs towards adjacent edges,
providing more convenient ways to edit UV maps.

[0]: https://blender.community/c/rightclickselect/66dbbc

Pull Request: https://projects.blender.org/blender/blender/pulls/119041

scripts/startup/bl_ui/space_image.py

@@ -531,6 +531,16 @@ class IMAGE_MT_uvs_context_menu(Menu):
 
         # UV Edit Mode
         if sima.show_uvedit:
+            ts = context.tool_settings
+            if ts.use_uv_select_sync:
+                is_vert_mode, is_edge_mode, _ = ts.mesh_select_mode
+            else:
+                uv_select_mode = ts.uv_select_mode
+                is_vert_mode = uv_select_mode == 'VERTEX'
+                is_edge_mode = uv_select_mode == 'EDGE'
+                # is_face_mode = uv_select_mode == 'FACE'
+                # is_island_mode = uv_select_mode == 'ISLAND'
+
             # Add
             layout.operator("uv.unwrap")
             layout.operator("uv.follow_active_quads")
@@ -552,6 +562,16 @@ class IMAGE_MT_uvs_context_menu(Menu):
 
             layout.operator_enum("uv.align", "axis")  # W, 2/3/4.
 
+            layout.operator_context = 'INVOKE_DEFAULT'
+
+            if is_vert_mode:
+                layout.operator("transform.vert_slide")
+
+            if is_edge_mode:
+                layout.operator("transform.edge_slide")
+
+            layout.operator_context = 'EXEC_REGION_WIN'
+
             layout.separator()
 
             # Remove

source/blender/editors/transform/transform.cc

@@ -674,7 +674,7 @@ static bool transform_modal_item_poll(const wmOperator *op, int value)
         return t->data_type == &TransConvertType_Tracking;
       }
       if (value == TFM_MODAL_VERT_EDGE_SLIDE &&
-          (t->data_type != &TransConvertType_Mesh ||
+          (!ELEM(t->data_type, &TransConvertType_Mesh, &TransConvertType_MeshUV) ||
            /* WORKAROUND: Avoid repeated keys in status bar.
             *
             * Previously, `Vert/Edge Slide` and `Move` were triggered by the same modal key.
@@ -1386,7 +1386,7 @@ int transformEvent(TransInfo *t, const wmEvent *event)
         break;
       case EVT_LEFTALTKEY:
       case EVT_RIGHTALTKEY:
-        if (ELEM(t->spacetype, SPACE_SEQ, SPACE_VIEW3D)) {
+        if (ELEM(t->spacetype, SPACE_SEQ, SPACE_VIEW3D, SPACE_IMAGE)) {
           t->flag |= T_ALT_TRANSFORM;
           t->redraw |= TREDRAW_HARD;
           handled = true;
@@ -1403,8 +1403,8 @@ int transformEvent(TransInfo *t, const wmEvent *event)
     switch (event->type) {
       case EVT_LEFTALTKEY:
       case EVT_RIGHTALTKEY:
-        /* TODO: Modal Map. */
-        if (ELEM(t->spacetype, SPACE_SEQ, SPACE_VIEW3D)) {
+        /* TODO: Modal Map */
+        if (ELEM(t->spacetype, SPACE_SEQ, SPACE_VIEW3D, SPACE_IMAGE)) {
           t->flag &= ~T_ALT_TRANSFORM;
           t->redraw |= TREDRAW_HARD;
           handled = true;

source/blender/editors/transform/transform_convert.hh

@@ -46,6 +46,7 @@ struct TransConvertTypeInfo {
 
 /**
  * Structure used for Edge Slide operation.
+ * The data is filled based on the 'transform_convert_' type.
  */
 struct TransDataEdgeSlideVert {
   TransData *td;
@@ -61,6 +62,7 @@ struct TransDataEdgeSlideVert {
 
 /**
  * Structure used for Vert Slide operation.
+ * The data is filled based on the 'transform_convert_' type.
  */
 struct TransDataVertSlideVert {
   TransData *td;
@@ -292,6 +294,14 @@ extern TransConvertTypeInfo TransConvertType_MeshSkin;
 
 extern TransConvertTypeInfo TransConvertType_MeshUV;
 
+blender::Array<TransDataVertSlideVert> transform_mesh_uv_vert_slide_data_create(
+    const TransInfo *t,
+    TransDataContainer *tc,
+    blender::Vector<blender::float3> &r_loc_dst_buffer);
+
+blender::Array<TransDataEdgeSlideVert> transform_mesh_uv_edge_slide_data_create(
+    const TransInfo *t, TransDataContainer *tc, int *r_group_len);
+
 /* `transform_convert_mesh_vert_cdata.cc` */
 
 extern TransConvertTypeInfo TransConvertType_MeshVertCData;

source/blender/editors/transform/transform_convert_mesh_uv.cc

@@ -9,6 +9,7 @@
 #include "MEM_guardedalloc.h"
 
 #include "BLI_linklist_stack.h"
+#include "BLI_math_geom.h"
 #include "BLI_math_matrix.h"
 #include "BLI_math_vector.h"
 
@@ -26,6 +27,8 @@
 #include "transform.hh"
 #include "transform_convert.hh"
 
+using namespace blender;
+
 /* -------------------------------------------------------------------- */
 /** \name UVs Transform Creation
  * \{ */
@@ -35,6 +38,7 @@ static void UVsToTransData(const float aspect[2],
                            const float *center,
                            const float calc_dist,
                            const bool selected,
+                           BMLoop *l,
                            TransData *r_td,
                            TransData2D *r_td2d)
 {
@@ -68,6 +72,7 @@ static void UVsToTransData(const float aspect[2],
   }
   unit_m3(r_td->mtx);
   unit_m3(r_td->smtx);
+  r_td->extra = l;
 }
 
 /**
@@ -380,7 +385,7 @@ static void createTransUVs(bContext *C, TransInfo *t)
         }
 
         luv = (float(*)[2])BM_ELEM_CD_GET_FLOAT_P(l, offsets.uv);
-        UVsToTransData(t->aspect, *luv, center, prop_distance, selected, td++, td2d++);
+        UVsToTransData(t->aspect, *luv, center, prop_distance, selected, l, td++, td2d++);
       }
     }
 
@@ -472,6 +477,675 @@ static void recalcData_uv(TransInfo *t)
 
 /** \} */
 
+/* -------------------------------------------------------------------- */
+/** \name API for Vert and Edge Slide
+ * \{ */
+
+struct UVGroups {
+  int sd_len;
+
+ private:
+  Vector<int> groups_offs_buffer;
+  Vector<int> groups_offs_indices;
+
+ public:
+  void init(const TransDataContainer *tc, BMesh *bm, const BMUVOffsets &offsets)
+  {
+    /* To identify #TransData by the corner, we first need to set all values in `index` to `-1`. */
+    BMIter fiter;
+    BMIter liter;
+    BMFace *f;
+    BM_ITER_MESH (f, &fiter, bm, BM_FACES_OF_MESH) {
+      BMLoop *l;
+      BM_ITER_ELEM (l, &liter, f, BM_LOOPS_OF_FACE) {
+        BM_elem_index_set(l, -1);
+      }
+    }
+
+    /* Now, count and set the index for the corners being transformed. */
+    this->sd_len = 0;
+    TransData *td = tc->data;
+    for (int i = 0; i < tc->data_len; i++, td++) {
+      if (!(td->flag & TD_SELECTED)) {
+        /* The selected ones are sorted at the beginning. */
+        break;
+      }
+      this->sd_len++;
+
+      BMLoop *l = static_cast<BMLoop *>(td->extra);
+      BM_elem_index_set(l, i);
+    }
+    bm->elem_index_dirty |= BM_LOOP;
+
+    /* Create the groups. */
+    this->groups_offs_buffer.reserve(this->sd_len);
+    this->groups_offs_indices.reserve((this->sd_len / 4) + 2);
+
+    td = tc->data;
+    for (int i = 0; i < tc->data_len; i++, td++) {
+      BMLoop *l_orig = static_cast<BMLoop *>(td->extra);
+      if (BM_elem_index_get(l_orig) == -1) {
+        /* Already added to a group. */
+        continue;
+      }
+
+      const float2 &uv_orig = BM_ELEM_CD_GET_FLOAT_P(l_orig, offsets.uv);
+      this->groups_offs_indices.append(this->groups_offs_buffer.size());
+
+      BMIter liter;
+      BMLoop *l_iter;
+      BM_ITER_ELEM (l_iter, &liter, l_orig->v, BM_LOOPS_OF_VERT) {
+        if (BM_elem_index_get(l_iter) == -1) {
+          /* Already added to a group or not participating in the transformation. */
+          continue;
+        }
+
+        if (l_orig != l_iter &&
+            !compare_v2v2(uv_orig, BM_ELEM_CD_GET_FLOAT_P(l_iter, offsets.uv), FLT_EPSILON))
+        {
+          /* Non-connected. */
+          continue;
+        }
+
+        this->groups_offs_buffer.append(BM_elem_index_get(l_iter));
+        BM_elem_index_set(l_iter, -1);
+      }
+    }
+    this->groups_offs_indices.append(this->groups_offs_buffer.size());
+  }
+
+  OffsetIndices<int> groups() const
+  {
+    return OffsetIndices<int>(this->groups_offs_indices);
+  }
+
+  Span<int> td_indices_get(const int group_index) const
+  {
+    return this->groups_offs_buffer.as_span().slice(this->groups()[group_index]);
+  }
+
+  Array<TransDataVertSlideVert> sd_array_create_and_init(TransDataContainer *tc)
+  {
+    Array<TransDataVertSlideVert> r_sv(this->sd_len);
+    TransDataVertSlideVert *sv = &r_sv[0];
+    for (const int group_index : this->groups().index_range()) {
+      for (int td_index : this->td_indices_get(group_index)) {
+        TransData *td = &tc->data[td_index];
+        sv->td = td;
+        sv++;
+      }
+    }
+
+    return r_sv;
+  }
+
+  Array<TransDataEdgeSlideVert> sd_array_create_and_init_edge(TransDataContainer *tc)
+  {
+    Array<TransDataEdgeSlideVert> r_sv(this->sd_len);
+    TransDataEdgeSlideVert *sv = &r_sv[0];
+    for (const int group_index : this->groups().index_range()) {
+      for (int td_index : this->td_indices_get(group_index)) {
+        TransData *td = &tc->data[td_index];
+        sv->td = td;
+        sv->dir_side[0] = float3(0);
+        sv->dir_side[1] = float3(0);
+        sv->loop_nr = -1;
+        sv++;
+      }
+    }
+
+    return r_sv;
+  }
+
+  MutableSpan<TransDataVertSlideVert> sd_group_get(MutableSpan<TransDataVertSlideVert> sd_array,
+                                                   const int group_index)
+  {
+    return sd_array.slice(this->groups()[group_index]);
+  }
+
+  MutableSpan<TransDataEdgeSlideVert> sd_group_get(MutableSpan<TransDataEdgeSlideVert> sd_array,
+                                                   const int group_index)
+  {
+    return sd_array.slice(this->groups()[group_index]);
+  }
+};
+
+static UVGroups *mesh_uv_groups_get(TransDataContainer *tc, BMesh *bm, const BMUVOffsets &offsets)
+{
+  UVGroups *uv_groups = static_cast<UVGroups *>(tc->custom.type.data);
+  if (uv_groups == nullptr) {
+    uv_groups = MEM_new<UVGroups>(__func__);
+    uv_groups->init(tc, bm, offsets);
+
+    /* Edge Slide and Vert Slide are often called in sequence, so, to avoid recalculating the
+     * groups, save them in the #TransDataContainer. */
+
+    tc->custom.type.data = uv_groups;
+    tc->custom.type.free_cb = [](TransInfo *, TransDataContainer *, TransCustomData *custom_data) {
+      UVGroups *data = static_cast<UVGroups *>(custom_data->data);
+      MEM_delete(data);
+      custom_data->data = nullptr;
+    };
+  }
+
+  return uv_groups;
+}
+
+/** \} */
+
+/* -------------------------------------------------------------------- */
+/** \name API for Vert Slide
+ * \{ */
+
+Array<TransDataVertSlideVert> transform_mesh_uv_vert_slide_data_create(
+    const TransInfo *t, TransDataContainer *tc, Vector<float3> &r_loc_dst_buffer)
+{
+
+  BMEditMesh *em = BKE_editmesh_from_object(tc->obedit);
+  BMesh *bm = em->bm;
+  const BMUVOffsets offsets = BM_uv_map_get_offsets(bm);
+
+  UVGroups *uv_groups = mesh_uv_groups_get(tc, bm, offsets);
+
+  Array<TransDataVertSlideVert> r_sv = uv_groups->sd_array_create_and_init(tc);
+
+  r_loc_dst_buffer.reserve(r_sv.size() * 4);
+
+  for (const int group_index : uv_groups->groups().index_range()) {
+    const int size_prev = r_loc_dst_buffer.size();
+
+    for (int td_index : uv_groups->td_indices_get(group_index)) {
+      TransData *td = &tc->data[td_index];
+      BMLoop *l = static_cast<BMLoop *>(td->extra);
+
+      for (BMLoop *l_dst : {l->prev, l->next}) {
+        const float2 &uv_dest = BM_ELEM_CD_GET_FLOAT_P(l_dst, offsets.uv);
+        Span<float3> uvs_added = r_loc_dst_buffer.as_span().drop_front(size_prev);
+
+        bool skip = std::any_of(
+            uvs_added.begin(), uvs_added.end(), [&](const float3 &uv_dest_added) {
+              return compare_v2v2(uv_dest, uv_dest_added, FLT_EPSILON);
+            });
+
+        if (!skip) {
+          r_loc_dst_buffer.append(float3(uv_dest, 0.0f));
+        }
+      }
+    }
+
+    const int size_new = r_loc_dst_buffer.size() - size_prev;
+    for (TransDataVertSlideVert &sv : uv_groups->sd_group_get(r_sv, group_index)) {
+      /* The buffer address may change as the vector is resized. Avoid setting #Span now. */
+      // sv.targets = r_loc_dst_buffer.as_span().drop_front(size_prev);
+
+      /* Store the buffer slice temporarily in `target_curr`. */
+      sv.co_link_orig_3d = {static_cast<float3 *>(POINTER_FROM_INT(size_prev)), size_new};
+      sv.co_link_curr = 0;
+    }
+  }
+
+  if (t->aspect[0] != 1.0f || t->aspect[1] != 1.0f) {
+    for (float3 &dest : r_loc_dst_buffer) {
+      dest[0] *= t->aspect[0];
+      dest[1] *= t->aspect[1];
+    }
+  }
+
+  for (TransDataVertSlideVert &sv : r_sv) {
+    int start = POINTER_AS_INT(sv.co_link_orig_3d.data());
+    sv.co_link_orig_3d = r_loc_dst_buffer.as_span().slice(start, sv.co_link_orig_3d.size());
+  }
+
+  return r_sv;
+}
+
+/** \} */
+
+/* -------------------------------------------------------------------- */
+/** \name API for Edge Slide
+ * \{ */
+
+/* Check if the UV group is a vertex between 2 faces. */
+static bool mesh_uv_group_is_inner(const TransDataContainer *tc,
+                                   const BMUVOffsets &offsets,
+                                   Span<int> group)
+{
+  if (group.size() == 1) {
+    return false;
+  }
+  if (group.size() > 2) {
+    return false;
+  }
+
+  TransData *td_a = &tc->data[group[0]];
+  TransData *td_b = &tc->data[group[1]];
+  BMLoop *l_a = static_cast<BMLoop *>(td_a->extra);
+  BMLoop *l_b = static_cast<BMLoop *>(td_b->extra);
+  BMLoop *l_a_prev = l_a->prev;
+  BMLoop *l_a_next = l_a->next;
+  BMLoop *l_b_prev = l_b->next;
+  BMLoop *l_b_next = l_b->prev;
+  if (l_a_prev->v != l_b_prev->v) {
+    std::swap(l_b_prev, l_b_next);
+    if (l_a_prev->v != l_b_prev->v) {
+      return false;
+    }
+  }
+
+  if (l_a_next->v != l_b_next->v) {
+    return false;
+  }
+
+  const float2 &uv_a_prev = BM_ELEM_CD_GET_FLOAT_P(l_a_prev, offsets.uv);
+  const float2 &uv_b_prev = BM_ELEM_CD_GET_FLOAT_P(l_b_prev, offsets.uv);
+  if (!compare_v2v2(uv_a_prev, uv_b_prev, FLT_EPSILON)) {
+    return false;
+  }
+
+  const float2 &uv_a_next = BM_ELEM_CD_GET_FLOAT_P(l_a_next, offsets.uv);
+  const float2 &uv_b_next = BM_ELEM_CD_GET_FLOAT_P(l_b_next, offsets.uv);
+  if (!compare_v2v2(uv_a_next, uv_b_next, FLT_EPSILON)) {
+    return false;
+  }
+
+  return true;
+}
+
+/**
+ * Find the closest point on the ngon on the opposite side.
+ * used to set the edge slide distance for ngons.
+ */
+static bool bm_loop_uv_calc_opposite_co(const BMLoop *l_tmp,
+                                        const float2 &uv_tmp,
+                                        const BMUVOffsets &offsets,
+                                        const float2 &ray_direction,
+                                        float2 &r_co)
+{
+  /* skip adjacent edges */
+  BMLoop *l_first = l_tmp->next;
+  BMLoop *l_last = l_tmp->prev;
+  BMLoop *l_iter;
+  float dist_sq_best = FLT_MAX;
+  bool found = false;
+
+  l_iter = l_first;
+  do {
+    const float2 &uv_iter = BM_ELEM_CD_GET_FLOAT_P(l_iter, offsets.uv);
+    const float2 &uv_iter_next = BM_ELEM_CD_GET_FLOAT_P(l_iter->next, offsets.uv);
+    float lambda;
+    if (isect_ray_seg_v2(uv_tmp, ray_direction, uv_iter, uv_iter_next, &lambda, nullptr) ||
+        isect_ray_seg_v2(uv_tmp, -ray_direction, uv_iter, uv_iter_next, &lambda, nullptr))
+    {
+      float2 isect_co = uv_tmp + ray_direction * lambda;
+      /* likelihood of multiple intersections per ngon is quite low,
+       * it would have to loop back on itself, but better support it
+       * so check for the closest opposite edge */
+      const float dist_sq_test = math::distance_squared(uv_tmp, isect_co);
+      if (dist_sq_test < dist_sq_best) {
+        r_co = isect_co;
+        dist_sq_best = dist_sq_test;
+        found = true;
+      }
+    }
+  } while ((l_iter = l_iter->next) != l_last);
+
+  return found;
+}
+
+static float2 isect_face_dst(const BMLoop *l,
+                             const float2 &uv,
+                             const float2 &aspect,
+                             const BMUVOffsets &offsets)
+{
+  BMFace *f = l->f;
+  BMLoop *l_next = l->next;
+  if (f->len == 4) {
+    /* we could use code below, but in this case
+     * sliding diagonally across the quad works well */
+    return BM_ELEM_CD_GET_FLOAT_P(l_next->next, offsets.uv);
+  }
+
+  BMLoop *l_prev = l->prev;
+  const float2 &uv_prev = BM_ELEM_CD_GET_FLOAT_P(l_prev, offsets.uv);
+  const float2 &uv_next = BM_ELEM_CD_GET_FLOAT_P(l_next, offsets.uv);
+
+  float2 ray_dir = (uv - uv_prev) + (uv_next - uv);
+  ray_dir = math::orthogonal(ray_dir * aspect);
+  ray_dir[0] /= aspect[0];
+  ray_dir[1] /= aspect[1];
+
+  float2 isect_co;
+  if (!bm_loop_uv_calc_opposite_co(l, uv, offsets, ray_dir, isect_co)) {
+    /* Rare case. */
+    mid_v3_v3v3(isect_co, l->prev->v->co, l_next->v->co);
+  }
+  return isect_co;
+}
+
+Array<TransDataEdgeSlideVert> transform_mesh_uv_edge_slide_data_create(const TransInfo *t,
+                                                                       TransDataContainer *tc,
+                                                                       int *r_group_len)
+{
+  Array<TransDataEdgeSlideVert> r_sv;
+  BMEditMesh *em = BKE_editmesh_from_object(tc->obedit);
+  BMesh *bm = em->bm;
+  const BMUVOffsets offsets = BM_uv_map_get_offsets(bm);
+
+  const bool check_edge = ED_uvedit_select_mode_get(t->scene) == UV_SELECT_EDGE;
+
+  UVGroups *uv_groups = mesh_uv_groups_get(tc, bm, offsets);
+  Array<int2> groups_linked(uv_groups->groups().size(), int2(-1, -1));
+
+  {
+    /* Identify the group to which a loop belongs through the element's index value. */
+
+    /* First we just need to "clean up" the neighboring loops.
+     * This way we can identify where a group of sliding edges starts and where it ends. */
+    TransData *td = tc->data;
+    for (int i = 0; i < tc->data_len; i++, td++) {
+      if (!(td->flag & TD_SELECTED)) {
+        /* The selected ones are sorted at the beginning. */
+        break;
+      }
+      BMLoop *l = static_cast<BMLoop *>(td->extra);
+      BM_elem_index_set(l->prev, -1);
+      BM_elem_index_set(l->next, -1);
+    }
+
+    /* Now set the group indexes. */
+    for (const int group_index : uv_groups->groups().index_range()) {
+      for (int td_index : uv_groups->td_indices_get(group_index)) {
+        TransData *td = &tc->data[td_index];
+        BMLoop *l = static_cast<BMLoop *>(td->extra);
+        BM_elem_index_set(l, group_index);
+      }
+    }
+    bm->elem_index_dirty |= BM_LOOP;
+  }
+
+  for (const int group_index : uv_groups->groups().index_range()) {
+    int2 &group_linked_pair = groups_linked[group_index];
+
+    for (int td_index : uv_groups->td_indices_get(group_index)) {
+      TransData *td = &tc->data[td_index];
+      BMLoop *l = static_cast<BMLoop *>(td->extra);
+
+      for (BMLoop *l_dst : {l->prev, l->next}) {
+        const int group_index_dst = BM_elem_index_get(l_dst);
+        if (group_index_dst == -1) {
+          continue;
+        }
+
+        if (ELEM(group_index_dst, group_linked_pair[0], group_linked_pair[1])) {
+          continue;
+        }
+
+        if (check_edge) {
+          BMLoop *l_edge = l_dst == l->prev ? l_dst : l;
+          if (!uvedit_edge_select_test_ex(t->settings, l_edge, offsets)) {
+            continue;
+          }
+        }
+
+        if (group_linked_pair[1] != -1) {
+          /* For Edge Slide, the vertex can only be connected to a maximum of 2 sliding edges. */
+          return r_sv;
+        }
+        const int slot = int(group_linked_pair[0] != -1);
+        group_linked_pair[slot] = group_index_dst;
+      }
+    }
+
+    if (group_linked_pair[0] == -1) {
+      /* For Edge Slide, the vertex must be connected to at least 1 sliding edge. */
+      return r_sv;
+    }
+  }
+
+  /* Alloc and initialize the #TransDataEdgeSlideVert. */
+  r_sv = uv_groups->sd_array_create_and_init_edge(tc);
+
+  /* Compute the sliding groups. */
+  int loop_nr = 0;
+  for (int i : r_sv.index_range()) {
+    if (r_sv[i].loop_nr != -1) {
+      /* This vertex has already been computed. */
+      continue;
+    }
+
+    BMLoop *l = static_cast<BMLoop *>(r_sv[i].td->extra);
+    int group_index = BM_elem_index_get(l);
+
+    /* Start from a vertex connected to just a single edge or any if it doesn't exist. */
+    int i_curr = group_index;
+    int i_prev = groups_linked[group_index][1];
+    while (!ELEM(i_prev, -1, group_index)) {
+      int tmp = groups_linked[i_prev][0] != i_curr ? groups_linked[i_prev][0] :
+                                                     groups_linked[i_prev][1];
+      i_curr = i_prev;
+      i_prev = tmp;
+    }
+
+    /**
+     * We need at least 3 points to calculate the intersection of
+     * `prev`-`curr` and `next`-`curr` destinations.
+     *
+     *  |         |         |
+     *  |         |         |
+     * prev ---- curr ---- next
+     */
+    struct SlideTempDataUV {
+      int i; /* The group index. */
+      struct {
+        BMFace *f;
+        float2 dst;
+      } fdata[2];
+      bool vert_is_inner; /* In the middle of two faces. */
+      /**
+       * Find the best direction to slide among the ones already computed.
+       *
+       * \param curr_prev: prev state of the #SlideTempDataUV where the faces are linked to the
+                           previous edge.
+       * \param l_src: the source corner in the edge to slide.
+       * \param l_dst: the current destination corner.
+       */
+      int find_best_dir(const SlideTempDataUV *curr_side_other,
+                        const BMLoop *l_src,
+                        const BMLoop *l_dst,
+                        const float2 &src,
+                        const float2 &dst,
+                        bool *r_do_isect_curr_dirs) const
+      {
+        *r_do_isect_curr_dirs = false;
+        const BMFace *f_curr = l_src->f;
+        if (curr_side_other->fdata[0].f &&
+            (curr_side_other->fdata[0].f == f_curr ||
+             compare_v2v2(dst, curr_side_other->fdata[0].dst, FLT_EPSILON)))
+        {
+          return 0;
+        }
+
+        if (curr_side_other->fdata[1].f &&
+            (curr_side_other->fdata[1].f == f_curr ||
+             compare_v2v2(dst, curr_side_other->fdata[1].dst, FLT_EPSILON)))
+        {
+          return 1;
+        }
+
+        if (curr_side_other->fdata[0].f || curr_side_other->fdata[1].f) {
+          /* Find the best direction checking the edges that share faces between them. */
+          int best_dir = -1;
+          const BMLoop *l_edge_dst = l_src->prev == l_dst ? l_src->prev : l_src;
+          const BMLoop *l_other = l_edge_dst->radial_next;
+          while (l_other != l_edge_dst) {
+            if (l_other->f == curr_side_other->fdata[0].f) {
+              best_dir = 0;
+              break;
+            }
+            if (l_other->f == curr_side_other->fdata[1].f) {
+              best_dir = 1;
+              break;
+            }
+            l_other = (l_other->v == l_src->v ? l_other->prev : l_other->next)->radial_next;
+          }
+
+          if (best_dir != -1) {
+            *r_do_isect_curr_dirs = true;
+            return best_dir;
+          }
+        }
+
+        if (ELEM(nullptr, this->fdata[0].f, this->fdata[1].f)) {
+          return int(this->fdata[0].f != nullptr);
+        }
+
+        /* Find the closest direction. */
+        *r_do_isect_curr_dirs = true;
+
+        float2 dir_curr = dst - src;
+        float2 dir0 = math::normalize(this->fdata[0].dst - src);
+        float2 dir1 = math::normalize(this->fdata[1].dst - src);
+        float dot0 = math::dot(dir_curr, dir0);
+        float dot1 = math::dot(dir_curr, dir1);
+        return int(dot0 < dot1);
+      }
+    } prev = {}, curr = {}, next = {}, tmp = {};
+
+    curr.i = i_curr;
+    curr.vert_is_inner = mesh_uv_group_is_inner(tc, offsets, uv_groups->td_indices_get(curr.i));
+
+    /* Do not compute `prev` for now. Let the loop calculate `curr` twice. */
+    prev.i = -1;
+
+    while (curr.i != -1) {
+      int tmp_i = prev.i == -1 ? i_prev : prev.i;
+      next.i = groups_linked[curr.i][0] != tmp_i ? groups_linked[curr.i][0] :
+                                                   groups_linked[curr.i][1];
+      if (next.i != -1) {
+        next.vert_is_inner = mesh_uv_group_is_inner(
+            tc, offsets, uv_groups->td_indices_get(next.i));
+
+        tmp = curr;
+        Span<int> td_indices_next = uv_groups->td_indices_get(next.i);
+
+        for (int td_index_curr : uv_groups->td_indices_get(curr.i)) {
+          BMLoop *l_curr = static_cast<BMLoop *>(tc->data[td_index_curr].extra);
+          const float2 &src = BM_ELEM_CD_GET_FLOAT_P(l_curr, offsets.uv);
+
+          for (int td_index_next : td_indices_next) {
+            BMLoop *l_next = static_cast<BMLoop *>(tc->data[td_index_next].extra);
+            if (l_curr->f != l_next->f) {
+              continue;
+            }
+
+            BLI_assert(l_curr != l_next);
+
+            BMLoop *l1_dst, *l2_dst;
+            if (l_curr->next == l_next) {
+              l1_dst = l_curr->prev;
+              l2_dst = l_next->next;
+            }
+            else {
+              l1_dst = l_curr->next;
+              l2_dst = l_next->prev;
+            }
+
+            const float2 &dst = BM_ELEM_CD_GET_FLOAT_P(l1_dst, offsets.uv);
+
+            /* Sometimes the sliding direction may fork (`isect_curr_dirs` is `true`).
+             * In this case, the resulting direction is the intersection of the destinations. */
+            bool isect_curr_dirs = false;
+
+            /* Identify the slot to slide according to the directions already computed in `curr`.
+             */
+            int best_dir = curr.find_best_dir(&tmp, l_curr, l1_dst, src, dst, &isect_curr_dirs);
+
+            if (curr.fdata[best_dir].f == nullptr) {
+              curr.fdata[best_dir].f = l_curr->f;
+              if (curr.vert_is_inner) {
+                curr.fdata[best_dir].dst = isect_face_dst(l_curr, src, t->aspect, offsets);
+              }
+              else {
+                curr.fdata[best_dir].dst = dst;
+              }
+            }
+
+            /* Compute `next`. */
+            next.fdata[best_dir].f = l_curr->f;
+            if (BM_elem_index_get(l2_dst) != -1 || next.vert_is_inner) {
+              /* Case where the vertex slides over the face. */
+              const float2 &src_next = BM_ELEM_CD_GET_FLOAT_P(l_next, offsets.uv);
+              next.fdata[best_dir].dst = isect_face_dst(l_next, src_next, t->aspect, offsets);
+            }
+            else {
+              /* Case where the vertex slides over an edge. */
+              const float2 &dst_next = BM_ELEM_CD_GET_FLOAT_P(l2_dst, offsets.uv);
+              next.fdata[best_dir].dst = dst_next;
+            }
+
+            if (isect_curr_dirs) {
+              /* The `best_dir` can only have one direction. */
+              const float2 &dst0 = prev.fdata[best_dir].dst;
+              const float2 &dst1 = curr.fdata[best_dir].dst;
+              const float2 &dst2 = dst;
+              const float2 &dst3 = next.fdata[best_dir].dst;
+              if (isect_line_line_v2_point(dst0, dst1, dst2, dst3, curr.fdata[best_dir].dst) ==
+                  ISECT_LINE_LINE_COLINEAR)
+              {
+                curr.fdata[best_dir].dst = math::midpoint(dst1, dst2);
+              }
+            }
+            /* There is only one pair of corners to slide per face, we don't need to keep checking
+             * `if (f_curr != l_next->f)`. */
+            break;
+          }
+        }
+      }
+
+      TransDataEdgeSlideVert *sv_first = nullptr;
+      for (TransDataEdgeSlideVert &sv : uv_groups->sd_group_get(r_sv, curr.i)) {
+        if (sv_first) {
+          TransData *td = sv.td;
+          sv = *sv_first;
+          sv.td = td;
+        }
+        else {
+          sv_first = &sv;
+          float2 iloc = sv.td->iloc;
+          const float2 &aspect = t->aspect;
+          if (curr.fdata[0].f) {
+            float2 dst = curr.fdata[0].dst * aspect;
+            sv.dir_side[0] = float3(dst - iloc, 0.0f);
+          }
+          if (curr.fdata[1].f) {
+            float2 dst = curr.fdata[1].dst * aspect;
+            sv.dir_side[1] = float3(dst - iloc, 0.0f);
+          }
+          sv.edge_len = math::distance(sv.dir_side[0], sv.dir_side[1]);
+          sv.loop_nr = loop_nr;
+        }
+      }
+
+      if (i_prev != -1 && prev.i == i_prev) {
+        /* Cycle returned to the beginning.
+         * The data with index `i_curr` was computed twice to make sure the directions are
+         * correct the second time. */
+        break;
+      }
+
+      /* Move forward. */
+      prev = curr;
+      curr = next;
+      next.fdata[0].f = next.fdata[1].f = nullptr;
+    }
+    loop_nr++;
+  }
+  *r_group_len = loop_nr;
+  return r_sv;
+}
+
+/** \} */
+
 TransConvertTypeInfo TransConvertType_MeshUV = {
     /*flags*/ (T_EDIT | T_POINTS | T_2D_EDIT),
     /*create_trans_data*/ createTransUVs,

source/blender/editors/transform/transform_mode_edge_slide.cc

@@ -334,7 +334,12 @@ static EdgeSlideData *createEdgeSlideVerts(TransInfo *t,
 {
   int group_len;
   EdgeSlideData *sld = MEM_new<EdgeSlideData>("sld");
+  if (t->data_type == &TransConvertType_MeshUV) {
+    sld->sv = transform_mesh_uv_edge_slide_data_create(t, tc, &group_len);
+  }
+  else {
     sld->sv = transform_mesh_edge_slide_data_create(tc, &group_len);
+  }
 
   if (sld->sv.is_empty()) {
     MEM_delete(sld);

source/blender/editors/transform/transform_mode_vert_slide.cc

@@ -178,7 +178,12 @@ static void calcVertSlideCustomPoints(TransInfo *t)
 static VertSlideData *createVertSlideVerts(TransInfo *t, TransDataContainer *tc)
 {
   VertSlideData *sld = MEM_new<VertSlideData>(__func__);
+  if (t->data_type == &TransConvertType_MeshUV) {
+    sld->sv = transform_mesh_uv_vert_slide_data_create(t, tc, sld->targets_buffer);
+  }
+  else {
     sld->sv = transform_mesh_vert_slide_data_create(tc, sld->targets_buffer);
+  }
 
   if (sld->sv.is_empty()) {
     MEM_delete(sld);

source/blender/editors/transform/transform_ops.cc

@@ -1197,7 +1197,7 @@ static void TRANSFORM_OT_edge_slide(wmOperatorType *ot)
   ot->exec = transform_exec;
   ot->modal = transform_modal;
   ot->cancel = transform_cancel;
-  ot->poll = ED_operator_editmesh_region_view3d;
+  ot->poll = ED_operator_editmesh;
   ot->poll_property = transform_poll_property;
 
   RNA_def_float_factor(ot->srna, "value", 0, -10.0f, 10.0f, "Factor", "", -1.0f, 1.0f);
@@ -1235,7 +1235,7 @@ static void TRANSFORM_OT_vert_slide(wmOperatorType *ot)
   ot->exec = transform_exec;
   ot->modal = transform_modal;
   ot->cancel = transform_cancel;
-  ot->poll = ED_operator_editmesh_region_view3d;
+  ot->poll = ED_operator_editmesh;
   ot->poll_property = transform_poll_property;
 
   RNA_def_float_factor(ot->srna, "value", 0, -10.0f, 10.0f, "Factor", "", -1.0f, 1.0f);