GPv3: Canvas overlay

This implements the "Canvas" overlay for GPv3.

Some things of note:
- The color and subdivision are hard coded for now
- The scale and offset have been removed for now

Pull Request: https://projects.blender.org/blender/blender/pulls/126248

source/blender/draw/engines/overlay/overlay_engine.cc

@@ -229,6 +229,7 @@ static void OVERLAY_cache_init(void *vedata)
   OVERLAY_mode_transfer_cache_init(data);
   OVERLAY_extra_cache_init(data);
   OVERLAY_facing_cache_init(data);
+  OVERLAY_grease_pencil_cache_init(data);
   OVERLAY_gpencil_legacy_cache_init(data);
   OVERLAY_grid_cache_init(data);
   OVERLAY_image_cache_init(data);
@@ -697,6 +698,7 @@ static void OVERLAY_draw_scene(void *vedata)
   OVERLAY_particle_draw(data);
   OVERLAY_metaball_draw(data);
   OVERLAY_gpencil_legacy_draw(data);
+  OVERLAY_grease_pencil_draw(data);
   OVERLAY_extra_draw(data);
   if (pd->overlay.flag & V3D_OVERLAY_VIEWER_ATTRIBUTE) {
     OVERLAY_viewer_attribute_draw(data);

source/blender/draw/engines/overlay/overlay_grease_pencil.cc

@@ -13,6 +13,8 @@
 #include "BKE_attribute.hh"
 #include "BKE_grease_pencil.hh"
 
+#include "DNA_grease_pencil_types.h"
+
 #include "overlay_private.hh"
 
 void OVERLAY_edit_grease_pencil_cache_init(OVERLAY_Data *vedata)
@@ -70,6 +72,93 @@ void OVERLAY_edit_grease_pencil_cache_init(OVERLAY_Data *vedata)
   }
 }
 
+void OVERLAY_grease_pencil_cache_init(OVERLAY_Data *vedata)
+{
+  using namespace blender;
+  OVERLAY_PassList *psl = vedata->psl;
+  const DRWContextState *draw_ctx = DRW_context_state_get();
+  Scene *scene = draw_ctx->scene;
+  ToolSettings *ts = scene->toolsettings;
+  const bke::AttrDomain selection_domain = ED_grease_pencil_selection_domain_get(ts);
+  const View3D *v3d = draw_ctx->v3d;
+
+  GPUShader *sh;
+  DRWShadingGroup *grp;
+
+  /* Default: Display nothing. */
+  psl->grease_pencil_canvas_ps = nullptr;
+
+  Object *ob = draw_ctx->obact;
+  const bool show_overlays = (v3d->flag2 & V3D_HIDE_OVERLAYS) == 0;
+  const bool show_grid = (v3d->gp_flag & V3D_GP_SHOW_GRID) != 0 &&
+                         ((ts->gpencil_v3d_align &
+                           (GP_PROJECT_DEPTH_VIEW | GP_PROJECT_DEPTH_STROKE)) == 0);
+  const bool grid_xray = (v3d->gp_flag & V3D_GP_SHOW_GRID_XRAY);
+
+  if (show_grid && show_overlays) {
+    const float3 base_color = float3(0.5f);
+    const float4 col_grid = float4(base_color, v3d->overlay.gpencil_grid_opacity);
+
+    float4x4 mat = ob->object_to_world();
+
+    const GreasePencil &grease_pencil = *static_cast<GreasePencil *>(ob->data);
+    const blender::bke::greasepencil::Layer &layer = *grease_pencil.get_active_layer();
+
+    if (ts->gp_sculpt.lock_axis != GP_LOCKAXIS_CURSOR) {
+      mat = layer.to_world_space(*ob);
+    }
+    const View3DCursor *cursor = &scene->cursor;
+
+    /* Set the grid in the selected axis */
+    switch (ts->gp_sculpt.lock_axis) {
+      case GP_LOCKAXIS_X:
+        std::swap(mat[0], mat[2]);
+        break;
+      case GP_LOCKAXIS_Y:
+        std::swap(mat[1], mat[2]);
+        break;
+      case GP_LOCKAXIS_Z:
+        /* Default. */
+        break;
+      case GP_LOCKAXIS_CURSOR: {
+        mat = float4x4(cursor->matrix<float3x3>());
+        break;
+      }
+      case GP_LOCKAXIS_VIEW:
+        /* view aligned */
+        DRW_view_viewmat_get(nullptr, mat.ptr(), true);
+        break;
+    }
+
+    mat *= 2.0f;
+
+    if (ts->gpencil_v3d_align & GP_PROJECT_CURSOR) {
+      mat.location() = cursor->location;
+    }
+    else {
+      mat.location() = layer.to_world_space(*ob).location();
+    }
+
+    const int gridlines = 4;
+    const int line_count = gridlines * 4 + 2;
+
+    DRWState state = DRW_STATE_WRITE_COLOR | DRW_STATE_BLEND_ALPHA;
+    state |= (grid_xray) ? DRW_STATE_DEPTH_ALWAYS : DRW_STATE_DEPTH_LESS_EQUAL;
+
+    DRW_PASS_CREATE(psl->grease_pencil_canvas_ps, state);
+
+    sh = OVERLAY_shader_gpencil_canvas();
+    grp = DRW_shgroup_create(sh, psl->grease_pencil_canvas_ps);
+    DRW_shgroup_uniform_block(grp, "globalsBlock", G_draw.block_ubo);
+    DRW_shgroup_uniform_vec4_copy(grp, "color", col_grid);
+    DRW_shgroup_uniform_vec3_copy(grp, "xAxis", mat[0]);
+    DRW_shgroup_uniform_vec3_copy(grp, "yAxis", mat[1]);
+    DRW_shgroup_uniform_vec3_copy(grp, "origin", mat[3]);
+    DRW_shgroup_uniform_int_copy(grp, "halfLineCount", line_count / 2);
+    DRW_shgroup_call_procedural_lines(grp, nullptr, line_count);
+  }
+}
+
 void OVERLAY_edit_grease_pencil_cache_populate(OVERLAY_Data *vedata, Object *ob)
 {
   using namespace blender::draw;
@@ -140,6 +229,15 @@ void OVERLAY_weight_grease_pencil_cache_populate(OVERLAY_Data *vedata, Object *o
   }
 }
 
+void OVERLAY_grease_pencil_draw(OVERLAY_Data *vedata)
+{
+  OVERLAY_PassList *psl = vedata->psl;
+
+  if (psl->grease_pencil_canvas_ps) {
+    DRW_draw_pass(psl->grease_pencil_canvas_ps);
+  }
+}
+
 void OVERLAY_edit_grease_pencil_draw(OVERLAY_Data *vedata)
 {
   OVERLAY_PassList *psl = vedata->psl;

source/blender/draw/engines/overlay/overlay_private.hh

@@ -102,6 +102,7 @@ struct OVERLAY_PassList {
   DRWPass *extra_centers_ps;
   DRWPass *extra_grid_ps;
   DRWPass *gpencil_canvas_ps;
+  DRWPass *grease_pencil_canvas_ps;
   DRWPass *facing_ps[2];
   DRWPass *fade_ps[2];
   DRWPass *mode_transfer_ps[2];
@@ -562,10 +563,12 @@ void OVERLAY_gpencil_legacy_cache_populate(OVERLAY_Data *vedata, Object *ob);
 void OVERLAY_gpencil_legacy_draw(OVERLAY_Data *vedata);
 void OVERLAY_edit_gpencil_legacy_draw(OVERLAY_Data *vedata);
 
+void OVERLAY_grease_pencil_cache_init(OVERLAY_Data *vedata);
 void OVERLAY_edit_grease_pencil_cache_init(OVERLAY_Data *vedata);
 void OVERLAY_edit_grease_pencil_cache_populate(OVERLAY_Data *vedata, Object *ob);
 void OVERLAY_sculpt_grease_pencil_cache_populate(OVERLAY_Data *vedata, Object *ob);
 void OVERLAY_weight_grease_pencil_cache_populate(OVERLAY_Data *vedata, Object *ob);
+void OVERLAY_grease_pencil_draw(OVERLAY_Data *vedata);
 void OVERLAY_edit_grease_pencil_draw(OVERLAY_Data *vedata);
 
 void OVERLAY_edit_lattice_cache_init(OVERLAY_Data *vedata);