LineArt: Back face culling

Option to discard back faced triangles, this speeds up calculation especially for when you only want to show visible feature lines.

Reviewed By: Antonio Vazquez (antoniov), Aleš Jelovčan (frogstomp)

Differential Revision: https://developer.blender.org/D13848

source/blender/gpencil_modifiers/intern/MOD_gpencillineart.c

@@ -409,6 +409,7 @@ static void options_panel_draw(const bContext *UNUSED(C), Panel *panel)
   uiItemR(col, ptr, "use_clip_plane_boundaries", 0, NULL, ICON_NONE);
   uiItemR(col, ptr, "use_crease_on_smooth", 0, IFACE_("Crease On Smooth"), ICON_NONE);
   uiItemR(col, ptr, "use_crease_on_sharp", 0, IFACE_("Crease On Sharp"), ICON_NONE);
+  uiItemR(col, ptr, "use_back_face_culling", 0, NULL, ICON_NONE);
 }
 
 static void style_panel_draw(const bContext *UNUSED(C), Panel *panel)

source/blender/gpencil_modifiers/intern/lineart/MOD_lineart.h

@@ -300,6 +300,7 @@ typedef struct LineartRenderBuffer {
   bool use_loose_edge_chain;
   bool use_geometry_space_chain;
   bool use_image_boundary_trimming;
+  bool use_back_face_culling;
 
   bool filter_face_mark;
   bool filter_face_mark_invert;

source/blender/gpencil_modifiers/intern/lineart/lineart_cpu.c

@@ -1346,6 +1346,10 @@ static void lineart_main_cull_triangles(LineartRenderBuffer *rb, bool clip_far)
       /* Select the triangle in the array. */
       tri = (void *)(((uchar *)eln->pointer) + rb->triangle_size * i);
 
+      if (tri->flags & LRT_CULL_DISCARD) {
+        continue;
+      }
+
       LRT_CULL_DECIDE_INSIDE
       LRT_CULL_ENSURE_MEMORY
       lineart_triangle_cull_single(rb,
@@ -1536,8 +1540,31 @@ static uint16_t lineart_identify_feature_line(LineartRenderBuffer *rb,
   double dot_1 = 0, dot_2 = 0;
   double result;
 
-  if (rb->cam_is_persp) {
-    sub_v3_v3v3_db(view_vector, l->gloc, rb->camera_pos);
+  if (rb->use_contour || rb->use_back_face_culling) {
+
+    if (rb->cam_is_persp) {
+      sub_v3_v3v3_db(view_vector, rb->camera_pos, l->gloc);
+    }
+    else {
+      view_vector = rb->view_vector;
+    }
+
+    dot_1 = dot_v3v3_db(view_vector, tri1->gn);
+    dot_2 = dot_v3v3_db(view_vector, tri2->gn);
+
+    if (rb->use_contour && (result = dot_1 * dot_2) <= 0 && (dot_1 + dot_2)) {
+      edge_flag_result |= LRT_EDGE_FLAG_CONTOUR;
+    }
+
+    /* Because the ray points towards the camera, so backface is when dot value being negative.*/
+    if (rb->use_back_face_culling) {
+      if (dot_1 < 0) {
+        tri1->flags |= LRT_CULL_DISCARD;
+      }
+      if (dot_2 < 0) {
+        tri2->flags |= LRT_CULL_DISCARD;
+      }
+    }
   }
   else {
     view_vector = rb->view_vector;
@@ -3160,6 +3187,12 @@ static LineartRenderBuffer *lineart_create_render_buffer(Scene *scene,
 
   rb->chain_preserve_details = (lmd->calculation_flags & LRT_CHAIN_PRESERVE_DETAILS) != 0;
 
+  /* This is used to limit calculation to a certain level to save time, lines who have higher
+   * occlusion levels will get ignored. */
+  rb->max_occlusion_level = lmd->level_end_override;
+
+  rb->use_back_face_culling = (lmd->calculation_flags & LRT_USE_BACK_FACE_CULLING) != 0;
+
   int16_t edge_types = lmd->edge_types_override;
 
   rb->use_contour = (edge_types & LRT_EDGE_FLAG_CONTOUR) != 0;
@@ -4178,12 +4211,6 @@ bool MOD_lineart_compute_feature_lines(Depsgraph *depsgraph,
    * See definition of LineartTriangleThread for details. */
   rb->triangle_size = lineart_triangle_size_get(scene, rb);
 
-  /* This is used to limit calculation to a certain level to save time, lines who have higher
-   * occlusion levels will get ignored. */
-  rb->max_occlusion_level = (lmd->flags & LRT_GPENCIL_USE_CACHE) ?
-                                lmd->level_end_override :
-                                (lmd->use_multiple_levels ? lmd->level_end : lmd->level_start);
-
   /* FIXME(Yiming): See definition of int #LineartRenderBuffer::_source_type for detailed. */
   rb->_source_type = lmd->source_type;
   rb->_source_collection = lmd->source_collection;

source/blender/makesdna/DNA_lineart_types.h

@@ -50,6 +50,7 @@ typedef enum eLineartMainFlags {
   LRT_USE_CREASE_ON_SMOOTH_SURFACES = (1 << 15),
   LRT_USE_CREASE_ON_SHARP_EDGES = (1 << 16),
   LRT_USE_CUSTOM_CAMERA = (1 << 17),
+  LRT_USE_BACK_FACE_CULLING = (1 << 19),
   LRT_USE_IMAGE_BOUNDARY_TRIMMING = (1 << 20),
   LRT_CHAIN_PRESERVE_DETAILS = (1 << 22),
 } eLineartMainFlags;

source/blender/makesrna/intern/rna_gpencil_modifier.c

@@ -3450,6 +3450,14 @@ static void rna_def_modifier_gpencillineart(BlenderRNA *brna)
       prop,
       "Image Boundary Trimming",
       "Trim all edges right at the boundary of image(including overscan region)");
+
+  prop = RNA_def_property(srna, "use_back_face_culling", PROP_BOOLEAN, PROP_NONE);
+  RNA_def_property_boolean_sdna(prop, NULL, "calculation_flags", LRT_USE_BACK_FACE_CULLING);
+  RNA_def_property_ui_text(
+      prop,
+      "Back Face Culling",
+      "Remove all back faces to speed up calculation, this will create edges in "
+      "different occlusion levels than when disabled");
   RNA_def_property_update(prop, 0, "rna_GpencilModifier_update");
 
   prop = RNA_def_property(srna, "use_invert_collection", PROP_BOOLEAN, PROP_NONE);