Python API: option for render engines to delegate Freestyle render to Eevee

Eevee is now used for Freestyle rendering by default, since other engines are
unlikely to have support for this. Workbench and Cycles do their own rendering.

RenderEngine add-ons can do their own Freestyle rendering by setting
bl_use_custom_freestyle = True.

Differential Revision: https://developer.blender.org/D8335

intern/cycles/blender/addon/__init__.py

@@ -59,6 +59,7 @@ class CyclesRender(bpy.types.RenderEngine):
     bl_use_exclude_layers = True
     bl_use_save_buffers = True
     bl_use_spherical_stereo = True
+    bl_use_custom_freestyle = True
 
     def __init__(self):
         self.session = None

source/blender/makesrna/intern/rna_render.c

@@ -848,6 +848,14 @@ static void rna_def_render_engine(BlenderRNA *brna)
   RNA_def_property_ui_text(
       prop, "Use Eevee Viewport", "Uses Eevee for viewport shading in LookDev shading mode");
 
+  prop = RNA_def_property(srna, "bl_use_custom_freestyle", PROP_BOOLEAN, PROP_NONE);
+  RNA_def_property_boolean_sdna(prop, NULL, "type->flag", RE_USE_CUSTOM_FREESTYLE);
+  RNA_def_property_flag(prop, PROP_REGISTER_OPTIONAL);
+  RNA_def_property_ui_text(
+      prop,
+      "Use Custom Freestyle",
+      "Handles freestyle rendering on its own, instead of delegating it to EEVEE");
+
   prop = RNA_def_property(srna, "bl_use_gpu_context", PROP_BOOLEAN, PROP_NONE);
   RNA_def_property_boolean_sdna(prop, NULL, "type->flag", RE_USE_GPU_CONTEXT);
   RNA_def_property_flag(prop, PROP_REGISTER_OPTIONAL);

source/blender/render/RE_engine.h

@@ -64,6 +64,7 @@ extern "C" {
 #define RE_USE_SPHERICAL_STEREO 128
 #define RE_USE_STEREO_VIEWPORT 256
 #define RE_USE_GPU_CONTEXT 512
+#define RE_USE_CUSTOM_FREESTYLE 1024
 
 /* RenderEngine.flag */
 #define RE_ENGINE_ANIMATION 1

source/blender/render/intern/pipeline.c

@@ -2007,13 +2007,38 @@ void RE_RenderFrame(Render *re,
   G.is_rendering = false;
 }
 
+static void change_renderdata_engine(Render *re, const char *new_engine)
+{
+  if (!STREQ(re->r.engine, new_engine)) {
+    if (re->engine) {
+      RE_engine_free(re->engine);
+      re->engine = NULL;
+    }
+    BLI_strncpy(re->r.engine, new_engine, sizeof(re->r.engine));
+  }
+}
+
 #ifdef WITH_FREESTYLE
+static bool use_eevee_for_freestyle_render(Render *re)
+{
+  RenderEngineType *type = RE_engines_find(re->r.engine);
+  return !(type->flag & RE_USE_CUSTOM_FREESTYLE);
+}
+
 void RE_RenderFreestyleStrokes(Render *re, Main *bmain, Scene *scene, int render)
 {
   re->result_ok = 0;
   if (render_init_from_main(re, &scene->r, bmain, scene, NULL, NULL, 0, 0)) {
     if (render) {
+      char scene_engine[32];
+      BLI_strncpy(scene_engine, re->r.engine, sizeof(scene_engine));
+      if (use_eevee_for_freestyle_render(re)) {
+        change_renderdata_engine(re, RE_engine_id_BLENDER_EEVEE);
+      }
+
       do_render_3d(re);
+
+      change_renderdata_engine(re, scene_engine);
     }
   }
   re->result_ok = 1;