From da78dd47e37d01565d9604dd5e55d86fd0ed85bc Mon Sep 17 00:00:00 2001
From: Falk David <falk@blender.org>
Date: Thu, 21 Dec 2023 11:30:16 +0100
Subject: [PATCH] Fix #116343: Drawing on frame with auto-keying off gives
 error

When drawing on a frame that didn't have a keyframe, but did have a drawing,
Blender would report that there was no drawing to draw on.
The issue was some faulty logic on the invoke of the drawing code.

Now, this properly checks if a new frame needs to be inserted and
only reports an error in case auto-key is off and there is no drawing.

Pull Request: https://projects.blender.org/blender/blender/pulls/116417
---
 .../blender/blenkernel/BKE_grease_pencil.hh   |  5 ++++
 .../blenkernel/intern/grease_pencil.cc        |  5 ++++
 .../sculpt_paint/grease_pencil_draw_ops.cc    | 23 +++++++++++--------
 3 files changed, 24 insertions(+), 9 deletions(-)

diff --git a/source/blender/blenkernel/BKE_grease_pencil.hh b/source/blender/blenkernel/BKE_grease_pencil.hh
index dfc0e363196..6cfce30a4b0 100644
--- a/source/blender/blenkernel/BKE_grease_pencil.hh
+++ b/source/blender/blenkernel/BKE_grease_pencil.hh
@@ -394,6 +394,11 @@ class Layer : public ::GreasePencilLayer {
    * drawing. */
   int drawing_index_at(const int frame_number) const;
 
+  /**
+   * \returns true if there is a drawing on this layer at \a frame_number.
+   */
+  bool has_drawing_at(const int frame_number) const;
+
   /**
    * \returns the key of the active frame at \a frame_number or -1 if there is no frame.
    */
diff --git a/source/blender/blenkernel/intern/grease_pencil.cc b/source/blender/blenkernel/intern/grease_pencil.cc
index 4a1d41a7fbf..bb0610ad5f8 100644
--- a/source/blender/blenkernel/intern/grease_pencil.cc
+++ b/source/blender/blenkernel/intern/grease_pencil.cc
@@ -808,6 +808,11 @@ int Layer::drawing_index_at(const int frame_number) const
   return (frame != nullptr) ? frame->drawing_index : -1;
 }
 
+bool Layer::has_drawing_at(const int frame_number) const
+{
+  return frame_at(frame_number) != nullptr;
+}
+
 int Layer::get_frame_duration_at(const int frame_number) const
 {
   const FramesMapKey frame_key = this->frame_key_at(frame_number);
diff --git a/source/blender/editors/sculpt_paint/grease_pencil_draw_ops.cc b/source/blender/editors/sculpt_paint/grease_pencil_draw_ops.cc
index c250c9be0a6..78069ec8872 100644
--- a/source/blender/editors/sculpt_paint/grease_pencil_draw_ops.cc
+++ b/source/blender/editors/sculpt_paint/grease_pencil_draw_ops.cc
@@ -141,16 +141,19 @@ static int grease_pencil_stroke_invoke(bContext *C, wmOperator *op, const wmEven
   }
 
   const int current_frame = scene->r.cfra;
-  // bke::greasepencil::Layer &active_layer = *grease_pencil.get_active_layer();
-  // grease_pencil.get_editable_drawing_at()
+  bke::greasepencil::Layer &active_layer = *grease_pencil.get_active_layer();
+  /* If there is no drawing at the current frame and auto-key is off, then */
+  if (!active_layer.has_drawing_at(current_frame) && !blender::animrig::is_autokey_on(scene)) {
+    BKE_report(op->reports, RPT_ERROR, "No Grease Pencil frame to draw on");
+    return OPERATOR_CANCELLED;
+  }
 
-  if (!grease_pencil.get_active_layer()->frames().contains(current_frame)) {
-    if (!blender::animrig::is_autokey_on(scene)) {
-      BKE_report(op->reports, RPT_ERROR, "No Grease Pencil frame to draw on");
-      return OPERATOR_CANCELLED;
-    }
+  /* If auto-key is on and the drawing at the current frame starts before the current frame a new
+   * keyframe needs to be inserted. */
+  if (blender::animrig::is_autokey_on(scene) &&
+      active_layer.frame_key_at(current_frame) < current_frame)
+  {
     const ToolSettings *ts = CTX_data_tool_settings(C);
-    bke::greasepencil::Layer &active_layer = *grease_pencil.get_active_layer();
     if ((ts->gpencil_flags & GP_TOOL_FLAG_RETAIN_LAST) != 0) {
       /* For additive drawing, we duplicate the frame that's currently visible and insert it at the
        * current frame. */
@@ -158,10 +161,12 @@ static int grease_pencil_stroke_invoke(bContext *C, wmOperator *op, const wmEven
           active_layer, active_layer.frame_key_at(current_frame), current_frame, false);
     }
     else {
-      /* Otherwise we just insert a blank keyframe. */
+      /* Otherwise we just insert a blank keyframe at the current frame. */
       grease_pencil.insert_blank_frame(active_layer, current_frame, 0, BEZT_KEYTYPE_KEYFRAME);
     }
   }
+  /* There should now always be a drawing at the current frame. */
+  BLI_assert(active_layer.has_drawing_at(current_frame));
 
   op->customdata = paint_stroke_new(C,
                                     op,