Fix T101517: GPencil strokes snap to origin in a Scale value is on 0

The problem was the conversion to object space converted the
points to zero.

Now, the new function `zero_axis_bias_m4` is used in order to add
a small bias in the inverse matrix and avoid the zero points.

A known math issue is the stroke can be offsetted if the scale is set to 1 
again. In this case apply the scale to reset to 1.

Differential Revision: https://developer.blender.org/D16162

2 files changed, 12 insertions, 3 deletions

diff --git a/source/blender/editors/gpencil/gpencil_sculpt_paint.c b/source/blender/editors/gpencil/gpencil_sculpt_paint.c
index e27cd255217..424544c8085 100644
--- a/source/blender/editors/gpencil/gpencil_sculpt_paint.c
+++ b/source/blender/editors/gpencil/gpencil_sculpt_paint.c
@@ -544,8 +544,10 @@ static void gpencil_brush_grab_apply_cached(tGP_BrushEditData *gso,
     return;
   }
 
-  float inverse_diff_mat[4][4];
-  invert_m4_m4(inverse_diff_mat, diff_mat);
+  float matrix[4][4], inverse_diff_mat[4][4];
+  copy_m4_m4(matrix, diff_mat);
+  zero_axis_bias_m4(matrix);
+  invert_m4_m4(inverse_diff_mat, matrix);
 
   /* Apply dvec to all of the stored points */
   for (int i = 0; i < data->size; i++) {
@@ -1169,7 +1171,10 @@ static bool gpencil_sculpt_brush_init(bContext *C, wmOperator *op)
   gso->scene = scene;
   gso->object = ob;
   if (ob) {
-    invert_m4_m4(gso->inv_mat, ob->obmat);
+    float matrix[4][4];
+    copy_m4_m4(matrix, ob->obmat);
+    zero_axis_bias_m4(matrix);
+    invert_m4_m4(gso->inv_mat, matrix);
     gso->vrgroup = gso->gpd->vertex_group_active_index - 1;
     if (!BLI_findlink(&gso->gpd->vertex_group_names, gso->vrgroup)) {
       gso->vrgroup = -1;
diff --git a/source/blender/editors/gpencil/gpencil_utils.c b/source/blender/editors/gpencil/gpencil_utils.c
index 7b659511aaa..b554c81ab00 100644
--- a/source/blender/editors/gpencil/gpencil_utils.c
+++ b/source/blender/editors/gpencil/gpencil_utils.c
@@ -633,6 +633,7 @@ void gpencil_apply_parent(Depsgraph *depsgraph, Object *obact, bGPDlayer *gpl, b
   float fpt[3];
 
   BKE_gpencil_layer_transform_matrix_get(depsgraph, obact, gpl, diff_mat);
+  zero_axis_bias_m4(diff_mat);
   invert_m4_m4(inverse_diff_mat, diff_mat);
 
   for (i = 0; i < gps->totpoints; i++) {
@@ -653,6 +654,7 @@ void gpencil_apply_parent_point(Depsgraph *depsgraph,
   float fpt[3];
 
   BKE_gpencil_layer_transform_matrix_get(depsgraph, obact, gpl, diff_mat);
+  zero_axis_bias_m4(diff_mat);
   invert_m4_m4(inverse_diff_mat, diff_mat);
 
   mul_v3_m4v3(fpt, inverse_diff_mat, &pt->x);
@@ -935,6 +937,7 @@ void ED_gpencil_project_stroke_to_view(bContext *C, bGPDlayer *gpl, bGPDstroke *
   gpencil_point_conversion_init(C, &gsc);
 
   BKE_gpencil_layer_transform_matrix_get(depsgraph, ob, gpl, diff_mat);
+  zero_axis_bias_m4(diff_mat);
   invert_m4_m4(inverse_diff_mat, diff_mat);
 
   /* Adjust each point */
@@ -1050,6 +1053,7 @@ void ED_gpencil_stroke_reproject(Depsgraph *depsgraph,
 
   float diff_mat[4][4], inverse_diff_mat[4][4];
   BKE_gpencil_layer_transform_matrix_get(depsgraph, gsc->ob, gpl, diff_mat);
+  zero_axis_bias_m4(diff_mat);
   invert_m4_m4(inverse_diff_mat, diff_mat);
 
   float origin[3];