From f5c0ef52cf2f4ae333269eec33e5bd7a89a00a23 Mon Sep 17 00:00:00 2001
From: Robert Guetzkow <rjg>
Date: Tue, 12 Jan 2021 12:12:33 +0100
Subject: Fix T84588: Cache access in rna_Particle_uv_on_emitter

The function `rna_Particle_uv_on_emitter` did not handle the case where
`particle->num_dmcache` was `DMCACHE_ISCHILD`. This resulted in an
incorrect offset for the `mtface` pointer. The commit checks for the
case and sets the offset accordingly, similar to existing code in
e.g. `particle_calculate_parent_uvs`.

Reviewed By: JacquesLucke

Differential Revision: https://developer.blender.org/D10072
---
 source/blender/makesdna/DNA_particle_types.h  | 5 ++++-
 source/blender/makesrna/intern/rna_particle.c | 8 +++-----
 2 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/source/blender/makesdna/DNA_particle_types.h b/source/blender/makesdna/DNA_particle_types.h
index d48144f90e8..5310ec56b26 100644
--- a/source/blender/makesdna/DNA_particle_types.h
+++ b/source/blender/makesdna/DNA_particle_types.h
@@ -132,7 +132,10 @@ typedef struct ParticleData {
    */
   /** Index to vert/edge/face. */
   int num;
-  /** Index to derived mesh data (face) to avoid slow lookups. */
+  /** 
+   * Index to derived mesh data (face) to avoid slow lookups. It can also have negative
+   * values DMCACHE_NOTFOUND and DMCACHE_ISCHILD.
+   */
   int num_dmcache;
 
   /** Coordinates on face/edge number "num" and depth along. */
diff --git a/source/blender/makesrna/intern/rna_particle.c b/source/blender/makesrna/intern/rna_particle.c
index 057f48c498c..bc9eda3cb5f 100644
--- a/source/blender/makesrna/intern/rna_particle.c
+++ b/source/blender/makesrna/intern/rna_particle.c
@@ -403,16 +403,15 @@ static void rna_Particle_uv_on_emitter(ParticleData *particle,
   }
   BKE_mesh_tessface_ensure(modifier->mesh_final); /* BMESH - UNTIL MODIFIER IS UPDATED FOR MPoly */
 
-  if (num == DMCACHE_NOTFOUND) {
+  if (ELEM(num, DMCACHE_NOTFOUND, DMCACHE_ISCHILD)) {
     if (particle->num < modifier->mesh_final->totface) {
       num = particle->num;
     }
   }
 
   /* get uvco */
-  if (r_uv && ELEM(from, PART_FROM_FACE, PART_FROM_VOLUME)) {
-
-    if (num != DMCACHE_NOTFOUND) {
+  if (r_uv && ELEM(from, PART_FROM_FACE, PART_FROM_VOLUME) &&
+      !ELEM(num, DMCACHE_NOTFOUND, DMCACHE_ISCHILD)) {
       MFace *mface;
       MTFace *mtface;
 
@@ -424,7 +423,6 @@ static void rna_Particle_uv_on_emitter(ParticleData *particle,
         psys_interpolate_uvs(mtface, mface->v4, particle->fuv, r_uv);
         return;
       }
-    }
   }
 
   r_uv[0] = 0.0f;
-- 
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