Fix T93084: Area stretch overlay full red on large scale mesh

Issue arises when face areas are really large combined with small UV
areas (report has a mesh ~1.5 km), then precission of shorts is
insufficient.

Now use floats instead.
This also removes this negative signed version of the total area ratio
(since with floats it is no longer used).

Thx @brecht for a lot of hand-holding!

NOTE: this is an alternative to D15805 (and quick tests show this does
not introduce the tiny performance hit as D15805 did).

Maniphest Tasks: T93084

Differential Revision: https://developer.blender.org/D15810

1 files changed, 7 insertions, 14 deletions

diff --git a/source/blender/draw/intern/mesh_extractors/extract_mesh_vbo_edituv_stretch_area.cc b/source/blender/draw/intern/mesh_extractors/extract_mesh_vbo_edituv_stretch_area.cc
index 2bb786303c4..a1737f8590e 100644
--- a/source/blender/draw/intern/mesh_extractors/extract_mesh_vbo_edituv_stretch_area.cc
+++ b/source/blender/draw/intern/mesh_extractors/extract_mesh_vbo_edituv_stretch_area.cc
@@ -27,7 +27,7 @@ static void extract_edituv_stretch_area_init(const MeshRenderData *mr,
   GPUVertBuf *vbo = static_cast<GPUVertBuf *>(buf);
   static GPUVertFormat format = {0};
   if (format.attr_len == 0) {
-    GPU_vertformat_attr_add(&format, "ratio", GPU_COMP_I16, 1, GPU_FETCH_INT_TO_FLOAT_UNIT);
+    GPU_vertformat_attr_add(&format, "ratio", GPU_COMP_F32, 1, GPU_FETCH_FLOAT);
   }
 
   GPU_vertbuf_init_with_format(vbo, &format);
@@ -37,15 +37,14 @@ static void extract_edituv_stretch_area_init(const MeshRenderData *mr,
 BLI_INLINE float area_ratio_get(float area, float uvarea)
 {
   if (area >= FLT_EPSILON && uvarea >= FLT_EPSILON) {
-    /* Tag inversion by using the sign. */
-    return (area > uvarea) ? (uvarea / area) : -(area / uvarea);
+    return uvarea / area;
   }
   return 0.0f;
 }
 
-BLI_INLINE float area_ratio_to_stretch(float ratio, float tot_ratio, float inv_tot_ratio)
+BLI_INLINE float area_ratio_to_stretch(float ratio, float tot_ratio)
 {
-  ratio *= (ratio > 0.0f) ? tot_ratio : -inv_tot_ratio;
+  ratio *= tot_ratio;
   return (ratio > 1.0f) ? (1.0f / ratio) : ratio;
 }
 
@@ -97,14 +96,8 @@ static void extract_edituv_stretch_area_finish(const MeshRenderData *mr,
   float *area_ratio = static_cast<float *>(MEM_mallocN(sizeof(float) * mr->poly_len, __func__));
   compute_area_ratio(mr, area_ratio, cache->tot_area, cache->tot_uv_area);
 
-  /* Convert in place to avoid an extra allocation */
-  uint16_t *poly_stretch = (uint16_t *)area_ratio;
-  for (int mp_index = 0; mp_index < mr->poly_len; mp_index++) {
-    poly_stretch[mp_index] = area_ratio[mp_index] * SHRT_MAX;
-  }
-
   /* Copy face data for each loop. */
-  uint16_t *loop_stretch = (uint16_t *)GPU_vertbuf_get_data(vbo);
+  float *loop_stretch = (float *)GPU_vertbuf_get_data(vbo);
 
   if (mr->extract_type == MR_EXTRACT_BMESH) {
     BMFace *efa;
@@ -112,7 +105,7 @@ static void extract_edituv_stretch_area_finish(const MeshRenderData *mr,
     int f, l_index = 0;
     BM_ITER_MESH_INDEX (efa, &f_iter, mr->bm, BM_FACES_OF_MESH, f) {
       for (int i = 0; i < efa->len; i++, l_index++) {
-        loop_stretch[l_index] = poly_stretch[f];
+        loop_stretch[l_index] = area_ratio[f];
       }
     }
   }
@@ -121,7 +114,7 @@ static void extract_edituv_stretch_area_finish(const MeshRenderData *mr,
     const MPoly *mp = mr->mpoly;
     for (int mp_index = 0, l_index = 0; mp_index < mr->poly_len; mp_index++, mp++) {
       for (int i = 0; i < mp->totloop; i++, l_index++) {
-        loop_stretch[l_index] = poly_stretch[mp_index];
+        loop_stretch[l_index] = area_ratio[mp_index];
       }
     }
   }