Fix T90782: add uv pack option to specify margin as a fraction

A refactor of the margin calculation of UV packing, in anticipation
of multiple packing methods soon becoming available.

Three margin scaling methods are now available:

* "Add", Simple method, just add the margin. [0]
    (The default margin scale from Blender 2.8 and earlier.)
* "Scaled", Use scale of existing UVs to multiply margin.
    (The default from Blender 3.3+)
* "Fraction", a new (slow) method to precisely specify
    a fraction of the UV unit square for margin. [1]

The "fraction" code path implements a novel combined search / secant
root finding method which exploits domain knowledge to accelerate
convergence while remaining robust against bad input.

[0]: Resolves T85978
[1]: Resolves T90782

Differential Revision: https://developer.blender.org/D16121

1 files changed, 8 insertions, 0 deletions

diff --git a/source/blender/editors/include/ED_uvedit.h b/source/blender/editors/include/ED_uvedit.h
index b499ae0ce59..e485fd2b061 100644
--- a/source/blender/editors/include/ED_uvedit.h
+++ b/source/blender/editors/include/ED_uvedit.h
@@ -339,12 +339,20 @@ bool ED_uvedit_udim_params_from_image_space(const struct SpaceImage *sima,
                                             bool use_active,
                                             struct UVMapUDIM_Params *udim_params);
 
+typedef enum {
+  ED_UVPACK_MARGIN_SCALED = 0, /* Use scale of existing UVs to multiply margin. */
+  ED_UVPACK_MARGIN_ADD,        /* Just add the margin, ignoring any UV scale. */
+  ED_UVPACK_MARGIN_FRACTION,   /* Specify a precise fraction of final UV output. */
+} eUVPackIsland_MarginMethod;
+
 struct UVPackIsland_Params {
   uint rotate : 1;
   uint only_selected_uvs : 1;
   uint only_selected_faces : 1;
   uint use_seams : 1;
   uint correct_aspect : 1;
+  eUVPackIsland_MarginMethod margin_method; /* Which formula to use when scaling island margin. */
+  float margin;                             /* Additional space to add around each island. */
 };
 
 /**