obj: vertex colors support in importer and exporter

Adds support for vertex colors to OBJ I/O.

Importer:

- Supports both "xyzrgb" and "MRGB" vertex color formats.
- Whenever vertex color is present in the file for a model, it is
  imported and a Color attribute is created (per-vertex, full float
  color data type). Color coming from the file is assumed to be sRGB,
  and is converted to linear upon import.

Exporter:

- Option to export the vertex colors. Defaults to "off", since not
  all 3rd party software supports vertex colors.
- When the option is "on", if a mesh has a color attribute layer,
  the active one is exported in "xyzrgb" form. If the mesh has
  per-face-corner colors, they are averaged on the vertices.
  Colors are converted from linear to sRGB upon export.

Reviewed By: Howard Trickey
Differential Revision: https://developer.blender.org/D15159

1 files changed, 50 insertions, 1 deletions

diff --git a/source/blender/io/wavefront_obj/importer/obj_import_file_reader.cc b/source/blender/io/wavefront_obj/importer/obj_import_file_reader.cc
index a627e7261e3..4a27d734819 100644
--- a/source/blender/io/wavefront_obj/importer/obj_import_file_reader.cc
+++ b/source/blender/io/wavefront_obj/importer/obj_import_file_reader.cc
@@ -5,12 +5,15 @@
  */
 
 #include "BLI_map.hh"
+#include "BLI_math_color.h"
 #include "BLI_string_ref.hh"
 #include "BLI_vector.hh"
 
 #include "obj_import_file_reader.hh"
 #include "obj_import_string_utils.hh"
 
+#include <charconv>
+
 namespace blender::io::obj {
 
 using std::string;
@@ -34,6 +37,7 @@ static Geometry *create_geometry(Geometry *const prev_geometry,
     g->geom_type_ = new_type;
     g->geometry_name_ = name.is_empty() ? "New object" : name;
     g->vertex_start_ = global_vertices.vertices.size();
+    g->vertex_color_start_ = global_vertices.vertex_colors.size();
     r_offset.set_index_offset(g->vertex_start_);
     return g;
   };
@@ -71,9 +75,51 @@ static void geom_add_vertex(Geometry *geom,
                             GlobalVertices &r_global_vertices)
 {
   float3 vert;
-  parse_floats(p, end, 0.0f, vert, 3);
+  p = parse_floats(p, end, 0.0f, vert, 3);
   r_global_vertices.vertices.append(vert);
   geom->vertex_count_++;
+  /* OBJ extension: "xyzrgb" vertex colors, when the vertex position
+   * is followed by 3 more RGB color components. See
+   * http://paulbourke.net/dataformats/obj/colour.html */
+  if (p < end) {
+    float3 srgb;
+    p = parse_floats(p, end, -1.0f, srgb, 3);
+    if (srgb.x >= 0 && srgb.y >= 0 && srgb.z >= 0) {
+      float3 linear;
+      srgb_to_linearrgb_v3_v3(linear, srgb);
+      r_global_vertices.vertex_colors.append(linear);
+      geom->vertex_color_count_++;
+    }
+  }
+}
+
+static void geom_add_mrgb_colors(Geometry *geom,
+                                 const char *p,
+                                 const char *end,
+                                 GlobalVertices &r_global_vertices)
+{
+  /* MRGB color extension, in the form of
+   * "#MRGB MMRRGGBBMMRRGGBB ..."
+   * http://paulbourke.net/dataformats/obj/colour.html */
+  p = drop_whitespace(p, end);
+  const int mrgb_length = 8;
+  while (p + mrgb_length <= end) {
+    uint32_t value = 0;
+    std::from_chars_result res = std::from_chars(p, p + mrgb_length, value, 16);
+    if (res.ec == std::errc::invalid_argument || res.ec == std::errc::result_out_of_range) {
+      return;
+    }
+    unsigned char srgb[4];
+    srgb[0] = (value >> 16) & 0xFF;
+    srgb[1] = (value >> 8) & 0xFF;
+    srgb[2] = value & 0xFF;
+    srgb[3] = 0xFF;
+    float linear[4];
+    srgb_to_linearrgb_uchar4(linear, srgb);
+    r_global_vertices.vertex_colors.append({linear[0], linear[1], linear[2]});
+    geom->vertex_color_count_++;
+    p += mrgb_length;
+  }
 }
 
 static void geom_add_vertex_normal(Geometry *geom,
@@ -482,6 +528,9 @@ void OBJParser::parse(Vector<std::unique_ptr<Geometry>> &r_all_geometries,
       else if (parse_keyword(p, end, "mtllib")) {
         add_mtl_library(StringRef(p, end).trim());
       }
+      else if (parse_keyword(p, end, "#MRGB")) {
+        geom_add_mrgb_colors(curr_geom, p, end, r_global_vertices);
+      }
       /* Comments. */
       else if (*p == '#') {
         /* Nothing to do. */