Fix T97757: Some MTL import correctness issues in the new OBJ importer

Fix several correctness issues where the new OBJ/MTL importer was not
producing the same results as the old one, mostly because the code for
some reason had slightly different logic. Fixes T97757:

- When .obj file tries to use a material that does not exist, the code
  was continuing to use the previous material, instead of creating new
  default one, as the previous importer did.
- Previous importer was always searching/parsing "foo.mtl" for a
  "foo.obj" file, even if the file itself does not contain
  "mtllib foo.mtl" statement. One file from T97757 repros happens to
  depend on that, so resurrect that behavior.
- When IOR (Ni) or Alpha (d) are not specified in .mtl file, do not
  wrongly set -1 values to the blender material.
- When base (Kd) or emissive (Ke) colors are not specified in the .mtl
  file, do not set them on the blender material.
- Roughness and metallic values used by viewport shading were not set
  onto blender material.
- The logic for when metallic was set to zero was incorrect; it should
  be set to zero when "not using reflection", not when "mtl file does
  not contain metallic".
- Do not produce a warning when illum value is not spelled out in .mtl
  file, treat as default (1).
- Parse illum as a float just like python importer does, as to not
  reintroduce part of T60135.

Reviewed By: Howard Trickey
Differential Revision: https://developer.blender.org/D14822

1 files changed, 40 insertions, 28 deletions

diff --git a/source/blender/io/wavefront_obj/importer/obj_import_mtl.cc b/source/blender/io/wavefront_obj/importer/obj_import_mtl.cc
index 56e3a062cb6..c2ecd8a37de 100644
--- a/source/blender/io/wavefront_obj/importer/obj_import_mtl.cc
+++ b/source/blender/io/wavefront_obj/importer/obj_import_mtl.cc
@@ -197,10 +197,10 @@ void ShaderNodetreeWrap::set_bsdf_socket_values(Material *mat)
   bool do_glass = false;
   /* See https://wikipedia.org/wiki/Wavefront_.obj_file for possible values of illum. */
   switch (illum) {
-    case 1: {
+    case -1:
+    case 1:
       /* Base color on, ambient on. */
       break;
-    }
     case 2: {
       /* Highlight on. */
       do_highlight = true;
@@ -257,28 +257,30 @@ void ShaderNodetreeWrap::set_bsdf_socket_values(Material *mat)
    * Principled BSDF: */
   /* Specular: average of Ks components. */
   float specular = (mtl_mat_.Ks[0] + mtl_mat_.Ks[1] + mtl_mat_.Ks[2]) / 3;
-  /* Roughness: map 0..1000 range to 1..0 and apply non-linearity. */
-  float clamped_ns = std::max(0.0f, std::min(1000.0f, mtl_mat_.Ns));
-  float roughness = 1.0f - sqrt(clamped_ns / 1000.0f);
-  /* Metallic: average of Ka components. */
-  float metallic = (mtl_mat_.Ka[0] + mtl_mat_.Ka[1] + mtl_mat_.Ka[2]) / 3;
-  float ior = mtl_mat_.Ni;
-  float alpha = mtl_mat_.d;
-
   if (specular < 0.0f) {
-    specular = static_cast<float>(do_highlight);
+    specular = do_highlight ? 1.0f : 0.0f;
   }
+  /* Roughness: map 0..1000 range to 1..0 and apply non-linearity. */
+  float roughness;
   if (mtl_mat_.Ns < 0.0f) {
-    roughness = static_cast<float>(!do_highlight);
+    roughness = do_highlight ? 0.0f : 1.0f;
   }
-  if (metallic < 0.0f) {
-    if (do_reflection) {
+  else {
+    float clamped_ns = std::max(0.0f, std::min(1000.0f, mtl_mat_.Ns));
+    roughness = 1.0f - sqrt(clamped_ns / 1000.0f);
+  }
+  /* Metallic: average of Ka components. */
+  float metallic = (mtl_mat_.Ka[0] + mtl_mat_.Ka[1] + mtl_mat_.Ka[2]) / 3;
+  if (do_reflection) {
+    if (metallic < 0.0f) {
       metallic = 1.0f;
     }
   }
   else {
     metallic = 0.0f;
   }
+
+  float ior = mtl_mat_.Ni;
   if (ior < 0) {
     if (do_tranparency) {
       ior = 1.0f;
@@ -287,28 +289,38 @@ void ShaderNodetreeWrap::set_bsdf_socket_values(Material *mat)
       ior = 1.5f;
     }
   }
-  if (alpha < 0) {
-    if (do_tranparency) {
-      alpha = 1.0f;
-    }
+  float alpha = mtl_mat_.d;
+  if (do_tranparency && alpha < 0) {
+    alpha = 1.0f;
   }
-  float3 base_color = {std::max(0.0f, mtl_mat_.Kd[0]),
-                       std::max(0.0f, mtl_mat_.Kd[1]),
-                       std::max(0.0f, mtl_mat_.Kd[2])};
-  float3 emission_color = {std::max(0.0f, mtl_mat_.Ke[0]),
-                           std::max(0.0f, mtl_mat_.Ke[1]),
-                           std::max(0.0f, mtl_mat_.Ke[2])};
 
-  set_property_of_socket(SOCK_RGBA, "Base Color", {base_color, 3}, bsdf_);
-  set_property_of_socket(SOCK_RGBA, "Emission", {emission_color, 3}, bsdf_);
+  float3 base_color = {mtl_mat_.Kd[0], mtl_mat_.Kd[1], mtl_mat_.Kd[2]};
+  if (base_color.x >= 0 && base_color.y >= 0 && base_color.z >= 0) {
+    set_property_of_socket(SOCK_RGBA, "Base Color", {base_color, 3}, bsdf_);
+    /* Viewport shading uses legacy r,g,b base color. */
+    mat->r = base_color.x;
+    mat->g = base_color.y;
+    mat->b = base_color.z;
+  }
+
+  float3 emission_color = {mtl_mat_.Ke[0], mtl_mat_.Ke[1], mtl_mat_.Ke[2]};
+  if (emission_color.x >= 0 && emission_color.y >= 0 && emission_color.z >= 0) {
+    set_property_of_socket(SOCK_RGBA, "Emission", {emission_color, 3}, bsdf_);
+  }
   if (mtl_mat_.texture_maps.contains_as(eMTLSyntaxElement::map_Ke)) {
     set_property_of_socket(SOCK_FLOAT, "Emission Strength", {1.0f}, bsdf_);
   }
   set_property_of_socket(SOCK_FLOAT, "Specular", {specular}, bsdf_);
   set_property_of_socket(SOCK_FLOAT, "Roughness", {roughness}, bsdf_);
+  mat->roughness = roughness;
   set_property_of_socket(SOCK_FLOAT, "Metallic", {metallic}, bsdf_);
-  set_property_of_socket(SOCK_FLOAT, "IOR", {ior}, bsdf_);
-  set_property_of_socket(SOCK_FLOAT, "Alpha", {alpha}, bsdf_);
+  mat->metallic = metallic;
+  if (ior != -1) {
+    set_property_of_socket(SOCK_FLOAT, "IOR", {ior}, bsdf_);
+  }
+  if (alpha != -1) {
+    set_property_of_socket(SOCK_FLOAT, "Alpha", {alpha}, bsdf_);
+  }
   if (do_tranparency) {
     mat->blend_method = MA_BM_BLEND;
   }