Collada import: use black for Base Color when missing <diffuse>

Treat a missing <diffuse> the same as a black diffuse color.

The easiest way to see this bug is with a Collada shader like

```
          <constant>
            <emission>
              <color sid="emission">1 0 0 1</color>
            </emission>
          </constant>
```

The Collada spec says this should be just

```
color = <emission>
```

ie. red everywhere. The importer slots the red into the Principled Emission socket, but since it leaves the Base Color as the default off-white, this is added to red, and the material looks white-pink in the light and red only in the shadows.

Putting black in the Base Color makes it look red everywhere.

D10939 will also eliminate the much-less-noticeable specular term for this case.

Reviewed By: gaiaclary

Differential Revision: https://developer.blender.org/D10941

1 files changed, 20 insertions, 10 deletions

diff --git a/source/blender/io/collada/Materials.cpp b/source/blender/io/collada/Materials.cpp
index 4b90e459e61..ac4c65464c8 100644
--- a/source/blender/io/collada/Materials.cpp
+++ b/source/blender/io/collada/Materials.cpp
@@ -231,22 +231,32 @@ void MaterialNode::set_alpha(COLLADAFW::EffectCommon::OpaqueMode mode,
 void MaterialNode::set_diffuse(COLLADAFW::ColorOrTexture &cot)
 {
   int locy = -300 * (node_map.size() - 2);
-  if (cot.isColor()) {
-    COLLADAFW::Color col = cot.getColor();
-    bNodeSocket *socket = nodeFindSocket(shader_node, SOCK_IN, "Base Color");
-    float *fcol = (float *)socket->default_value;
 
-    fcol[0] = material->r = col.getRed();
-    fcol[1] = material->g = col.getGreen();
-    fcol[2] = material->b = col.getBlue();
-    fcol[3] = material->a = col.getAlpha();
-  }
-  else if (cot.isTexture()) {
+  if (cot.isTexture()) {
     bNode *texture_node = add_texture_node(cot, -300, locy, "Base Color");
     if (texture_node != nullptr) {
       add_link(texture_node, 0, shader_node, 0);
     }
   }
+  else {
+    bNodeSocket *socket = nodeFindSocket(shader_node, SOCK_IN, "Base Color");
+    float *fcol = (float *)socket->default_value;
+
+    if (cot.isColor()) {
+      COLLADAFW::Color col = cot.getColor();
+      fcol[0] = material->r = col.getRed();
+      fcol[1] = material->g = col.getGreen();
+      fcol[2] = material->b = col.getBlue();
+      fcol[3] = material->a = col.getAlpha();
+    }
+    else {
+      /* no diffuse term = same as black */
+      fcol[0] = material->r = 0.0f;
+      fcol[1] = material->g = 0.0f;
+      fcol[2] = material->b = 0.0f;
+      fcol[3] = material->a = 1.0f;
+    }
+  }
 }
 
 Image *MaterialNode::get_diffuse_image()