Minor changes as a follow up to CL 2248469

Fixes #1786

(Internal change: 2248713)

pxr/imaging/hdMtlx/hdMtlx.cpp

@@ -230,7 +230,6 @@ _AddMaterialXNode(
 
     // For each of the HdNode parameters add the corresponding parameter/input 
     // to the mxNode
-    bool hasFilenameInput = false;
     TfTokenVector hdNodeParamNames =
         netInterface->GetAuthoredNodeParameterNames(hdNodeName);
     for (TfToken const &paramName : hdNodeParamNames) {
@@ -240,7 +239,6 @@ _AddMaterialXNode(
         mx::InputPtr mxInput = mxNodeDef->getActiveInput(mxInputName);
         if (mxInput) {
             mxInputType = mxInput->getType();
-            hasFilenameInput |= mxInputType == "filename";
         }
         std::string mxInputValue = HdMtlxConvertToString(
             netInterface->GetNodeParameterValue(hdNodeName, paramName));
@@ -249,7 +247,7 @@ _AddMaterialXNode(
     }
 
     // MaterialX nodes that use textures are assumed to have a filename input
-    if (hasFilenameInput) {
+    if (mxNodeDef->getNodeGroup() == "texture2d") {
         if (mxHdData) {
             // Save the corresponding MaterialX and Hydra names for ShaderGen
             mxHdData->mxHdTextureMap[mxNodeName] = connectionName;

pxr/usd/usdMtlx/overview.dox

@@ -63,9 +63,8 @@ get mapped is still undecided.
     containing folder is indicated with the PXR_MTLX_PLUGIN_SEARCH_PATHS 
     environment variable. 
     + Library defined custom nodes that use textures need to set the nodegroup
-    on the nodedef to 'texture2d', and make sure the 'filename' input is 
-    exposed so the texture coordinates are available, and textures are properly 
-    initialized for rendering. 
+    on the nodedef to 'texture2d', and make sure the 'filename' typed input is 
+    exposed so renderers can recognize the texture references. 
 
 \section usdMtlx_unsupported Unsupported MaterialX Features