WorldPositionOffset not working on custom expression additional outputs

Development Rendering
, ,
1

Custom expression additional outputs are considered (0,0,0) when connected to WPO.

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2

Steps to Reproduce
Create a custom node, add an additional output, set the additional output to a not zero number and return this value as custom expression result too. Connect the custom expression’s output to WPO: it works. Connect the custom expression’s additional output: it doesn’t work.

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3

Hello,

Thanks for reporting this bug. I’ve verified the problem still exists in latest and created the following issue for tracking which should be visible soon https://issues.unrealengine.com/issue/UE\-385745

It looks likely that this issue was caused by CL#17030334 which was added to fix a FXC.exe ICE. One potential fix is:

+++ b/HLSLMaterialTranslator.cpp
[Content removed]23 @@
 			// If we found the definition of the return value, there is no need to add more definitions as they won't contribute to the outcome
 			if (ReturnValueSymbolName != nullptr && CodeChunk.SymbolName == ReturnValueSymbolName)
 			{
-				break;
+				// Don't break early if a later chunk modifies this value via inout
+				// (e.g. a CustomExpression function call that takes this variable as
+				// an 'inout' parameter). Without this check, that call gets truncated
+				// and the variable is never modified, producing a zero result.
+				bool bHasUnemittedDependency = false;
+				for (int32 DepIdx : CodeChunk.ReferencedCodeChunks)
+				{
+					if (DepIdx > ChunkIndex && DepIdx < EndChunk)
+					{
+						bHasUnemittedDependency = true;
+						break;
+					}
+				}
+				if (!bHasUnemittedDependency)
+				{
+					break;
+				}
 			}
 		}

with

--- a/HLSLMaterialTranslator.cpp
+++ b/HLSLMaterialTranslator.cpp
[Content removed]17 @@
 	}
 	int32 Result = CustomEntry->OutputCodeIndex[OutputIndex];
+
+	// When an additional output is the result, mark the main function call as
+	// a dependency so GetDefinitions() will continue past this chunk and emit
+	// the call that modifies this variable via inout. 
+	if (OutputIndex > 0 && CustomEntry->OutputCodeIndex[0] != INDEX_NONE)
+	{
+		ReferencedCodeChunks.AddUnique(CustomEntry->OutputCodeIndex[0]);
+	}
+
 	if (Custom->IsResultMaterialAttributes(OutputIndex))
 	{

However, this fix hasn’t been thoroughly tested.

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