Fix T39799: Backdrop (compositor) ignores alpha.

This issue is because of a somewhat "special" behavior in old code, which got lost during rB09874df:

There was a variant of the `relinkConnections` function which would leave the socket completely unconnected. This is not a valid state really (given that each unconnected input must otherwise connected to a constant `Set` type node), but was used as a way to distinguish connected alpha/depth sockets in composite and viewer output nodes.
https://developer.blender.org/diffusion/B/browse/master/source/blender/compositor/intern/COM_InputSocket.cpp;28a829893c702918afc5ac1945a06eaefa611594$69

After the large cleanup patch ({D309}) every socket is now automatically connected to a constant, such that `getInputSocketReader` will never return a NULL pointer. This breaks the previous test method, which needs to be replaced by more explicit flags. Luckily this was done only for very few output nodes (Composite, Viewer, Output-File). These now use the regular SetValueOperation default in case "use alpha" is disabled, but set this to an explicit 1.0 value instead of mapping to the node socket.

1 files changed, 8 insertions, 2 deletions

diff --git a/source/blender/compositor/nodes/COM_CompositorNode.cpp b/source/blender/compositor/nodes/COM_CompositorNode.cpp
index 868528ade81..3d79eb693ea 100644
--- a/source/blender/compositor/nodes/COM_CompositorNode.cpp
+++ b/source/blender/compositor/nodes/COM_CompositorNode.cpp
@@ -34,6 +34,7 @@ void CompositorNode::convertToOperations(NodeConverter &converter, const Composi
 	bNode *editorNode = this->getbNode();
 	bool is_active = (editorNode->flag & NODE_DO_OUTPUT_RECALC) ||
 	                 context.isRendering();
+	bool ignore_alpha = editorNode->custom2 & CMP_NODE_OUTPUT_IGNORE_ALPHA;
 
 	NodeInput *imageSocket = this->getInputSocket(0);
 	NodeInput *alphaSocket = this->getInputSocket(1);
@@ -43,12 +44,17 @@ void CompositorNode::convertToOperations(NodeConverter &converter, const Composi
 	compositorOperation->setSceneName(context.getScene()->id.name);
 	compositorOperation->setRenderData(context.getRenderData());
 	compositorOperation->setbNodeTree(context.getbNodeTree());
-	compositorOperation->setIgnoreAlpha(editorNode->custom2 & CMP_NODE_OUTPUT_IGNORE_ALPHA);
+	/* alpha socket gives either 1 or a custom alpha value if "use alpha" is enabled */
+	compositorOperation->setUseAlphaInput(ignore_alpha || alphaSocket->isLinked());
 	compositorOperation->setActive(is_active);
 	
 	converter.addOperation(compositorOperation);
 	converter.mapInputSocket(imageSocket, compositorOperation->getInputSocket(0));
-	converter.mapInputSocket(alphaSocket, compositorOperation->getInputSocket(1));
+	/* only use alpha link if "use alpha" is enabled */
+	if (ignore_alpha)
+		converter.addInputValue(compositorOperation->getInputSocket(1), 1.0f);
+	else
+		converter.mapInputSocket(alphaSocket, compositorOperation->getInputSocket(1));
 	converter.mapInputSocket(depthSocket, compositorOperation->getInputSocket(2));
 	
 	converter.addNodeInputPreview(imageSocket);