Forge pipeline will now take original model lightmap into account, if present.

src/main/java/net/minecraftforge/client/model/pipeline/QuadGatheringTransformer.java

@@ -8,6 +8,7 @@ public abstract class QuadGatheringTransformer implements IVertexConsumer
     protected VertexFormat format;
     protected int vertices = 0;
 
+    protected byte[] dataLength = null;
     protected float[][][] quadData = null;
 
     public void setParent(IVertexConsumer parent)
@@ -18,6 +19,7 @@ public abstract class QuadGatheringTransformer implements IVertexConsumer
     public void setVertexFormat(VertexFormat format)
     {
         this.format = format;
+        dataLength = new byte[format.getElementCount()];
         quadData = new float[format.getElementCount()][4][4];
     }
 
@@ -32,7 +34,11 @@ public abstract class QuadGatheringTransformer implements IVertexConsumer
     {
         System.arraycopy(data, 0, quadData[element][vertices], 0, data.length);
         if(element == getVertexFormat().getElementCount() - 1) vertices++;
-        if(vertices == 4)
+        if(vertices == 0)
+        {
+            dataLength[element] = (byte)data.length;
+        }
+        else if(vertices == 4)
         {
             vertices = 0;
             processQuad();

src/main/java/net/minecraftforge/client/model/pipeline/VertexLighterFlat.java

@@ -131,7 +131,13 @@ public class VertexLighterFlat extends QuadGatheringTransformer
                 z += normal[v][2] * .5f;
             }
 
+            float blockLight = lightmap[v][0], skyLight = lightmap[v][1];
             updateLightmap(normal[v], lightmap[v], x, y, z);
+            if(dataLength[lightmapIndex] > 1)
+            {
+                if(blockLight > lightmap[v][0]) lightmap[v][0] = blockLight;
+                if(skyLight > lightmap[v][1]) lightmap[v][1] = skyLight;
+            }
             updateColor(normal[v], color[v], x, y, z, tint, multiplier);
             if(EntityRenderer.anaglyphEnable)
             {