ImageGadget : Fix unnecessary texture updates

When an image tile doesn't need updating, `Tile::computeUpdate()` still returns an `Update` struct, but with `channelData == null`. When processing that in `Tile::applyUpdates()`, we were zeroing out `Tile::m_channelDataHash`, which would cause the next update to do completely unnecessary work. Now we don't do that.

Changes.md

@@ -9,6 +9,7 @@ Improvements
 Fixes
 -----
 
+- Viewer, ImageGadget : Fixed unnecessary texture updates when specific image tiles don't change.
 - Constraint : The `target` browser now shows locations from the `targetScene` if it has an input connection. Before it always showed locations from the main input.
 - ImageInspector : Fixed broken UI caused by double-clicking in the Image tab.
 

python/GafferImageUITest/ImageGadgetTest.py

@@ -34,6 +34,7 @@
 #
 ##########################################################################
 
+import time
 import unittest
 import imath
 
@@ -120,5 +121,30 @@ def testStateChangedSignal( self ) :
 		self.assertEqual( len( cs ), 2 )
 		self.assertNotEqual( gadget.state(), gadget.State.Paused )
 
+	def testNoUnecessaryUpdates( self ) :
+
+		script = Gaffer.ScriptNode()
+		script["image"] = GafferImage.Checkerboard()
+		script["image"]["format"].setValue( GafferImage.Format( GafferImage.ImagePlug.tileSize(), GafferImage.ImagePlug.tileSize() ) )
+
+		gadget = GafferImageUI.ImageGadget()
+		gadget.setImage( script["image"]["out"] )
+		gadget.setContext( script.context() )
+
+		with GafferUI.Window() as window :
+			GafferUI.GadgetWidget( gadget )
+
+		window.setVisible( True )
+		while gadget.tileUpdateCount() < 4 :
+			self.waitForIdle()
+
+		for frame in range( 2, 4 ) :
+			script.context().setFrame( frame )
+			t = time.time() + 0.5
+			while time.time() < t :
+				self.waitForIdle()
+
+			self.assertEqual( gadget.tileUpdateCount(), 4 )
+
 if __name__ == "__main__":
 	unittest.main()

src/GafferImageUI/ImageGadget.cpp

@@ -747,7 +747,7 @@ void ImageGadget::Tile::applyUpdates( const std::vector<Update> &updates )
 
 	for( const auto &u : updates )
 	{
-		if( u.tile )
+		if( u.channelData )
 		{
 			u.tile->m_channelDataToConvert = u.channelData;
 			u.tile->m_channelDataHash = u.channelDataHash;