ViewportGadget : Avoid rendering selection ids with GL_BLEND enabled

Enabling GL_BLEND for selection renders on Intel graphics hardware leads to corrupted ids in the selection buffer, so we now ensure it is disabled for selection renders in `renderInternal`.

We also consolidate the enabling of GL_BLEND so it only occurs in `renderInternal`, rather than in various parts of StandardStyle and in `ViewportGadget::render`.

This fixes GafferHQ#901 and GafferHQ#2788 on the Intel graphics hardware we have available to test with.

Changes.md

@@ -21,6 +21,7 @@ Fixes
 - Dispatcher : Fixed shutdown crashes caused by Python slots connected to the dispatch signals [^1].
 - Display : Fixed shutdown crashes caused by Python slots connected to `driverCreatedSignal()` and `imageReceivedSignal()` [^1].
 - LightPositionTool : Fixed crash when changing the tool mode with nothing selected [^1].
+- ViewportGadget : Fixed selection issues with Intel GPUs (#901, #2788).
 
 API
 ---

src/GafferUI/StandardStyle.cpp

@@ -581,7 +581,6 @@ void StandardStyle::bind( const Style *currentStyle ) const
 		return;
 	}
 
-	glEnable( GL_BLEND );
 	glBlendFuncSeparate( GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, GL_ONE, GL_ONE_MINUS_SRC_ALPHA );
 	glUseProgram( shader()->program() );
 
@@ -1130,7 +1129,6 @@ void StandardStyle::renderImage( const Imath::Box2f &box, const IECoreGL::Textur
 	glPushAttrib( GL_COLOR_BUFFER_BIT );
 
 	// As the image is already pre-multiplied we need to change our blend mode.
-	glEnable( GL_BLEND );
 	if( !IECoreGL::Selector::currentSelector() )
 	{
 		// Some users have reported crashes that were traced back to this call

src/GafferUI/ViewportGadget.cpp

@@ -1193,7 +1193,6 @@ void ViewportGadget::render() const
 	glClearColor( 0.26f, 0.26f, 0.26f, 0.0f );
 	glClearDepth( 1.0f );
 	glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );
-	glEnable( GL_BLEND );
 
 	// Set up the camera to world matrix in gl_TextureMatrix[0] so that we can
 	// reference world space positions in shaders
@@ -1288,13 +1287,16 @@ void ViewportGadget::renderInternal( RenderReason reason, Gadget::Layer filterLa
 		if( reason != RenderReason::Draw )
 		{
 			// We're doing selection so post-processing doesn't matter. Just
-			// render direct to output buffer.
+			// render direct to output buffer without blending as that can
+			// corrupt the selection buffer on some graphics hardware.
+			glDisable( GL_BLEND );
 			renderLayerInternal( reason, layer, viewTransform, bound, selector );
 			continue;
 		}
 
-		// Render to intemediate framebuffer.
+		// Render to intermediate framebuffer.
 
+		glEnable( GL_BLEND );
 		glBindFramebuffer( GL_DRAW_FRAMEBUFFER, acquireFramebuffer() );
 		glClearColor( 0.0f, 0.0f, 0.0f, 0.0f );
 		glClear( GL_COLOR_BUFFER_BIT );