Workaround: resolve MSAA by copying to backbuffer

- On Nvidia, MSAA resolve uses a different, higher-quality sampling filter when resolving to the window backbuffer.
- This difference is very perceptible when rendering thin lines with high contrast.
- This does not present a significant performance cost on mid-high spec systems. Ideally this workaround is dynamically enabled based on driver version etc.

src/graphics/opengl/CommandBufferGL.cpp

@@ -330,7 +330,8 @@ void CommandList::ExecuteBlitRenderTargetCmd(const BlitRenderTargetCmd &cmd)
 	// invalidate cached render target state
 	stateCache->SetRenderTarget(nullptr);
 
-	cmd.srcTarget->Bind(RenderTarget::READ);
+	if (cmd.srcTarget)
+		cmd.srcTarget->Bind(RenderTarget::READ);
 
 	// dstTarget can be null if blitting to the window implicit backbuffer
 	if (cmd.dstTarget)
@@ -343,7 +344,8 @@ void CommandList::ExecuteBlitRenderTargetCmd(const BlitRenderTargetCmd &cmd)
 		cmd.dstExtents.x, cmd.dstExtents.y, cmd.dstExtents.x + cmd.dstExtents.w, cmd.dstExtents.y + cmd.dstExtents.h,
 		mask, cmd.linearFilter ? GL_LINEAR : GL_NEAREST);
 
-	cmd.srcTarget->Unbind(RenderTarget::READ);
+	if (cmd.srcTarget)
+		cmd.srcTarget->Unbind(RenderTarget::READ);
 
 	// dstTarget can be null if blitting to the window implicit backbuffer
 	if (cmd.dstTarget)

src/graphics/opengl/RendererGL.cpp

@@ -81,6 +81,11 @@ namespace Graphics {
 		SDL_GL_SetAttribute(SDL_GL_GREEN_SIZE, 8);
 		SDL_GL_SetAttribute(SDL_GL_BLUE_SIZE, 8);
 
+		// HACK (sturnclaw): request RGBA backbuffer specifically for the purpose of using
+		// it as an intermediate multisample resolve target with RGBA textures.
+		// See ResolveRenderTarget() for more details
+		SDL_GL_SetAttribute(SDL_GL_ALPHA_SIZE, 8);
+
 		winFlags |= (vs.hidden ? SDL_WINDOW_HIDDEN : SDL_WINDOW_SHOWN);
 		if (!vs.hidden && vs.fullscreen) // TODO: support for borderless fullscreen and changing window size
 			winFlags |= SDL_WINDOW_FULLSCREEN;
@@ -638,7 +643,21 @@ namespace Graphics {
 	{
 		bool hasDepthTexture = src->GetDepthTexture() && dst->GetDepthTexture();
 
-		m_drawCommandList->AddBlitRenderTargetCmd(src, dst, extents, extents, true, hasDepthTexture);
+		// HACK (sturnclaw): work around NVidia undocumented behavior of using a higher-quality filtering
+		// kernel when resolving to window backbuffer instead of offscreen FBO.
+		// Otherwise there's a distinct visual quality loss when performing MSAA resolve to offscreen FBO.
+		// Ideally this should be replaced by using a custom MSAA resolve shader; however builtin resolve
+		// usually has better performance (ref: https://therealmjp.github.io/posts/msaa-resolve-filters/)
+		// NOTE: this behavior appears to be independent of setting GL_MULTISAMPLE_FILTER_HINT_NV on Linux
+		if (!hasDepthTexture && extents.w <= m_width && extents.h <= m_height) {
+			ViewportExtents tmpExtents = { 0, 0, extents.w, extents.h };
+
+			m_drawCommandList->AddBlitRenderTargetCmd(src, nullptr, extents, tmpExtents, true);
+			m_drawCommandList->AddBlitRenderTargetCmd(nullptr, dst, tmpExtents, extents, true);
+		} else {
+			m_drawCommandList->AddBlitRenderTargetCmd(src, dst, extents, extents, true, hasDepthTexture);
+		}
+
 		m_drawCommandList->AddRenderPassCmd(m_activeRenderTarget, m_viewport);
 	}