Render via an intermediate frame buffer.

OpenRA.Game/Graphics/PlatformInterfaces.cs

@@ -41,6 +41,8 @@ namespace OpenRA
 
 		Size WindowSize { get; }
 		float WindowScale { get; }
+		Size SurfaceSize { get; }
+
 		event Action<float, float> OnWindowScaleChanged;
 
 		void PumpInput(IInputHandler inputHandler);
@@ -62,7 +64,7 @@ namespace OpenRA
 		IFrameBuffer CreateFrameBuffer(Size s);
 		IFrameBuffer CreateFrameBuffer(Size s, Color clearColor);
 		IShader CreateShader(string name);
-		void EnableScissor(int left, int top, int width, int height);
+		void EnableScissor(int x, int y, int width, int height);
 		void DisableScissor();
 		void SaveScreenshot(string path);
 		void Present();

OpenRA.Game/Graphics/SpriteRenderer.cs

@@ -160,9 +160,9 @@ namespace OpenRA.Graphics
 			shader.SetVec("Scroll", scroll.X, scroll.Y, scroll.Y);
 			shader.SetVec("r1",
 				zoom * 2f / screen.Width,
-				-zoom * 2f / screen.Height,
+				zoom * 2f / screen.Height,
 				-depthScale * zoom / screen.Height);
-			shader.SetVec("r2", -1, 1, 1 - depthOffset);
+			shader.SetVec("r2", -1, -1, 1 - depthOffset);
 
 			// Texture index is sampled as a float, so convert to pixels then scale
 			shader.SetVec("DepthTextureScale", 128 * depthScale * zoom / screen.Height);

OpenRA.Game/Renderer.cs

@@ -38,13 +38,16 @@ namespace OpenRA
 		readonly IVertexBuffer<Vertex> tempBuffer;
 		readonly Stack<Rectangle> scissorState = new Stack<Rectangle>();
 
+		IFrameBuffer screenBuffer;
+		Sprite screenSprite;
+
 		SheetBuilder fontSheetBuilder;
 		readonly IPlatform platform;
 
 		float depthScale;
 		float depthOffset;
 
-		Size lastResolution = new Size(-1, -1);
+		Size lastBufferSize = new Size(-1, -1);
 		int2 lastScroll = new int2(-1, -1);
 		float lastZoom = -1f;
 		ITexture currentPaletteTexture;
@@ -123,26 +126,55 @@ namespace OpenRA
 		public void BeginFrame(int2 scroll, float zoom)
 		{
 			Context.Clear();
+
+			var surfaceSize = Window.SurfaceSize;
+			var surfaceBufferSize = surfaceSize.NextPowerOf2();
+
+			if (screenSprite == null || screenSprite.Sheet.Size != surfaceBufferSize)
+			{
+				if (screenBuffer != null)
+					screenBuffer.Dispose();
+
+				// Render the screen into a frame buffer to simplify reading back screenshots
+				screenBuffer = Context.CreateFrameBuffer(surfaceBufferSize, Color.FromArgb(0xFF, 0, 0, 0));
+			}
+
+			if (screenSprite == null || surfaceSize.Width != screenSprite.Bounds.Width || -surfaceSize.Height != screenSprite.Bounds.Height)
+			{
+				var screenSheet = new Sheet(SheetType.BGRA, screenBuffer.Texture);
+
+				// Flip sprite in Y to match OpenGL's bottom-left origin
+				var screenBounds = Rectangle.FromLTRB(0, surfaceSize.Height, surfaceSize.Width, 0);
+				screenSprite = new Sprite(screenSheet, screenBounds, TextureChannel.RGBA);
+			}
+
+			screenBuffer.Bind();
 			SetViewportParams(scroll, zoom);
 		}
 
 		public void SetViewportParams(int2 scroll, float zoom)
 		{
-			// PERF: Calling SetViewportParams on each renderer is slow. Only call it when things change.
-			var resolutionChanged = lastResolution != Resolution;
-			if (resolutionChanged)
-			{
-				lastResolution = Resolution;
-				SpriteRenderer.SetViewportParams(lastResolution, 0f, 0f, 1f, int2.Zero);
-			}
+			// In HiDPI windows we follow Apple's convention of defining window coordinates as for standard resolution windows
+			// but to have a higher resolution backing surface with more than 1 texture pixel per viewport pixel.
+			// We must convert the surface buffer size to a viewport size - in general this is NOT just the window size
+			// rounded to the next power of two, as the NextPowerOf2 calculation is done in the surface pixel coordinates
+			var scale = Window.WindowScale;
+			var surfaceBufferSize = Window.SurfaceSize.NextPowerOf2();
+			var bufferSize = new Size((int)(surfaceBufferSize.Width / scale), (int)(surfaceBufferSize.Height / scale));
 
-			// If zoom evaluates as different due to floating point weirdness that's OK, setting the parameters again is harmless.
-			if (resolutionChanged || lastScroll != scroll || lastZoom != zoom)
+			// PERF: Calling SetViewportParams on each renderer is slow. Only call it when things change.
+			// If zoom evaluates as different due to floating point weirdness that's OK, it will be going away soon
+			if (lastBufferSize != bufferSize || lastScroll != scroll || lastZoom != zoom)
 			{
+				if (lastBufferSize != bufferSize)
+					SpriteRenderer.SetViewportParams(bufferSize, 0f, 0f, 1f, int2.Zero);
+
+				WorldSpriteRenderer.SetViewportParams(bufferSize, depthScale, depthOffset, zoom, scroll);
+				WorldModelRenderer.SetViewportParams(bufferSize, zoom, scroll);
+
+				lastBufferSize = bufferSize;
 				lastScroll = scroll;
 				lastZoom = zoom;
-				WorldSpriteRenderer.SetViewportParams(lastResolution, depthScale, depthOffset, zoom, scroll);
-				WorldModelRenderer.SetViewportParams(lastResolution, zoom, scroll);
 			}
 		}
 
@@ -162,6 +194,15 @@ namespace OpenRA
 		public void EndFrame(IInputHandler inputHandler)
 		{
 			Flush();
+
+			screenBuffer.Unbind();
+
+			// Render the compositor buffer to the screen
+			// HACK / PERF: Fudge the coordinates to cover the actual window while keeping the buffer viewport parameters
+			// This saves us two redundant (and expensive) SetViewportParams each frame
+			RgbaSpriteRenderer.DrawSprite(screenSprite, new float3(0, lastBufferSize.Height, 0), new float3(lastBufferSize.Width, -lastBufferSize.Height, 0));
+			Flush();
+
 			Window.PumpInput(inputHandler);
 			Context.Present();
 		}

OpenRA.Platforms.Default/Sdl2GraphicsContext.cs

@@ -83,7 +83,7 @@ namespace OpenRA.Platforms.Default
 			return new Shader(name);
 		}
 
-		public void EnableScissor(int left, int top, int width, int height)
+		public void EnableScissor(int x, int y, int width, int height)
 		{
 			VerifyThreadAffinity();
 
@@ -97,16 +97,15 @@ namespace OpenRA.Platforms.Default
 			var windowScale = window.WindowScale;
 			var surfaceSize = window.SurfaceSize;
 
-			var bottom = windowSize.Height - (top + height);
 			if (windowSize != surfaceSize)
 			{
-				left = (int)Math.Round(windowScale * left);
-				bottom = (int)Math.Round(windowScale * bottom);
+				x = (int)Math.Round(windowScale * x);
+				y = (int)Math.Round(windowScale * y);
 				width = (int)Math.Round(windowScale * width);
 				height = (int)Math.Round(windowScale * height);
 			}
 
-			OpenGL.glScissor(left, bottom, width, height);
+			OpenGL.glScissor(x, y, width, height);
 			OpenGL.CheckGLError();
 			OpenGL.glEnable(OpenGL.GL_SCISSOR_TEST);
 			OpenGL.CheckGLError();

OpenRA.Platforms.Default/Sdl2PlatformWindow.cs

@@ -59,7 +59,7 @@ namespace OpenRA.Platforms.Default
 			}
 		}
 
-		internal Size SurfaceSize
+		public Size SurfaceSize
 		{
 			get
 			{