Added a built-in variable readable in shaders: ‘love_ScreenSize’. Its x and y components contain the width and height of the current viewport. Resolves issue #841.

src/modules/graphics/opengl/OpenGL.h

@@ -87,6 +87,11 @@ public:
 		Viewport(int _x, int _y, int _w, int _h)
 			: x(_x), y(_y), w(_w), h(_h)
 		{}
+
+		bool operator == (const Viewport &rhs) const
+		{
+			return x == rhs.x && y == rhs.y && w == rhs.w && h == rhs.h;
+		}
 	};
 
 	OpenGL();

src/modules/graphics/opengl/Shader.cpp

@@ -72,6 +72,7 @@ Shader::Shader(const ShaderSources &sources)
 	, builtinUniforms()
 	, vertexAttributes()
 	, lastCanvas((Canvas *) -1)
+	, lastViewport()
 {
 	if (shaderSources.empty())
 		throw love::Exception("Cannot create shader: no source code!");
@@ -693,30 +694,37 @@ bool Shader::sendBuiltinFloat(BuiltinExtern builtin, int size, const GLfloat *ve
 
 void Shader::checkSetScreenParams()
 {
-	if (lastCanvas == Canvas::current)
+	OpenGL::Viewport view = gl.getViewport();
+
+	if (view == lastViewport && lastCanvas == Canvas::current)
 		return;
 
-	// In the shader, we do pixcoord.y = gl_FragCoord.y * params[0] + params[1].
+	// In the shader, we do pixcoord.y = gl_FragCoord.y * params.z + params.w.
 	// This lets us flip pixcoord.y when needed, to be consistent (Canvases
 	// have flipped y-values for pixel coordinates.)
-	GLfloat params[] = {0.0f, 0.0f};
+	GLfloat params[] = {
+		(GLfloat) view.w, (GLfloat) view.h,
+		0.0f, 0.0f,
+	};
 
 	if (Canvas::current != nullptr)
 	{
 		// gl_FragCoord.y is flipped in Canvases, so we un-flip:
 		// pixcoord.y = gl_FragCoord.y * -1.0 + height.
-		params[0] = -1.0f;
-		params[1] = (float) Canvas::current->getHeight();
+		params[2] = -1.0f;
+		params[3] = (GLfloat) view.h;
 	}
 	else
 	{
 		// No flipping: pixcoord.y = gl_FragCoord.y * 1.0 + 0.0.
-		params[0] = 1.0f;
-		params[1] = 0.0f;
+		params[2] = 1.0f;
+		params[3] = 0.0f;
 	}
 
-	sendBuiltinFloat(BUILTIN_SCREEN_PARAMS, 2, params, 1);
+	sendBuiltinFloat(BUILTIN_SCREEN_SIZE, 4, params, 1);
+
 	lastCanvas = Canvas::current;
+	lastViewport = view;
 }
 
 const std::map<std::string, Object *> &Shader::getBoundRetainables() const
@@ -767,7 +775,7 @@ StringMap<OpenGL::VertexAttrib, OpenGL::ATTRIB_MAX_ENUM> Shader::attribNames(Sha
 
 StringMap<Shader::BuiltinExtern, Shader::BUILTIN_MAX_ENUM>::Entry Shader::builtinNameEntries[] =
 {
-	{"love_ScreenParams", Shader::BUILTIN_SCREEN_PARAMS},
+	{"love_ScreenSize", Shader::BUILTIN_SCREEN_SIZE},
 };
 
 StringMap<Shader::BuiltinExtern, Shader::BUILTIN_MAX_ENUM> Shader::builtinNames(Shader::builtinNameEntries, sizeof(Shader::builtinNameEntries));

src/modules/graphics/opengl/Shader.h

@@ -59,7 +59,7 @@ public:
 	// Built-in extern (uniform) variables.
 	enum BuiltinExtern
 	{
-		BUILTIN_SCREEN_PARAMS,
+		BUILTIN_SCREEN_SIZE,
 		BUILTIN_MAX_ENUM
 	};
 
@@ -216,6 +216,7 @@ private:
 
 	// Pointer to the active Canvas when the screen params were last checked.
 	Canvas *lastCanvas;
+	OpenGL::Viewport lastViewport;
 
 	// Max GPU texture units available for sent images
 	static GLint maxTexUnits;