Add glfwGetWindowOpacity and glfwSetWindowOpacity

This adds support for setting the opacity of the whole window, including
any decorations.

Fixes #1089.

src/cocoa_window.m

@@ -1493,6 +1493,16 @@ void _glfwPlatformSetWindowFloating(_GLFWwindow* window, GLFWbool enabled)
         [window->ns.object setLevel:NSNormalWindowLevel];
 }
 
+float _glfwPlatformGetWindowOpacity(_GLFWwindow* window)
+{
+    return (float) [window->ns.object alphaValue];
+}
+
+void _glfwPlatformSetWindowOpacity(_GLFWwindow* window, float opacity)
+{
+    [window->ns.object setAlphaValue:opacity];
+}
+
 void _glfwPlatformPollEvents(void)
 {
     for (;;)

src/internal.h

@@ -687,9 +687,11 @@ int _glfwPlatformWindowIconified(_GLFWwindow* window);
 int _glfwPlatformWindowVisible(_GLFWwindow* window);
 int _glfwPlatformWindowMaximized(_GLFWwindow* window);
 int _glfwPlatformFramebufferTransparent(_GLFWwindow* window);
+float _glfwPlatformGetWindowOpacity(_GLFWwindow* window);
 void _glfwPlatformSetWindowResizable(_GLFWwindow* window, GLFWbool enabled);
 void _glfwPlatformSetWindowDecorated(_GLFWwindow* window, GLFWbool enabled);
 void _glfwPlatformSetWindowFloating(_GLFWwindow* window, GLFWbool enabled);
+void _glfwPlatformSetWindowOpacity(_GLFWwindow* window, float opacity);
 
 void _glfwPlatformPollEvents(void);
 void _glfwPlatformWaitEvents(void);

src/mir_window.c

@@ -648,6 +648,15 @@ void _glfwPlatformSetWindowFloating(_GLFWwindow* window, GLFWbool enabled)
                     "Mir: Unsupported function %s", __PRETTY_FUNCTION__);
 }
 
+float _glfwPlatformGetWindowOpacity(_GLFWwindow* window)
+{
+    return 1.f;
+}
+
+void _glfwPlatformSetWindowOpacity(_GLFWwindow* window, float opacity)
+{
+}
+
 void _glfwPlatformPollEvents(void)
 {
     EventNode* node = NULL;

src/null_window.c

@@ -182,6 +182,15 @@ void _glfwPlatformSetWindowFloating(_GLFWwindow* window, GLFWbool enabled)
 {
 }
 
+float _glfwPlatformGetWindowOpacity(_GLFWwindow* window)
+{
+    return 1.f;
+}
+
+void _glfwPlatformSetWindowOpacity(_GLFWwindow* window, float opacity)
+{
+}
+
 void _glfwPlatformShowWindow(_GLFWwindow* window)
 {
 }

src/win32_window.c

@@ -1591,6 +1591,39 @@ void _glfwPlatformSetWindowFloating(_GLFWwindow* window, GLFWbool enabled)
                  SWP_NOACTIVATE | SWP_NOMOVE | SWP_NOSIZE);
 }
 
+float _glfwPlatformGetWindowOpacity(_GLFWwindow* window)
+{
+    BYTE alpha;
+    DWORD flags;
+
+    if ((GetWindowLongW(window->win32.handle, GWL_EXSTYLE) & WS_EX_LAYERED) &&
+        GetLayeredWindowAttributes(window->win32.handle, NULL, &alpha, &flags))
+    {
+        if (flags & LWA_ALPHA)
+            return alpha / 255.f;
+    }
+
+    return 1.f;
+}
+
+void _glfwPlatformSetWindowOpacity(_GLFWwindow* window, float opacity)
+{
+    if (opacity < 1.f)
+    {
+        const BYTE alpha = (BYTE) (255 * opacity);
+        DWORD style = GetWindowLongW(window->win32.handle, GWL_EXSTYLE);
+        style |= WS_EX_LAYERED;
+        SetWindowLongW(window->win32.handle, GWL_EXSTYLE, style);
+        SetLayeredWindowAttributes(window->win32.handle, 0, alpha, LWA_ALPHA);
+    }
+    else
+    {
+        DWORD style = GetWindowLongW(window->win32.handle, GWL_EXSTYLE);
+        style &= ~WS_EX_LAYERED;
+        SetWindowLongW(window->win32.handle, GWL_EXSTYLE, style);
+    }
+}
+
 void _glfwPlatformPollEvents(void)
 {
     MSG msg;

src/window.c

@@ -648,6 +648,34 @@ GLFWAPI void glfwGetWindowContentScale(GLFWwindow* handle,
     _glfwPlatformGetWindowContentScale(window, xscale, yscale);
 }
 
+GLFWAPI float glfwGetWindowOpacity(GLFWwindow* handle)
+{
+    _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window != NULL);
+
+    _GLFW_REQUIRE_INIT_OR_RETURN(1.f);
+    return _glfwPlatformGetWindowOpacity(window);
+}
+
+GLFWAPI void glfwSetWindowOpacity(GLFWwindow* handle, float opacity)
+{
+    _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window != NULL);
+    assert(opacity == opacity);
+    assert(opacity >= 0.f);
+    assert(opacity <= 1.f);
+
+    _GLFW_REQUIRE_INIT();
+
+    if (opacity != opacity || opacity < 0.f || opacity > 1.f)
+    {
+        _glfwInputError(GLFW_INVALID_VALUE, "Invalid window opacity %f", opacity);
+        return;
+    }
+
+    _glfwPlatformSetWindowOpacity(window, opacity);
+}
+
 GLFWAPI void glfwIconifyWindow(GLFWwindow* handle)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;

src/wl_window.c

@@ -692,6 +692,15 @@ void _glfwPlatformSetWindowFloating(_GLFWwindow* window, GLFWbool enabled)
                     "Wayland: Window attribute setting not implemented yet");
 }
 
+float _glfwPlatformGetWindowOpacity(_GLFWwindow* window)
+{
+    return 1.f;
+}
+
+void _glfwPlatformSetWindowOpacity(_GLFWwindow* window, float opacity)
+{
+}
+
 void _glfwPlatformPollEvents(void)
 {
     handleEvents(0);

src/x11_init.c

@@ -737,9 +737,18 @@ static GLFWbool initExtensions(void)
         XInternAtom(_glfw.x11.display, "_NET_WM_ICON_NAME", False);
     _glfw.x11.NET_WM_BYPASS_COMPOSITOR =
         XInternAtom(_glfw.x11.display, "_NET_WM_BYPASS_COMPOSITOR", False);
+    _glfw.x11.NET_WM_WINDOW_OPACITY =
+        XInternAtom(_glfw.x11.display, "_NET_WM_WINDOW_OPACITY", False);
     _glfw.x11.MOTIF_WM_HINTS =
         XInternAtom(_glfw.x11.display, "_MOTIF_WM_HINTS", False);
 
+    // The compositing manager selection name contains the screen number
+    {
+        char name[32];
+        snprintf(name, sizeof(name), "_NET_WM_CM_S%u", _glfw.x11.screen);
+        _glfw.x11.NET_WM_CM_Sx = XInternAtom(_glfw.x11.display, name, False);
+    }
+
     return GLFW_TRUE;
 }
 

src/x11_platform.h

@@ -257,6 +257,8 @@ typedef struct _GLFWlibraryX11
     Atom            NET_WM_STATE_DEMANDS_ATTENTION;
     Atom            NET_WM_BYPASS_COMPOSITOR;
     Atom            NET_WM_FULLSCREEN_MONITORS;
+    Atom            NET_WM_WINDOW_OPACITY;
+    Atom            NET_WM_CM_Sx;
     Atom            NET_ACTIVE_WINDOW;
     Atom            NET_FRAME_EXTENTS;
     Atom            NET_REQUEST_FRAME_EXTENTS;

src/x11_window.c

@@ -2449,11 +2449,7 @@ int _glfwPlatformFramebufferTransparent(_GLFWwindow* window)
     if (!window->x11.transparent)
         return GLFW_FALSE;
 
-    // Check whether a compositing manager is running
-    char name[32];
-    snprintf(name, sizeof(name), "_NET_WM_CM_S%u", _glfw.x11.screen);
-    const Atom selection = XInternAtom(_glfw.x11.display, name, False);
-    return XGetSelectionOwner(_glfw.x11.display, selection) != None;
+    return XGetSelectionOwner(_glfw.x11.display, _glfw.x11.NET_WM_CM_Sx) != None;
 }
 
 void _glfwPlatformSetWindowResizable(_GLFWwindow* window, GLFWbool enabled)
@@ -2559,6 +2555,37 @@ void _glfwPlatformSetWindowFloating(_GLFWwindow* window, GLFWbool enabled)
     XFlush(_glfw.x11.display);
 }
 
+float _glfwPlatformGetWindowOpacity(_GLFWwindow* window)
+{
+    float opacity = 1.f;
+
+    if (XGetSelectionOwner(_glfw.x11.display, _glfw.x11.NET_WM_CM_Sx))
+    {
+        CARD32* value = NULL;
+
+        if (_glfwGetWindowPropertyX11(window->x11.handle,
+                                      _glfw.x11.NET_WM_WINDOW_OPACITY,
+                                      XA_CARDINAL,
+                                      (unsigned char**) &value))
+        {
+            opacity = (float) (*value / (double) 0xffffffffu);
+        }
+
+        if (value)
+            XFree(value);
+    }
+
+    return opacity;
+}
+
+void _glfwPlatformSetWindowOpacity(_GLFWwindow* window, float opacity)
+{
+    const CARD32 value = (CARD32) (0xffffffffu * (double) opacity);
+    XChangeProperty(_glfw.x11.display, window->x11.handle,
+                    _glfw.x11.NET_WM_WINDOW_OPACITY, XA_CARDINAL, 32,
+                    PropModeReplace, (unsigned char*) &value, 1);
+}
+
 void _glfwPlatformPollEvents(void)
 {
     _GLFWwindow* window;