graphics/widget: introduce opacity attribute on the canvas and widget.

You can use the opacity attribute to control the opacity of the widget
and its children. The attribute is cumulative, and multiplied to the
current context color. By default, the opacity is 1.0, and doesn't act
at all.

kivy/data/glsl/default.vs

@@ -1,6 +1,6 @@
 $HEADER$
 void main (void) {
-  frag_color = color;
+  frag_color = color * vec4(1.0, 1.0, 1.0, opacity);
   tex_coord0 = vTexCoords0;
   gl_Position = projection_mat * modelview_mat * vec4(vPosition.xy, 0.0, 1.0);
 }

kivy/data/glsl/header.vs

@@ -14,3 +14,4 @@ attribute vec2     vTexCoords0;
 uniform mat4       modelview_mat;
 uniform mat4       projection_mat;
 uniform vec4       color;
+uniform float      opacity;

kivy/graphics/instructions.pxd

@@ -82,6 +82,7 @@ cdef class CanvasBase(InstructionGroup):
 
 cdef class Canvas(CanvasBase):
     cdef object __weakref__
+    cdef float _opacity
     cdef CanvasBase _before
     cdef CanvasBase _after
     cdef void reload(self)
@@ -89,6 +90,7 @@ cdef class Canvas(CanvasBase):
     cpdef add(self, Instruction c)
     cpdef remove(self, Instruction c)
     cpdef draw(self)
+    cdef void apply(self)
 
 
 cdef class RenderContext(Canvas):

kivy/graphics/instructions.pyx

@@ -499,6 +499,7 @@ cdef class Canvas(CanvasBase):
     def __init__(self, **kwargs):
         get_context().register_canvas(self)
         CanvasBase.__init__(self, **kwargs)
+        self._opacity = kwargs.get('opacity', 1.0)
         self._before = None
         self._after = None
 
@@ -530,6 +531,19 @@ cdef class Canvas(CanvasBase):
         '''
         self.apply()
 
+    cdef void apply(self):
+        cdef float opacity = self._opacity
+        cdef float rc_opacity
+        cdef RenderContext rc
+        if opacity != 1.0:
+            rc = getActiveContext()
+            rc_opacity = rc['opacity']
+            rc.push_state('opacity')
+            rc['opacity'] = rc_opacity * opacity
+        InstructionGroup.apply(self)
+        if opacity != 1.0:
+            rc.pop_state('opacity')
+
     cpdef add(self, Instruction c):
         # the after group must remain the last one.
         if self._after is None:
@@ -569,6 +583,15 @@ cdef class Canvas(CanvasBase):
                 self._after = c
             return self._after
 
+    property opacity:
+        '''Property for getting the opacity value
+        '''
+        def __get__(self):
+            return self._opacity
+        def __set__(self, value):
+            self._opacity = value
+            self.flag_update()
+
 # Active Canvas and getActiveCanvas function is used
 # by instructions, so they know which canvas to add
 # tehmselves to
@@ -629,8 +652,8 @@ cdef class RenderContext(Canvas):
         self.default_texture = tex
 
         self.state_stacks = {
+            'opacity': [1.0],
             'texture0' : [0],
-            'linewidth': [1.0],
             'color'    : [[1.0,1.0,1.0,1.0]],
             'projection_mat': [Matrix()],
             'modelview_mat' : [Matrix()],

kivy/uix/widget.py

@@ -532,6 +532,30 @@ class Widget(EventDispatcher):
     dict.
     '''
 
+    opacity = NumericProperty(1.0)
+    '''Opacity of the widget and all the children.
+
+    .. versionadded:: 1.4.1
+
+    The opacity attribute control the opacity of the widget and the children.
+    Take care, it's a cumulative attribute: the value is multiplied to the
+    current global opacity, and the result is applied to the current context
+    color.
+
+    For example: if your parent have an opacity of 0.5, and one children have an
+    opacity of 0.2, the real opacity of the children will be 0.5 * 0.2 = 0.1.
+
+    Then, the opacity is applied on the shader as::
+
+        frag_color = color * vec4(1.0, 1.0, 1.0, opacity);
+
+    :data:`opacity` is a :class:`~kivy.properties.NumericProperty`, default to
+    1.0.
+    '''
+
+    def on_opacity(self, instance, value):
+        self.canvas.opacity = value
+
     canvas = None
     '''Canvas of the widget.