examples: add plasma shader example

examples/shader/plasma.kv

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+#:kivy 1.0
+
+<ShaderWidget>:
+	canvas:
+		Color:
+			rgb: 1, 0, 0
+		Rectangle:
+			pos: self.pos
+			size: self.size

examples/shader/plasma.py

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+'''
+Plasma Shader
+=============
+
+This shader example have been taken from http://www.iquilezles.org/apps/shadertoy/
+with some adapation.
+
+This might become a Kivy widget when experimentation will be done.
+'''
+
+
+from kivy.clock import Clock
+from kivy.app import App
+from kivy.uix.floatlayout import FloatLayout
+from kivy.core.window import Window
+from kivy.graphics import RenderContext
+from kivy.properties import StringProperty
+
+# This header must be not changed, it contain the minimum information from Kivy.
+header = '''
+#ifdef GL_ES
+precision highp float;
+#endif
+
+/* Outputs from the vertex shader */
+varying vec4 frag_color;
+varying vec2 tex_coord0;
+
+/* uniform texture samplers */
+uniform sampler2D texture0;
+'''
+
+# Plasma shader
+plasma_shader = header + '''
+uniform vec2 resolution;
+uniform float time;
+
+void main(void)
+{
+   float x = gl_FragCoord.x;
+   float y = gl_FragCoord.y;
+   float mov0 = x+y+cos(sin(time)*2.)*100.+sin(x/100.)*1000.;
+   float mov1 = y / resolution.y / 0.2 + time;
+   float mov2 = x / resolution.x / 0.2;
+   float c1 = abs(sin(mov1+time)/2.+mov2/2.-mov1-mov2+time);
+   float c2 = abs(sin(c1+sin(mov0/1000.+time)+sin(y/40.+time)+sin((x+y)/100.)*3.));
+   float c3 = abs(sin(c2+cos(mov1+mov2+c2)+cos(mov2)+sin(x/1000.)));
+   gl_FragColor = vec4( c1,c2,c3,1.0);
+}
+'''
+
+class ShaderWidget(FloatLayout):
+
+    # property to set the source code for fragment shader
+    fs = StringProperty(None)
+
+    def __init__(self, **kwargs):
+        # Instead of using Canvas, we will use a RenderContext,
+        # and change the default shader used.
+        self.canvas = RenderContext()
+
+        # call the constructor of parent
+        # if they are any graphics object, they will be added on our new canvas
+        super(ShaderWidget, self).__init__(**kwargs)
+
+        # We'll update our glsl variables in a clock
+        Clock.schedule_interval(self.update_glsl, 1 / 60.)
+
+    def on_fs(self, instance, value):
+        # set the fragment shader to our source code
+        shader = self.canvas.shader
+        old_value = shader.fs
+        shader.fs = value
+        if not shader.success:
+            shader.fs = old_value
+            raise Exception('failed')
+
+    def update_glsl(self, *largs):
+        self.canvas['time'] = Clock.get_boottime()
+        self.canvas['resolution'] = map(float, self.size)
+        # This is needed for the default vertex shader.
+        self.canvas['projection_mat'] = Window.render_context['projection_mat']
+
+
+class PlasmaApp(App):
+    def build(self):
+        return ShaderWidget(fs=plasma_shader)
+
+if __name__ == '__main__':
+    PlasmaApp().run()