Adding intro and skipIntro option

shaders/glsl/rainPass.intro.frag.glsl

@@ -0,0 +1,59 @@
+precision highp float;
+
+// This shader governs the "intro"— the initial stream of rain from a blank screen.
+// It writes falling rain to the channels of a data texture:
+// 		R: raindrop length
+// 		G: unused
+// 		B: unused
+// 		A: unused
+
+#define PI 3.14159265359
+#define SQRT_2 1.4142135623730951
+#define SQRT_5 2.23606797749979
+
+uniform sampler2D previousIntroState;
+uniform float numColumns, numRows;
+uniform float time, tick;
+uniform float animationSpeed, fallSpeed;
+
+uniform bool skipIntro;
+
+// Helper functions for generating randomness, borrowed from elsewhere
+
+highp float randomFloat( const in vec2 uv ) {
+	const highp float a = 12.9898, b = 78.233, c = 43758.5453;
+	highp float dt = dot( uv.xy, vec2( a,b ) ), sn = mod( dt, PI );
+	return fract(sin(sn) * c);
+}
+
+vec2 randomVec2( const in vec2 uv ) {
+	return fract(vec2(sin(uv.x * 591.32 + uv.y * 154.077), cos(uv.x * 391.32 + uv.y * 49.077)));
+}
+
+float wobble(float x) {
+	return x + 0.3 * sin(SQRT_2 * x) + 0.2 * sin(SQRT_5 * x);
+}
+
+// Main function
+
+vec4 computeResult(float simTime, bool isFirstFrame, vec2 glyphPos, vec2 screenPos, vec4 previous) {
+	if (skipIntro) {
+		return vec4(1., 0., 0., 0.);
+	}
+
+	float columnTimeOffset = randomFloat(vec2(glyphPos.x, 0.)) * -10.;
+	columnTimeOffset += sin(glyphPos.x / numColumns * PI) - 1.;
+	float introTime = (simTime + columnTimeOffset) * fallSpeed / numRows * 100.;
+
+	vec4 result = vec4(introTime, 0., 0., 0.);
+	return result;
+}
+
+void main()	{
+	float simTime = time * animationSpeed;
+	bool isFirstFrame = tick <= 1.;
+	vec2 glyphPos = gl_FragCoord.xy;
+	vec2 screenPos = glyphPos / vec2(numColumns, numRows);
+	vec4 previous = texture2D( previousIntroState, screenPos );
+	gl_FragColor = computeResult(simTime, isFirstFrame, glyphPos, screenPos, previous);
+}