Crt, Ripple and Glitch take a level, 0 to 3, tunable at run time

Coordinator's request: `Crt 0..3` (likewise Ripple, Glitch), 0 off, `on`
the default level 2 (the polished rewrite), `off` and a bare word as
before. The level is one uniform (pardesLevel, beside pardesOpaque in the
prefix's own block); each pass picks its constants by it. Level 3 is at
least as strong as the old effects, measured as pixels changed by more
than 8 in a channel against a plain frame: Crt 18.3% (old 17.4%), Ripple
11.1% (9.5%), Glitch 9.3% a frame (8.1%); Glitch's level raises how often
it bursts as well as how hard. Config and ctl report `Crt 2` / `Crt off`.

Shared files touched: ninep/ctl.zig (the report line). Not touched:
pardes.zig, Messages.zig, mouse.zig, gui.zig, detached/*.

1 files changed, 29 insertions, 18 deletions

diff --git a/shaders/prebuilt/post-glitch.frag.glsl b/shaders/prebuilt/post-glitch.frag.glsl
index 46538452..7b288237 100644
--- a/shaders/prebuilt/post-glitch.frag.glsl
+++ b/shaders/prebuilt/post-glitch.frag.glsl
@@ -39,12 +39,19 @@ layout(set = 3, binding = 0, std140) uniform Globals {
     uniform vec3  iSelectionBackgroundColor;
 };
 
-// pardes's own: 1 when the window is opaque, so a pass cannot punch holes
-// in it through its alpha.
+// pardes's own: pardesOpaque is 1 when the window is opaque, so a pass
+// cannot punch holes in it through its alpha; pardesLevel is a bundled
+// pass's strength, 1 to 3 (`Crt 3`), and 0 for a user's file.
 layout(set = 3, binding = 1, std140) uniform PardesPost {
     float pardesOpaque;
+    float pardesLevel;
 };
 
+// A bundled pass's constant at levels 1, 2 and 3.
+float pardesByLevel(float one, float two, float three) {
+    return pardesLevel < 1.5 ? one : (pardesLevel < 2.5 ? two : three);
+}
+
 #define CURSORSTYLE_BLOCK        0
 #define CURSORSTYLE_BLOCK_HOLLOW 1
 #define CURSORSTYLE_BAR          2
@@ -65,7 +72,8 @@ void main() {
 
 // The shader's own lines count from 1, so glslc's errors name them.
 #line 1
-// Glitch: now and then the signal slips. Every couple of seconds a short
+// Glitch: now and then the signal slips. At the default level, every couple
+// of seconds a short
 // burst tears a few horizontal bands sideways and pulls their red and blue
 // apart; the bands are sized in pixels and the burst eases in and out over
 // a fraction of a second, so it reads as one event rather than a flicker,
@@ -74,12 +82,15 @@ void main() {
 // A torn band's brightness dips in linear light. It is a Shadertoy file:
 // pardes puts its prefix before it.
 
-const float PERIOD = 2.4;           // seconds between bursts, on average
-const float BURST = 0.28;           // seconds a burst lasts
-const float STEP_HZ = 24.0;         // how often a burst re-tears
-const float SHIFT_PX = 14.0;
-const float SPLIT_PX = 2.5;
-const float DIP = 0.12;
+// Each at levels 1, 2 and 3 (`Glitch 1`..`Glitch 3`); 2 is the default. A
+// higher level slips more often and harder.
+float period() { return pardesByLevel(4.5, 2.4, 0.55); }      // seconds between bursts, on average
+float burst() { return pardesByLevel(0.2, 0.28, 0.5); }       // seconds a burst lasts
+float stepHz() { return pardesByLevel(20.0, 24.0, 30.0); }    // how often a burst re-tears
+float shiftPx() { return pardesByLevel(8.0, 14.0, 34.0); }
+float splitPx() { return pardesByLevel(1.5, 2.5, 6.0); }
+float dip() { return pardesByLevel(0.08, 0.12, 0.22); }
+float tearFrom() { return pardesByLevel(0.8, 0.72, 0.35); }   // how few bands tear
 
 float hash(vec2 p) {
     vec3 q = fract(vec3(p.xyx) * 0.1031);
@@ -101,27 +112,27 @@ void mainImage(out vec4 fragColor, in vec2 fragCoord) {
     vec2 uv = fragCoord / size;
     // Which burst window this is, when in it the burst starts, and how far
     // into the burst we are: 0 outside one.
-    float window = floor(iTime / PERIOD);
-    float start = window * PERIOD + hash(vec2(window, 7.0)) * (PERIOD - BURST);
-    float t = (iTime - start) / BURST;
+    float window = floor(iTime / period());
+    float start = window * period() + hash(vec2(window, 7.0)) * (period() - burst());
+    float t = (iTime - start) / burst();
     float live = step(0.0, t) * step(t, 1.0);
     float envelope = live * sin(3.14159265 * clamp(t, 0.0, 1.0));
     if (envelope <= 0.0) {
         fragColor = texture(iChannel0, uv);
         return;
     }
-    // Bands of 6 to 40 px, re-cut at STEP_HZ during the burst; about one
+    // Bands of 6 to 40 px, re-cut at stepHz() during the burst; about one
     // band in four tears.
-    float tick = floor(iTime * STEP_HZ);
+    float tick = floor(iTime * stepHz());
     float band_px = mix(6.0, 40.0, hash(vec2(tick, 3.0)));
     float band = floor(fragCoord.y / band_px);
     float roll = hash(vec2(band, tick));
-    float tear = smoothstep(0.72, 0.9, roll) * envelope;
-    float shift = (hash(vec2(tick, band + 11.0)) - 0.5) * 2.0 * SHIFT_PX * tear;
+    float tear = smoothstep(tearFrom(), tearFrom() + 0.18, roll) * envelope;
+    float shift = (hash(vec2(tick, band + 11.0)) - 0.5) * 2.0 * shiftPx() * tear;
     vec2 at = vec2(fragCoord.x + shift, fragCoord.y);
-    vec2 split = vec2(SPLIT_PX * tear, 0.0);
+    vec2 split = vec2(splitPx() * tear, 0.0);
     vec4 center = texture(iChannel0, at / size);
     vec3 col = vec3(texture(iChannel0, (at + split) / size).r, center.g, texture(iChannel0, (at - split) / size).b);
-    col = toSrgb(toLinear(col) * (1.0 - DIP * tear));
+    col = toSrgb(toLinear(col) * (1.0 - dip() * tear));
     fragColor = vec4(col, center.a);
 }