A ripple-on-click shader

kitty/shaders/custom/pond-ripple.pipeline

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+startgroup
+    animation_start pointer-left-button-press
+    animation_stop 1500
+    animation_curve linear
+    animation_step 16
+    shaders pond-ripple
+endgroup

kitty/shaders/custom/pond-ripple.slang

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+#language slang 2026
+// Copyright (C) 2026 Kovid Goyal <kovid at kovidgoyal.net>
+// Distributed under terms of the GPLv3 license.
+//
+// Pond ripple effect — concentric expanding rings centered on pointer-left-button-press.
+// animation_progress (0 → 1) drives the wavefront expansion over the pipeline duration.
+
+import kitty_custom_shader_types;
+
+// Maximum wavefront radius at animation_progress=1 (aspect-corrected UV units).
+extern static const float WAVE_SPEED     = 0.45;
+// Spatial frequency of the rings (cycles per aspect-corrected UV unit).
+extern static const float WAVE_FREQUENCY = 14.0;
+// Starting half-width of the Gaussian envelope (expands as animation_progress increases).
+extern static const float WAVE_WIDTH     = 0.055;
+// Peak radial UV displacement at the wavecrest.
+extern static const float AMPLITUDE      = 0.010;
+// Multiplicative brightness modulation at the wave crests (water-surface sheen).
+extern static const float CREST_BRIGHT   = 0.12;
+
+public float4 fragment_main(
+    float4 color,
+    KittyTextures t,
+    KittyCustomShaderData d,
+    float4 viewport,
+    float animation_progress
+) {
+    float2 pixelDims = float2(d.viewport_size_pixels) * viewport.zw;
+    float  aspect    = pixelDims.x / pixelDims.y;
+
+    // d.mouse_pos.zw is the UV position of the last left-button press (y=0 at bottom).
+    float2 center = d.mouse_pos.zw;
+
+    // Aspect-corrected vector from the click center to this pixel.
+    float2 delta = (t.pos - center) * float2(aspect, 1.0);
+    float  dist  = length(delta);
+    float2 dir   = delta / max(dist, 1e-5);
+
+    // Wavefront radius advances linearly with animation_progress.
+    float front = WAVE_SPEED * animation_progress;
+
+    // Signed distance from the wavefront (negative = wave has passed, positive = not yet reached).
+    float behind = dist - front;
+
+    // Gaussian spatial envelope centered at the wavefront; grows in width over time.
+    float sigma = WAVE_WIDTH * (1.0 + animation_progress);
+    float env   = exp(-behind * behind / (2.0 * sigma * sigma));
+
+    // Causality: suppress any energy ahead of the wavefront.
+    env *= 1.0 - smoothstep(-0.01, 0.02, behind);
+
+    // Temporal amplitude decay as the wave expands outward.
+    float decay = pow(1.0 - animation_progress, 0.7);
+
+    // Traveling sinusoidal wave: phase = k * (dist − front).
+    float wave = sin(behind * WAVE_FREQUENCY * 6.28318530718);
+
+    // Radial UV displacement: push pixels away from center at crests, pull inward at troughs.
+    float2 warpedUV = clamp(
+        t.pos + dir * (wave * env * decay * AMPLITUDE),
+        float2(0.0), float2(1.0)
+    );
+
+    // Sample the terminal backbuffer at the displaced position.
+    float4 result = t.backbuffer.Sample(warpedUV);
+
+    // Multiplicative brightness sheen at the wave crests (simulates reflected surface light).
+    result.rgb *= 1.0 + abs(wave) * env * decay * CREST_BRIGHT;
+
+    return result;
+}