examples: subtle warm cursor-glow shader

README.md

@@ -115,12 +115,23 @@ a CRT look where each bell sends a split-flap wave across the screen:
   <img src="docs/screenshots/crt-splitflap.gif" alt="toastty with a curved, scanlined CRT shader; after a bell, the character cells flip like split-flap display tiles in a wave spreading out from the cursor" width="720">
 </p>
 
+Shaders are plain files, not built in. Copy one next to your config and
+list it; relative paths are relative to the config file's directory:
+
+```sh
+mkdir -p ~/.config/toastty/shaders
+cp examples/shaders/*.glsl ~/.config/toastty/shaders/
+```
+
 ```toml
 [shader]
-paths = ["~/.config/toastty/shaders/crt-splitflap.glsl"]   # run in order
+paths = ["shaders/crt-splitflap.glsl"]   # run in order
 animation = "auto"   # "auto" | "always" | "never"
 ```
 
+[`cursor-glow.glsl`](examples/shaders/cursor-glow.glsl) is a smaller
+example: a subtle warm glow around the cursor.
+
 Shaders that don't use time cost nothing when the screen is idle.
 toastty reads which uniforms a shader uses and only draws frames when
 they can change. A shader whose animation is triggered by events can

examples/shaders/cursor-glow.glsl

@@ -0,0 +1,43 @@
+// A soft, warm yellow glow around the cursor.
+//
+// Static: it reads only the cursor uniforms, which change only when the
+// cursor moves (and that redraws anyway), so it costs nothing at idle.
+// The glow follows the cursor's visibility, so it blinks with a blinking
+// cursor and disappears when an app hides the cursor.
+//
+//   [shader]
+//   paths = ["shaders/cursor-glow.glsl"]
+
+const vec3 GLOW_COLOR = vec3(1.0, 0.78, 0.38);  // warm yellow
+const float STRENGTH = 0.16;    // peak brightness next to the cursor, 0..1
+const float REACH_CELLS = 2.2;  // how far the glow fades out, in cell heights
+
+// Distance from `p` to the rectangle with corners `lo`, `hi` (0 inside).
+float rect_distance(vec2 p, vec2 lo, vec2 hi) {
+    vec2 d = max(max(lo - p, p - hi), 0.0);
+    return length(d);
+}
+
+void mainImage(out vec4 fragColor, in vec2 fragCoord) {
+    vec4 base = texture(iChannel0, fragCoord / iResolution.xy);
+    if (iCursorVisible == 0) {
+        fragColor = base;
+        return;
+    }
+
+    // iCurrentCursor: xy = left, bottom edge (y points down); zw = size.
+    vec2 lo = vec2(iCurrentCursor.x, iCurrentCursor.y - iCurrentCursor.w);
+    vec2 hi = vec2(iCurrentCursor.x + iCurrentCursor.z, iCurrentCursor.y);
+    float d = rect_distance(fragCoord, lo, hi);
+
+    // Smooth falloff to zero at the reach, and none on the cursor itself
+    // so the character under it keeps its colours.
+    float reach = REACH_CELLS * iCellSize.y;
+    float falloff = 1.0 - smoothstep(0.0, reach, d);
+    float g = STRENGTH * falloff * falloff * smoothstep(0.0, 1.5, d);
+
+    // Screen blend in premultiplied space: brightens without clipping,
+    // and stays a valid premultiplied colour over a transparent background.
+    vec4 glow = vec4(GLOW_COLOR * g, g);
+    fragColor = base + glow - base * glow;
+}