ghostty: add cursor trail shader

.config/ghostty/config

···
 window-padding-color = extend
 
 theme = "Kanagawa Wave"
+
+custom-shader = "cursor.glsl"

.config/ghostty/cursor.glsl

···
+// Adapted from: https://github.com/KroneCorylus/shader-playground/blob/main/shaders/cursor_smear_fade.glsl
+
+float getSdfRectangle(in vec2 p, in vec2 xy, in vec2 b)
+{
+    vec2 d = abs(p - xy) - b;
+    return length(max(d, 0.0)) + min(max(d.x, d.y), 0.0);
+}
+
+// Based on Inigo Quilez's 2D distance functions article: https://iquilezles.org/articles/distfunctions2d/
+// Potencially optimized by eliminating conditionals and loops to enhance performance and reduce branching
+
+float seg(in vec2 p, in vec2 a, in vec2 b, inout float s, float d) {
+    vec2 e = b - a;
+    vec2 w = p - a;
+    vec2 proj = a + e * clamp(dot(w, e) / dot(e, e), 0.0, 1.0);
+    float segd = dot(p - proj, p - proj);
+    d = min(d, segd);
+
+    float c0 = step(0.0, p.y - a.y);
+    float c1 = 1.0 - step(0.0, p.y - b.y);
+    float c2 = 1.0 - step(0.0, e.x * w.y - e.y * w.x);
+    float allCond = c0 * c1 * c2;
+    float noneCond = (1.0 - c0) * (1.0 - c1) * (1.0 - c2);
+    float flip = mix(1.0, -1.0, step(0.5, allCond + noneCond));
+    s *= flip;
+    return d;
+}
+
+float getSdfParallelogram(in vec2 p, in vec2 v0, in vec2 v1, in vec2 v2, in vec2 v3) {
+    float s = 1.0;
+    float d = dot(p - v0, p - v0);
+
+    d = seg(p, v0, v3, s, d);
+    d = seg(p, v1, v0, s, d);
+    d = seg(p, v2, v1, s, d);
+    d = seg(p, v3, v2, s, d);
+
+    return s * sqrt(d);
+}
+
+vec2 normalize(vec2 value, float isPosition) {
+    return (value * 2.0 - (iResolution.xy * isPosition)) / iResolution.y;
+}
+
+float antialising(float distance) {
+    return 1. - smoothstep(0., normalize(vec2(2., 2.), 0.).x, distance);
+}
+
+float determineStartVertexFactor(vec2 a, vec2 b) {
+    // Conditions using step
+    float condition1 = step(b.x, a.x) * step(a.y, b.y); // a.x < b.x && a.y > b.y
+    float condition2 = step(a.x, b.x) * step(b.y, a.y); // a.x > b.x && a.y < b.y
+
+    // If neither condition is met, return 1 (else case)
+    return 1.0 - max(condition1, condition2);
+}
+
+vec2 getRectangleCenter(vec4 rectangle) {
+    return vec2(rectangle.x + (rectangle.z / 2.), rectangle.y - (rectangle.w / 2.));
+}
+float ease(float x) {
+    return pow(1.0 - x, 3.0);
+}
+
+const float DURATION = 0.25; //IN SECONDS
+
+void mainImage(out vec4 fragColor, in vec2 fragCoord)
+{
+    #if !defined(WEB)
+    fragColor = texture(iChannel0, fragCoord.xy / iResolution.xy);
+    #endif
+    // Normalization for fragCoord to a space of -1 to 1;
+    vec2 vu = normalize(fragCoord, 1.);
+    vec2 offsetFactor = vec2(-.5, 0.5);
+
+    // Normalization for cursor position and size;
+    // cursor xy has the postion in a space of -1 to 1;
+    // zw has the width and height
+    vec4 currentCursor = vec4(normalize(iCurrentCursor.xy, 1.), normalize(iCurrentCursor.zw, 0.));
+    vec4 previousCursor = vec4(normalize(iPreviousCursor.xy, 1.), normalize(iPreviousCursor.zw, 0.));
+
+    // When drawing a parellelogram between cursors for the trail i need to determine where to start at the top-left or top-right vertex of the cursor
+    float vertexFactor = determineStartVertexFactor(currentCursor.xy, previousCursor.xy);
+    float invertedVertexFactor = 1.0 - vertexFactor;
+
+    // Set every vertex of my parellogram
+    vec2 v0 = vec2(currentCursor.x + currentCursor.z * vertexFactor, currentCursor.y - currentCursor.w);
+    vec2 v1 = vec2(currentCursor.x + currentCursor.z * invertedVertexFactor, currentCursor.y);
+    vec2 v2 = vec2(previousCursor.x + currentCursor.z * invertedVertexFactor, previousCursor.y);
+    vec2 v3 = vec2(previousCursor.x + currentCursor.z * vertexFactor, previousCursor.y - previousCursor.w);
+
+    float sdfCurrentCursor = getSdfRectangle(vu, currentCursor.xy - (currentCursor.zw * offsetFactor), currentCursor.zw * 0.5);
+    float sdfTrail = getSdfParallelogram(vu, v0, v1, v2, v3);
+
+    float progress = clamp((iTime - iTimeCursorChange) / DURATION, 0.0, 1.0);
+    float easedProgress = ease(progress);
+    // Distance between cursors determine the total length of the parallelogram;
+    vec2 centerCC = getRectangleCenter(currentCursor);
+    vec2 centerCP = getRectangleCenter(previousCursor);
+    float lineLength = distance(centerCC, centerCP);
+
+    vec4 newColor = vec4(fragColor);
+    vec4 trailColor = mix(iPreviousCursorColor * 0.9, iCurrentCursorColor * 0.9, easedProgress);
+    trailColor = mix(trailColor, vec4(trailColor.rgb * 0.1, 0), easedProgress);
+
+    newColor = mix(newColor, trailColor, antialising(sdfTrail));
+    // Draw current cursor
+    newColor = mix(newColor, trailColor, antialising(sdfCurrentCursor));
+    newColor = mix(newColor, fragColor, step(sdfCurrentCursor, 0.));
+    fragColor = mix(fragColor, newColor, step(sdfCurrentCursor, easedProgress * lineLength));
+}