From 8f19a9ce9789e3dd16327ab08a27b2d7a54915da Mon Sep 17 00:00:00 2001
From: Rick Choi <jinhwanlazy@gmail.com>
Date: Sun, 27 Oct 2024 13:55:53 +0900
Subject: [PATCH] fade out cursor trail where cursor is hidden

---
 kitty/cursor_trail.c      | 15 +++++++++++++++
 kitty/shaders.c           |  5 ++++-
 kitty/state.h             |  1 +
 kitty/trail_fragment.glsl |  3 ++-
 4 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/kitty/cursor_trail.c b/kitty/cursor_trail.c
index 21cb3a1c2..8f4474c3d 100644
--- a/kitty/cursor_trail.c
+++ b/kitty/cursor_trail.c
@@ -123,6 +123,21 @@ update_cursor_trail_state(CursorTrail *ct, Window *w, monotonic_t now, OSWindow
         }
     }
 
+
+    const bool cursor_trail_always_visible = false;
+    if (cursor_trail_always_visible) {
+        log_error("A: cursor trail always visible");
+        ct->opacity = 1.0f;
+    } else if (WD.screen->modes.mDECTCEM) {
+        log_error("B: cursor is visible");
+      ct->opacity += monotonic_t_to_s_double(now - ct->updated_at) / OPT(cursor_trail_decay_slow);
+      ct->opacity = fminf(ct->opacity, 1.0f);
+    } else {
+        log_error("C: cursor is invisible");
+      ct->opacity -= monotonic_t_to_s_double(now - ct->updated_at) / OPT(cursor_trail_decay_slow);
+      ct->opacity = fmaxf(ct->opacity, 0.0f);
+    }
+
     ct->updated_at = now;
     ct->needs_render = false;
 
diff --git a/kitty/shaders.c b/kitty/shaders.c
index f953fe71b..e58f781fd 100644
--- a/kitty/shaders.c
+++ b/kitty/shaders.c
@@ -1149,6 +1149,8 @@ init_trail_program(void) {
 void
 draw_cursor_trail(CursorTrail *trail, Window *active_window) {
     bind_program(TRAIL_PROGRAM);
+    glEnable(GL_BLEND);
+    BLEND_ONTO_OPAQUE;
 
     glUniform4fv(trail_program_layout.uniforms.x_coords, 1, trail->corner_x);
     glUniform4fv(trail_program_layout.uniforms.y_coords, 1, trail->corner_y);
@@ -1157,9 +1159,10 @@ draw_cursor_trail(CursorTrail *trail, Window *active_window) {
     glUniform2fv(trail_program_layout.uniforms.cursor_edge_y, 1, trail->cursor_edge_y);
 
     color_vec3(trail_program_layout.uniforms.trail_color, active_window ? active_window->render_data.screen->last_rendered.cursor_bg : OPT(foreground));
+    glUniform1fv(trail_program_layout.uniforms.trail_opacity, 1, &trail->opacity);
 
     glDrawArrays(GL_TRIANGLE_FAN, 0, 4);
-
+    glDisable(GL_BLEND);
     unbind_program();
 }
 
diff --git a/kitty/state.h b/kitty/state.h
index 4c5a6d6c1..56d9cd9b9 100644
--- a/kitty/state.h
+++ b/kitty/state.h
@@ -219,6 +219,7 @@ typedef struct {
 typedef struct {
     bool needs_render;
     monotonic_t updated_at;
+    float opacity;
     float corner_x[4];
     float corner_y[4];
     float cursor_edge_x[2];
diff --git a/kitty/trail_fragment.glsl b/kitty/trail_fragment.glsl
index b9dadb866..664dd6666 100644
--- a/kitty/trail_fragment.glsl
+++ b/kitty/trail_fragment.glsl
@@ -1,6 +1,7 @@
 uniform vec2 cursor_edge_x;
 uniform vec2 cursor_edge_y;
 uniform vec3 trail_color;
+uniform float trail_opacity;
 
 in vec2 frag_pos;
 out vec4 final_color;
@@ -10,6 +11,6 @@ void main() {
         cursor_edge_y[1] <= frag_pos.y && frag_pos.y <= cursor_edge_y[0]) {
         discard;
     } else {
-        final_color = vec4(trail_color, 1.0);
+        final_color = vec4(trail_color, trail_opacity);
     }
 }