From 3b0b1df9c3ffda4da543ef92611528393ecec4bd Mon Sep 17 00:00:00 2001
From: Kovid Goyal <kovid@kovidgoyal.net>
Date: Wed, 15 Jan 2025 15:05:03 +0530
Subject: [PATCH] Use textFetch() for underline exclusion sampling

Fixes sampling from non-bottom row
---
 kitty/cell_fragment.glsl |  6 ++++--
 kitty/cell_vertex.glsl   | 10 +++++-----
 2 files changed, 9 insertions(+), 7 deletions(-)

diff --git a/kitty/cell_fragment.glsl b/kitty/cell_fragment.glsl
index dc6a19e45..7d0ef3dc8 100644
--- a/kitty/cell_fragment.glsl
+++ b/kitty/cell_fragment.glsl
@@ -15,7 +15,7 @@ in vec3 sprite_pos;
 in vec3 underline_pos;
 in vec3 cursor_pos;
 in vec3 strike_pos;
-in vec3 underline_exclusion_pos;
+flat in uint underline_exclusion_pos;
 in vec3 foreground;
 in vec4 cursor_color_premult;
 in vec3 decoration_fg;
@@ -128,8 +128,10 @@ vec4 load_text_foreground_color() {
 
 vec4 calculate_premul_foreground_from_sprites(vec4 text_fg) {
     // Return premul foreground color from decorations (cursor, underline, strikethrough)
+    ivec3 sz = textureSize(sprites, 0);
     float underline_alpha = texture(sprites, underline_pos).a;
-    float underline_exclusion = texture(sprites, underline_exclusion_pos).a;
+    float underline_exclusion = texelFetch(sprites, ivec3(int(
+        sprite_pos.x * float(sz.x)), int(underline_exclusion_pos), int(sprite_pos.z)), 0).a;
     underline_alpha *= 1.0f - underline_exclusion;
     float strike_alpha = texture(sprites, strike_pos).a;
     float cursor_alpha = texture(sprites, cursor_pos).a;
diff --git a/kitty/cell_vertex.glsl b/kitty/cell_vertex.glsl
index a567119e5..12eb28f27 100644
--- a/kitty/cell_vertex.glsl
+++ b/kitty/cell_vertex.glsl
@@ -52,7 +52,7 @@ out vec3 underline_pos;
 out vec3 cursor_pos;
 out vec4 cursor_color_premult;
 out vec3 strike_pos;
-out vec3 underline_exclusion_pos;
+flat out uint underline_exclusion_pos;
 out vec3 foreground;
 out vec3 decoration_fg;
 out float colored_sprite;
@@ -121,10 +121,10 @@ vec3 to_sprite_pos(uvec2 pos, uint idx) {
     return vec3(s_xpos[pos.x], s_ypos[pos.y], c.z);
 }
 
-vec3 to_underline_exclusion_pos(uvec2 pos, vec3 sprite_pos) {
+uint to_underline_exclusion_pos() {
     uvec3 c = to_sprite_coords(sprite_idx[0]);
-    float y = (float(c.y) + 1.0f) * (1.0f / float(sprites_ynum));
-    return vec3(sprite_pos.x, y, sprite_pos.z);
+    uint cell_top_px = c.y * (cell_height + 1u);
+    return cell_top_px + cell_height;
 }
 
 uint read_sprite_decorations_idx() {
@@ -240,7 +240,7 @@ void main() {
     uvec2 decs = get_decorations_indices(uint(in_url), text_attrs);
     strike_pos = to_sprite_pos(cell_data.pos, decs[0]);
     underline_pos = to_sprite_pos(cell_data.pos, decs[1]);
-    underline_exclusion_pos = to_underline_exclusion_pos(cell_data.pos, sprite_pos);
+    underline_exclusion_pos = to_underline_exclusion_pos();
 
     // Cursor
     cursor_color_premult = vec4(color_to_vec(cursor_bg) * cursor_opacity, cursor_opacity);