Add support for drawing undercurls

Fixes #1628.

1 files changed, 44 insertions, 1 deletions

diff --git a/alacritty/res/rect.f.glsl b/alacritty/res/rect.f.glsl
index 945eaf2d..00c2de83 100644
--- a/alacritty/res/rect.f.glsl
+++ b/alacritty/res/rect.f.glsl
@@ -1,10 +1,53 @@
 #version 330 core
 
+// We're using `origin_upper_left`, since we only known about padding from left
+// and top. If we use default `origin_bottom_left` we won't be able to offset
+// `gl_FragCoord` properly to align with the terminal grid.
+layout(origin_upper_left) in vec4 gl_FragCoord;
+
 flat in vec4 color;
 
 out vec4 FragColor;
 
+uniform int isUndercurl;
+
+uniform float cellWidth;
+uniform float cellHeight;
+uniform float paddingY;
+uniform float paddingX;
+
+uniform float undercurlThickness;
+uniform float undercurlPosition;
+
+#define PI 3.1415926538
+
 void main()
 {
-    FragColor = color;
+    if (isUndercurl == 0) {
+        FragColor = color;
+        return;
+    }
+
+    int x = int(gl_FragCoord.x - paddingX) % int(cellWidth);
+    int y = int(gl_FragCoord.y - paddingY) % int(cellHeight);
+
+    // We use `undercurlPosition` as amplitude, since it's half of the descent
+    // value.
+    float undercurl = -1. * undercurlPosition / 2.
+        * cos(float(x) * 2 * PI / float(cellWidth))
+        + cellHeight - undercurlPosition;
+
+    float undercurl_top = undercurl + undercurlThickness / 2.;
+    float undercurl_bottom = undercurl - undercurlThickness / 2.;
+
+
+    // Compute resulted alpha based on distance from `gl_FragCoord.y` to the
+    // cosine curve.
+    float alpha = 1.;
+    if (y > undercurl_top || y < undercurl_bottom) {
+        alpha = 1. - min(abs(undercurl_top - y), abs(undercurl_bottom - y));
+    }
+
+    // The result is an alpha mask on a rect, which leaves only curve opaque.
+    FragColor = vec4(color.xyz, alpha);
 }