From bead9f0b78380d1428f40fdab339c1c8f88fa626 Mon Sep 17 00:00:00 2001
From: Gabriel Schneider <gbrls@0x4200.cafe>
Date: Thu, 2 Jul 2026 20:06:25 -0300
Subject: sdl ui backend

---
 shaders/ui.frag.glsl | 18 ++++++++++++++++++
 shaders/ui.vert.glsl | 44 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 62 insertions(+)
 create mode 100644 shaders/ui.frag.glsl
 create mode 100644 shaders/ui.vert.glsl

(limited to 'shaders')

diff --git a/shaders/ui.frag.glsl b/shaders/ui.frag.glsl
new file mode 100644
index 00000000..2284fa0b
--- /dev/null
+++ b/shaders/ui.frag.glsl
@@ -0,0 +1,18 @@
+#version 450
+
+// Sample the glyph atlas (R = coverage 0..1) and mix the cell's bg up to its fg
+// by that coverage. Fully opaque: the swapchain is SDR, no blending needed.
+
+layout(set = 2, binding = 0) uniform sampler2D u_atlas;
+
+layout(location = 0) in vec2 v_uv;
+layout(location = 1) in vec3 v_fg;
+layout(location = 2) in vec3 v_bg;
+
+layout(location = 0) out vec4 o_col;
+
+void main() {
+    float a = texture(u_atlas, v_uv).r;
+    vec3 col = mix(v_bg, v_fg, a);
+    o_col = vec4(col, 1.0);
+}
diff --git a/shaders/ui.vert.glsl b/shaders/ui.vert.glsl
new file mode 100644
index 00000000..c7567a8f
--- /dev/null
+++ b/shaders/ui.vert.glsl
@@ -0,0 +1,44 @@
+#version 450
+
+// Terminal cell renderer. The CPU uploads one instance per visible cell; this
+// shader expands each instance into a two-triangle quad with gl_VertexIndex.
+// The fragment shader mixes fg/bg by the atlas's alpha (a glyph mask in the R
+// channel), so background fills and glyph foregrounds draw in a single pass.
+
+layout(location = 0) in vec4 a_rect; // x0, y0, x1, y1
+layout(location = 1) in vec4 a_uv_rect; // u0, v0, u1, v1
+layout(location = 2) in vec3 a_fg;
+layout(location = 3) in vec3 a_bg;
+
+layout(location = 0) out vec2 v_uv;
+layout(location = 1) out vec3 v_fg;
+layout(location = 2) out vec3 v_bg;
+
+void main() {
+    vec2 pos;
+    vec2 uv;
+    int corner = gl_VertexIndex % 6;
+    if (corner == 0) {
+        pos = vec2(a_rect.x, a_rect.y);
+        uv = vec2(a_uv_rect.x, a_uv_rect.y);
+    } else if (corner == 1) {
+        pos = vec2(a_rect.z, a_rect.y);
+        uv = vec2(a_uv_rect.z, a_uv_rect.y);
+    } else if (corner == 2) {
+        pos = vec2(a_rect.z, a_rect.w);
+        uv = vec2(a_uv_rect.z, a_uv_rect.w);
+    } else if (corner == 3) {
+        pos = vec2(a_rect.x, a_rect.y);
+        uv = vec2(a_uv_rect.x, a_uv_rect.y);
+    } else if (corner == 4) {
+        pos = vec2(a_rect.z, a_rect.w);
+        uv = vec2(a_uv_rect.z, a_uv_rect.w);
+    } else {
+        pos = vec2(a_rect.x, a_rect.w);
+        uv = vec2(a_uv_rect.x, a_uv_rect.w);
+    }
+    v_uv = uv;
+    v_fg = a_fg;
+    v_bg = a_bg;
+    gl_Position = vec4(pos, 0.0, 1.0);
+}
-- 
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