1 files changed, 43 insertions, 0 deletions
diff --git a/src/crt.zig b/src/crt.zig
new file mode 100644
index 00000000..9cf81593
--- /dev/null
+++ b/src/crt.zig
@@ -0,0 +1,43 @@
+//! The Crt builtin's screen geometry, shared between the gui shell's input
+//! mapping and its shader: crt.frag.glsl shows the scene point
+//! uv' = cc*(1 + barrel*r2) at screen point cc, so a physical mouse position
+//! must pass through the SAME forward transform before it becomes a cell.
+//! Pure std, so the unit-test step can pin it against the shader formula.
+
+const std = @import("std");
+
+/// MUST match BARREL in shaders/crt.frag.glsl
+pub const barrel: f32 = 0.012;
+
+pub const Point = struct { x: f32, y: f32 };
+
+/// physical window pixels -> the scene pixels the shader displays there,
+/// clamped to the window. The shader's phosphor quantization and chromatic
+/// offset displace at most a few pixels — well under a cell — and are
+/// ignored here.
+pub fn map(x: f32, y: f32, w: f32, h: f32) Point {
+    const ccx = x / w * 2.0 - 1.0;
+    const ccy = y / h * 2.0 - 1.0;
+    const s = 1.0 + barrel * (ccx * ccx + ccy * ccy);
+    return .{
+        .x = std.math.clamp((ccx * s + 1.0) * 0.5 * w, 0, w - 1),
+        .y = std.math.clamp((ccy * s + 1.0) * 0.5 * h, 0, h - 1),
+    };
+}
+
+test "map matches the shader's barrel formula" {
+    // center is a fixed point
+    try std.testing.expectApproxEqAbs(@as(f32, 500), map(500, 250, 1000, 500).x, 0.001);
+    try std.testing.expectApproxEqAbs(@as(f32, 250), map(500, 250, 1000, 500).y, 0.001);
+    // corners: cc=(±1,±1), r2=2, scale 1.024 pushes past the tube -> clamped
+    try std.testing.expectEqual(@as(f32, 0), map(0, 0, 1000, 500).x);
+    try std.testing.expectEqual(@as(f32, 0), map(0, 0, 1000, 500).y);
+    try std.testing.expectEqual(@as(f32, 999), map(1000, 500, 1000, 500).x);
+    try std.testing.expectEqual(@as(f32, 499), map(1000, 500, 1000, 500).y);
+    // (100,100) of 1000x500: cc=(-0.8,-0.6), r2=1.0, s=1.012 ->
+    // cc'=(-0.8096,-0.6072) -> (95.2, 98.2)
+    try std.testing.expectApproxEqAbs(@as(f32, 95.2), map(100, 100, 1000, 500).x, 0.01);
+    try std.testing.expectApproxEqAbs(@as(f32, 98.2), map(100, 100, 1000, 500).y, 0.01);
+    // (900,250): cc=(0.8,0), r2=0.64, s=1.00768 -> x'=903.072
+    try std.testing.expectApproxEqAbs(@as(f32, 903.072), map(900, 250, 1000, 500).x, 0.01);
+}