diff --git a/crates/haku2/src/canvas.zig b/crates/haku2/src/canvas.zig
new file mode 100644
index 0000000..cfd32cb
--- /dev/null
+++ b/crates/haku2/src/canvas.zig
@@ -0,0 +1,48 @@
+const value = @import("value.zig");
+const Value = value.Value;
+
+/// Canvases are opaque handles passed from the host to the VM.
+/// The host creates and manages the canvas, hence no `new` function.
+pub const Canvas = opaque {
+    fn wrap(status: bool) !void {
+        if (!status) return error.Draw;
+    }
+
+    pub fn begin(c: *Canvas) !void {
+        try wrap(__haku2_canvas_begin(c));
+    }
+
+    pub fn line(c: *Canvas, start: value.Vec2, end: value.Vec2) !void {
+        const x1, const y1 = start;
+        const x2, const y2 = end;
+        try wrap(__haku2_canvas_line(c, x1, y1, x2, y2));
+    }
+
+    pub fn rect(c: *Canvas, top_left: value.Vec2, size: value.Vec2) !void {
+        const x, const y = top_left;
+        const width, const height = size;
+        try wrap(__haku2_canvas_rectangle(c, x, y, width, height));
+    }
+
+    pub fn circle(c: *Canvas, center: value.Vec2, r: f32) !void {
+        const x, const y = center;
+        try wrap(__haku2_canvas_circle(c, x, y, r));
+    }
+
+    pub fn fill(c: *Canvas, color: value.Rgba8) !void {
+        const r, const g, const b, const a = color;
+        try wrap(__haku2_canvas_fill(c, r, g, b, a));
+    }
+
+    pub fn stroke(c: *Canvas, color: value.Rgba8, thickness: f32) !void {
+        const r, const g, const b, const a = color;
+        try wrap(__haku2_canvas_stroke(c, r, g, b, a, thickness));
+    }
+};
+
+extern fn __haku2_canvas_begin(c: *Canvas) bool;
+extern fn __haku2_canvas_line(c: *Canvas, x1: f32, y1: f32, x2: f32, y2: f32) bool;
+extern fn __haku2_canvas_rectangle(c: *Canvas, x: f32, y: f32, width: f32, height: f32) bool;
+extern fn __haku2_canvas_circle(c: *Canvas, x: f32, y: f32, r: f32) bool;
+extern fn __haku2_canvas_fill(c: *Canvas, r: u8, g: u8, b: u8, a: u8) bool;
+extern fn __haku2_canvas_stroke(c: *Canvas, r: u8, g: u8, b: u8, a: u8, thickness: f32) bool;
diff --git a/crates/haku2/src/haku2.zig b/crates/haku2/src/haku2.zig
index f201fbd..9c5363a 100644
--- a/crates/haku2/src/haku2.zig
+++ b/crates/haku2/src/haku2.zig
@@ -1,21 +1,24 @@
 const std = @import("std");
 const mem = std.mem;
+const builtin = @import("builtin");
 
 const bytecode = @import("bytecode.zig");
+const Canvas = @import("canvas.zig").Canvas;
+const render = @import("render.zig");
 const Scratch = @import("scratch.zig");
 const value = @import("value.zig");
 const Vm = @import("vm.zig");
 
-const hostAllocator = @import("allocator.zig").hostAllocator;
+const allocator = if (builtin.cpu.arch == .wasm32) std.heap.wasm_allocator else @import("allocator.zig").hostAllocator;
 
 // Scratch
 
 export fn haku2_scratch_new(max: usize) ?*Scratch {
-    return Scratch.create(hostAllocator, max) catch return null;
+    return Scratch.create(allocator, max) catch return null;
 }
 
 export fn haku2_scratch_destroy(scratch: *Scratch) void {
-    scratch.destroy(hostAllocator);
+    scratch.destroy(allocator);
 }
 
 export fn haku2_scratch_reset(scratch: *Scratch) void {
@@ -25,11 +28,11 @@ export fn haku2_scratch_reset(scratch: *Scratch) void {
 // Limits
 
 export fn haku2_limits_new() ?*Vm.Limits {
-    return hostAllocator.create(Vm.Limits) catch null;
+    return allocator.create(Vm.Limits) catch null;
 }
 
 export fn haku2_limits_destroy(limits: *Vm.Limits) void {
-    hostAllocator.destroy(limits);
+    allocator.destroy(limits);
 }
 
 export fn haku2_limits_set_stack_capacity(limits: *Vm.Limits, new: usize) void {
@@ -53,27 +56,31 @@ export fn haku2_defs_parse(
     tags_len: usize,
 ) ?*bytecode.Defs {
     return bytecode.Defs.parse(
-        hostAllocator,
+        allocator,
         defs_string[0..defs_len],
         tags_string[0..tags_len],
     ) catch null;
 }
 
 export fn haku2_defs_destroy(defs: *bytecode.Defs) void {
-    defs.destroy(hostAllocator);
+    defs.destroy(allocator);
 }
 
 // VM
 
 export fn haku2_vm_new(s: *Scratch, defs: *const bytecode.Defs, limits: *const Vm.Limits) ?*Vm {
-    const vm = hostAllocator.create(Vm) catch return null;
-    errdefer hostAllocator.destroy(vm);
+    const vm = allocator.create(Vm) catch return null;
+    errdefer allocator.destroy(vm);
 
     vm.* = Vm.init(s.allocator(), defs, limits) catch return null;
 
     return vm;
 }
 
+export fn haku2_vm_destroy(vm: *Vm) void {
+    allocator.destroy(vm);
+}
+
 export fn haku2_vm_run_main(
     vm: *Vm,
     scratch: *Scratch,
@@ -95,6 +102,46 @@ export fn haku2_vm_run_main(
     return true;
 }
 
-export fn haku2_vm_destroy(vm: *Vm) void {
-    hostAllocator.destroy(vm);
+export fn haku2_vm_is_dotter(vm: *const Vm) bool {
+    if (vm.stack.len == 0) return false;
+    const top = vm.stack[vm.stack_top];
+    return top == .ref and top.ref.* == .reticle and top.ref.reticle == .dotter;
+}
+
+export fn haku2_vm_run_dotter(
+    vm: *Vm,
+    scratch: *Scratch,
+    from_x: f32,
+    from_y: f32,
+    to_x: f32,
+    to_y: f32,
+    num: f32,
+) bool {
+    vm.runDotter(
+        scratch.allocator(),
+        .{ from_x, from_y, 0, 0 },
+        .{ to_x, to_y, 0, 0 },
+        num,
+    ) catch return false;
+    return true;
+}
+
+export fn haku2_vm_exception_len(vm: *const Vm) usize {
+    if (vm.exception) |exn| {
+        return exn.len;
+    } else {
+        return 0;
+    }
+}
+
+export fn haku2_vm_exception_render(vm: *const Vm, buffer: [*]u8) void {
+    const exn = vm.exception.?;
+    _ = exn.format(buffer[0..exn.len], &exn.args);
+}
+
+// Renderer
+
+export fn haku2_render(vm: *Vm, canvas: *Canvas, max_depth: usize) bool {
+    render.render(vm, canvas, max_depth) catch return false;
+    return true;
 }
diff --git a/crates/haku2/src/lib.rs b/crates/haku2/src/lib.rs
index 489559c..f246983 100644
--- a/crates/haku2/src/lib.rs
+++ b/crates/haku2/src/lib.rs
@@ -1,6 +1,9 @@
 use std::{
     alloc::{self, Layout},
-    ptr,
+    error::Error,
+    fmt::{self, Display},
+    marker::{PhantomData, PhantomPinned},
+    ptr::{self, NonNull},
 };
 
 #[unsafe(no_mangle)]
@@ -35,3 +38,415 @@ unsafe extern "C" fn __haku2_dealloc(ptr: *mut u8, size: usize, align: usize) {
         }
     }
 }
+
+#[repr(C)]
+struct ScratchC {
+    _data: (),
+    _marker: PhantomData<(*mut u8, PhantomPinned)>,
+}
+
+#[repr(C)]
+struct LimitsC {
+    _data: (),
+    _marker: PhantomData<(*mut u8, PhantomPinned)>,
+}
+
+#[repr(C)]
+struct DefsC {
+    _data: (),
+    _marker: PhantomData<(*mut u8, PhantomPinned)>,
+}
+
+#[repr(C)]
+struct VmC {
+    _data: (),
+    _marker: PhantomData<(*mut u8, PhantomPinned)>,
+}
+
+extern "C" {
+    fn haku2_scratch_new(max: usize) -> *mut ScratchC;
+    fn haku2_scratch_destroy(scratch: *mut ScratchC);
+    fn haku2_scratch_reset(scratch: *mut ScratchC);
+
+    fn haku2_limits_new() -> *mut LimitsC;
+    fn haku2_limits_destroy(limits: *mut LimitsC);
+    fn haku2_limits_set_stack_capacity(limits: *mut LimitsC, new: usize);
+    fn haku2_limits_set_call_stack_capacity(limits: *mut LimitsC, new: usize);
+    fn haku2_limits_set_fuel(limits: *mut LimitsC, new: u32);
+
+    fn haku2_defs_parse(
+        defs_string: *const u8,
+        defs_len: usize,
+        tags_string: *const u8,
+        tags_len: usize,
+    ) -> *mut DefsC;
+    fn haku2_defs_destroy(defs: *mut DefsC);
+
+    fn haku2_vm_new(s: *mut ScratchC, defs: *const DefsC, limits: *const LimitsC) -> *mut VmC;
+    fn haku2_vm_destroy(vm: *mut VmC);
+    fn haku2_vm_run_main(
+        vm: *mut VmC,
+        scratch: *mut ScratchC,
+        code: *const u8,
+        code_len: usize,
+        local_count: u8,
+    ) -> bool;
+    fn haku2_vm_is_dotter(vm: *const VmC) -> bool;
+    fn haku2_vm_run_dotter(
+        vm: *mut VmC,
+        scratch: *mut ScratchC,
+        from_x: f32,
+        from_y: f32,
+        to_x: f32,
+        to_y: f32,
+        num: f32,
+    ) -> bool;
+    fn haku2_vm_exception_len(vm: *const VmC) -> usize;
+    fn haku2_vm_exception_render(vm: *const VmC, buffer: *mut u8);
+
+    // improper_ctypes is emitted for `*mut CanvasC`, which is an opaque {} on the Zig side and
+    // therefore FFI-safe.
+    #[expect(improper_ctypes)]
+    fn haku2_render(vm: *mut VmC, canvas: *mut CanvasC, max_depth: usize) -> bool;
+}
+
+#[derive(Debug)]
+pub struct Scratch {
+    raw: NonNull<ScratchC>,
+}
+
+impl Scratch {
+    pub fn new(max: usize) -> Scratch {
+        Scratch {
+            // SAFETY: haku2_scratch_new does not have any safety invariants.
+            raw: NonNull::new(unsafe { haku2_scratch_new(max) }).expect("out of memory"),
+        }
+    }
+
+    pub fn reset(&mut self) {
+        // SAFETY: The pointer passed is non-null.
+        unsafe {
+            haku2_scratch_reset(self.raw.as_ptr());
+        }
+    }
+}
+
+impl Drop for Scratch {
+    fn drop(&mut self) {
+        // SAFETY: The pointer passed is non-null.
+        unsafe {
+            haku2_scratch_destroy(self.raw.as_ptr());
+        }
+    }
+}
+
+#[derive(Debug, Clone, Copy)]
+pub struct LimitsSpec {
+    pub stack_capacity: usize,
+    pub call_stack_capacity: usize,
+    pub fuel: u32,
+}
+
+#[derive(Debug)]
+pub struct Limits {
+    raw: NonNull<LimitsC>,
+}
+
+// SAFETY: Limits's backing storage is only modified on creation.
+// Changing the limits requires creating a new instance.
+unsafe impl Send for Limits {}
+unsafe impl Sync for Limits {}
+
+impl Limits {
+    pub fn new(spec: LimitsSpec) -> Self {
+        // SAFETY: haku2_limits_new has no safety invariants.
+        let limits = NonNull::new(unsafe { haku2_limits_new() }).expect("out of memory");
+
+        // SAFETY: The following functions are called on a valid pointer.
+        unsafe {
+            haku2_limits_set_stack_capacity(limits.as_ptr(), spec.stack_capacity);
+            haku2_limits_set_call_stack_capacity(limits.as_ptr(), spec.call_stack_capacity);
+            haku2_limits_set_fuel(limits.as_ptr(), spec.fuel);
+        }
+
+        Self { raw: limits }
+    }
+}
+
+impl Drop for Limits {
+    fn drop(&mut self) {
+        // SAFETY: The pointer passed is non-null.
+        unsafe {
+            haku2_limits_destroy(self.raw.as_ptr());
+        }
+    }
+}
+
+#[derive(Debug)]
+pub struct Defs {
+    raw: NonNull<DefsC>,
+}
+
+// SAFETY: Defs' backing storage is not modified after creation.
+unsafe impl Send for Defs {}
+unsafe impl Sync for Defs {}
+
+impl Defs {
+    pub fn parse(defs: &str, tags: &str) -> Self {
+        Self {
+            raw: NonNull::new(unsafe {
+                haku2_defs_parse(defs.as_ptr(), defs.len(), tags.as_ptr(), tags.len())
+            })
+            .expect("out of memory"),
+        }
+    }
+}
+
+impl Drop for Defs {
+    fn drop(&mut self) {
+        // SAFETY: The pointer passed is non-null.
+        unsafe {
+            haku2_defs_destroy(self.raw.as_ptr());
+        }
+    }
+}
+
+#[derive(Debug)]
+pub struct Vm {
+    scratch: Scratch,
+    raw: NonNull<VmC>,
+}
+
+#[derive(Debug)]
+pub enum Cont<'vm> {
+    None,
+    Dotter(ContDotter<'vm>),
+}
+
+#[derive(Debug)]
+pub struct ContDotter<'vm> {
+    vm: &'vm mut Vm,
+}
+
+#[derive(Debug, Clone, Copy)]
+pub struct Dotter {
+    pub from: (f32, f32),
+    pub to: (f32, f32),
+    pub num: f32,
+}
+
+impl Vm {
+    pub fn new(scratch: Scratch, defs: &Defs, limits: &Limits) -> Self {
+        Self {
+            // SAFETY:
+            // - Ownership of s is passed to the VM, so the VM cannot outlive the scratch space.
+            //   - The VM never gives you any references back, so this is safe to do.
+            // - The other arguments are only borrowed immutably for construction.
+            raw: NonNull::new(unsafe {
+                haku2_vm_new(scratch.raw.as_ptr(), defs.raw.as_ptr(), limits.raw.as_ptr())
+            })
+            .expect("out of memory"),
+            scratch,
+        }
+    }
+
+    /// Begin running code. This makes the VM enter a "trampoline" state: after this call, you may
+    /// proceed to call `cont` as many times as it returns a value other than [`Cont::None`].
+    ///
+    /// Calling `begin` again during this process will work correctly, and result in another
+    /// continuation being stack on top of the old one---at the expense of a stack slot.
+    ///
+    /// # Safety
+    ///
+    /// The bytecode passed in must be valid, because bytecode validation is done on a best-effort
+    /// basis. Bytecode retrieved out of the compiler is guaranteed to be safe.
+    pub unsafe fn begin(&mut self, code: &[u8], local_count: u8) -> Result<(), Exception> {
+        let ok = unsafe {
+            haku2_vm_run_main(
+                self.raw.as_ptr(),
+                self.scratch.raw.as_ptr(),
+                code.as_ptr(),
+                code.len(),
+                local_count,
+            )
+        };
+        if ok {
+            Ok(())
+        } else {
+            Err(self.exception().expect("missing exception after !ok"))
+        }
+    }
+
+    fn is_dotter(&self) -> bool {
+        // SAFETY: The pointer is valid.
+        unsafe { haku2_vm_is_dotter(self.raw.as_ptr()) }
+    }
+
+    /// Returns how the VM should continue executing after the previous execution.
+    pub fn cont(&mut self) -> Cont<'_> {
+        match () {
+            _ if self.is_dotter() => Cont::Dotter(ContDotter { vm: self }),
+            _ => Cont::None,
+        }
+    }
+
+    /// Renders the current scribble on top of the stack.
+    /// If the value on top is not a scribble, throws an exception (indicated by the return type.)
+    ///
+    /// The rendering is performed by calling into the [`Canvas`] trait.
+    pub fn render(&mut self, canvas: &mut dyn Canvas, max_depth: usize) -> Result<(), Exception> {
+        let mut wrapped = CanvasC { inner: canvas };
+        let ok = unsafe { haku2_render(self.raw.as_ptr(), &mut wrapped, max_depth) };
+        if ok {
+            Ok(())
+        } else {
+            Err(self.exception().expect("missing exception after !ok"))
+        }
+    }
+
+    /// Render the current exception out to a string.
+    /// Returns `None` if there's no exception.
+    pub fn exception(&self) -> Option<Exception> {
+        // SAFETY: The pointer passed to this function is valid.
+        let len = unsafe { haku2_vm_exception_len(self.raw.as_ptr()) };
+        if len == 0 {
+            return None;
+        }
+
+        let mut buffer = vec![0; len];
+        // SAFETY: The length of the buffer is as indicated by haku2_vm_exception_len.
+        unsafe {
+            haku2_vm_exception_render(self.raw.as_ptr(), buffer.as_mut_ptr());
+        }
+        Some(Exception {
+            message: String::from_utf8_lossy(&buffer).into_owned(),
+        })
+    }
+}
+
+impl ContDotter<'_> {
+    pub fn run(self, dotter: &Dotter) -> Result<(), Exception> {
+        let Dotter {
+            from: (from_x, from_y),
+            to: (to_x, to_y),
+            num,
+        } = *dotter;
+
+        let ok = unsafe {
+            haku2_vm_run_dotter(
+                self.vm.raw.as_ptr(),
+                self.vm.scratch.raw.as_ptr(),
+                from_x,
+                from_y,
+                to_x,
+                to_y,
+                num,
+            )
+        };
+        if ok {
+            Ok(())
+        } else {
+            Err(self.vm.exception().expect("missing exception after !ok"))
+        }
+    }
+}
+
+impl Drop for Vm {
+    fn drop(&mut self) {
+        // SAFETY: The pointer passed is non-null.
+        unsafe {
+            haku2_vm_destroy(self.raw.as_ptr());
+        }
+    }
+}
+
+#[derive(Debug)]
+pub struct Exception {
+    message: String,
+}
+
+impl Display for Exception {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", self.message)
+    }
+}
+
+impl Error for Exception {}
+
+/// Marker for the VM to indicate that the rendering did not go down correctly.
+/// If this is encountered, it throws an exception and aborts rendering.
+#[derive(Debug)]
+pub struct RenderError;
+
+pub trait Canvas {
+    fn begin(&mut self) -> Result<(), RenderError>;
+    fn line(&mut self, x1: f32, y1: f32, x2: f32, y2: f32) -> Result<(), RenderError>;
+    fn rectangle(&mut self, x: f32, y: f32, width: f32, height: f32) -> Result<(), RenderError>;
+    fn circle(&mut self, x: f32, y: f32, r: f32) -> Result<(), RenderError>;
+    fn fill(&mut self, r: u8, g: u8, b: u8, a: u8) -> Result<(), RenderError>;
+    fn stroke(&mut self, r: u8, g: u8, b: u8, a: u8, thickness: f32) -> Result<(), RenderError>;
+}
+
+// SAFETY NOTE: I'm not sure the ownership model for this is quite correct.
+// Given how the &mut's ownership flows through the Zig side of the code, it _should_ be fine,
+// but I'm not an unsafe code expert to say this is the case for sure.
+
+#[repr(C)]
+struct CanvasC<'a> {
+    inner: &'a mut dyn Canvas,
+}
+
+#[unsafe(no_mangle)]
+unsafe extern "C" fn __haku2_canvas_begin(c: *mut CanvasC) -> bool {
+    let c = &mut *c;
+    c.inner.begin().is_ok()
+}
+
+#[unsafe(no_mangle)]
+unsafe extern "C" fn __haku2_canvas_line(
+    c: *mut CanvasC,
+    x1: f32,
+    y1: f32,
+    x2: f32,
+    y2: f32,
+) -> bool {
+    let c = &mut *c;
+    c.inner.line(x1, y1, x2, y2).is_ok()
+}
+
+#[unsafe(no_mangle)]
+unsafe extern "C" fn __haku2_canvas_rectangle(
+    c: *mut CanvasC,
+    x: f32,
+    y: f32,
+    width: f32,
+    height: f32,
+) -> bool {
+    let c = &mut *c;
+    c.inner.rectangle(x, y, width, height).is_ok()
+}
+
+#[unsafe(no_mangle)]
+unsafe extern "C" fn __haku2_canvas_circle(c: *mut CanvasC, x: f32, y: f32, r: f32) -> bool {
+    let c = &mut *c;
+    c.inner.circle(x, y, r).is_ok()
+}
+
+#[unsafe(no_mangle)]
+unsafe extern "C" fn __haku2_canvas_fill(c: *mut CanvasC, r: u8, g: u8, b: u8, a: u8) -> bool {
+    let c = &mut *c;
+    c.inner.fill(r, g, b, a).is_ok()
+}
+
+#[unsafe(no_mangle)]
+unsafe extern "C" fn __haku2_canvas_stroke(
+    c: *mut CanvasC,
+    r: u8,
+    g: u8,
+    b: u8,
+    a: u8,
+    thickness: f32,
+) -> bool {
+    let c = &mut *c;
+    c.inner.stroke(r, g, b, a, thickness).is_ok()
+}
diff --git a/crates/haku2/src/render.zig b/crates/haku2/src/render.zig
new file mode 100644
index 0000000..72ce279
--- /dev/null
+++ b/crates/haku2/src/render.zig
@@ -0,0 +1,59 @@
+const std = @import("std");
+
+const Canvas = @import("canvas.zig").Canvas;
+const value = @import("value.zig");
+const Value = value.Value;
+const Vm = @import("vm.zig");
+
+fn notAScribble(vm: *Vm, val: Value) Vm.Error {
+    return vm.throw(
+        "the brush returned a {s}, which cannot be drawn. return a scribble (e.g. fill, stroke, list) instead",
+        .{val.typeName()},
+    );
+}
+
+fn renderRec(vm: *Vm, canvas: *Canvas, val: Value, depth: usize, max_depth: usize) !void {
+    if (depth > max_depth) {
+        return vm.throw(
+            "the brush returned a scribble that's nested too deep ({} levels). try generating lists that aren't as deep using the (map (range min max) f) idiom, or flatten your lists using the flatten function",
+            .{max_depth},
+        );
+    }
+    if (val != .ref) return notAScribble(vm, val);
+
+    switch (val.ref.*) {
+        .scribble => {
+            try canvas.begin();
+
+            switch (val.ref.scribble.shape) {
+                .point => |point| try canvas.line(point, point),
+                .line => |line| try canvas.line(line.start, line.end),
+                .rect => |rect| try canvas.rect(rect.top_left, rect.size),
+                .circle => |circle| try canvas.circle(circle.center, circle.radius),
+            }
+
+            switch (val.ref.scribble.action) {
+                .stroke => |stroke| try canvas.stroke(value.rgbaTo8(stroke.color), stroke.thickness),
+                .fill => |fill| try canvas.fill(value.rgbaTo8(fill.color)),
+            }
+        },
+
+        .list => {
+            for (val.ref.list) |nested| {
+                try vm.consumeFuel(&vm.fuel, 1);
+                try renderRec(vm, canvas, nested, depth + 1, max_depth);
+            }
+        },
+
+        .shape => {
+            return vm.throw("the brush returned a bare shape, which cannot be drawn. try wrapping your shape in a fill or a stroke: (fill #000 <shape>)", .{});
+        },
+
+        else => return notAScribble(vm, val),
+    }
+}
+
+pub fn render(vm: *Vm, canvas: *Canvas, max_depth: usize) !void {
+    const val = try vm.pop();
+    try renderRec(vm, canvas, val, 0, max_depth);
+}
diff --git a/crates/haku2/src/system.zig b/crates/haku2/src/system.zig
index 3df802f..925afdb 100644
--- a/crates/haku2/src/system.zig
+++ b/crates/haku2/src/system.zig
@@ -3,10 +3,50 @@ const mem = std.mem;
 const meta = std.meta;
 const math = std.math;
 
+const bytecode = @import("bytecode.zig");
+const Opcode = bytecode.Opcode;
 const value = @import("value.zig");
 const Value = value.Value;
 const Vm = @import("vm.zig");
 
+fn recordBytecodeSize(fields: []const value.TagId) usize {
+    var size: usize = 0;
+
+    size += 1; // Opcode.field
+    size += 1; // count: u8
+    size += 2 * fields.len; // tags: [count]u16
+    size += 1; // Opcode.return
+
+    return size;
+}
+
+fn recordBytecode(comptime fields: []const value.TagId) [recordBytecodeSize(fields)]u8 {
+    if (fields.len > 255) @compileError("too many fields");
+
+    var code = [_]u8{undefined} ** recordBytecodeSize(fields);
+    var cursor: usize = 0;
+
+    code[cursor] = @intFromEnum(Opcode.field);
+    cursor += 1;
+    code[cursor] = @as(u8, @truncate(fields.len));
+    cursor += 1;
+
+    for (fields) |field| {
+        const tag_id = mem.toBytes(field);
+        code[cursor] = tag_id[0];
+        code[cursor + 1] = tag_id[1];
+        cursor += 2;
+    }
+
+    code[cursor] = @intFromEnum(Opcode.ret);
+    cursor += 1;
+
+    return code;
+}
+
+const record_dotter_bytecode = recordBytecode(&.{ .From, .To, .Num });
+pub const record_dotter: bytecode.Chunk = .{ .bytecode = &record_dotter_bytecode };
+
 pub const Context = struct {
     vm: *Vm,
     allocator: mem.Allocator,
@@ -692,18 +732,22 @@ fn circle(center: Vec4, radius: f32) value.Ref {
 }
 
 fn stroke(thickness: f32, color: Rgba, shape: *const value.Shape) value.Ref {
-    return .{ .scribble = .{ .stroke = .{
-        .thickness = thickness,
-        .color = color.value,
+    return .{ .scribble = .{
         .shape = shape.*,
-    } } };
+        .action = .{ .stroke = .{
+            .thickness = thickness,
+            .color = color.value,
+        } },
+    } };
 }
 
 fn fill(color: Rgba, shape: *const value.Shape) value.Ref {
-    return .{ .scribble = .{ .fill = .{
-        .color = color.value,
+    return .{ .scribble = .{
         .shape = shape.*,
-    } } };
+        .action = .{ .fill = .{
+            .color = color.value,
+        } },
+    } };
 }
 
 fn withDotter(cont: *const value.Closure, vm: *Vm) Vm.Error!value.Ref {
diff --git a/crates/haku2/src/value.zig b/crates/haku2/src/value.zig
index ec700d6..51a6e76 100644
--- a/crates/haku2/src/value.zig
+++ b/crates/haku2/src/value.zig
@@ -1,6 +1,7 @@
 const std = @import("std");
 const meta = std.meta;
 const mem = std.mem;
+const math = std.math;
 
 const bytecode = @import("bytecode.zig");
 
@@ -88,7 +89,12 @@ pub const Rgba8 = @Vector(4, u8);
 pub const Rgba = @Vector(4, f32);
 
 pub fn rgbaFrom8(rgba: Rgba8) Rgba {
-    return @as(Rgba, @floatFromInt(rgba)) / @as(Rgba, @splat(255.0));
+    return @as(Rgba, @floatFromInt(rgba)) / @as(Rgba, @splat(255));
+}
+
+pub fn rgbaTo8(rgba: Rgba) Rgba8 {
+    const clamped = math.clamp(rgba, @as(Rgba, @splat(0)), @as(Rgba, @splat(1)));
+    return @as(Rgba8, @intFromFloat(clamped * @as(Rgba, @splat(255))));
 }
 
 pub const Ref = union(enum) {
@@ -131,19 +137,22 @@ pub const Shape = union(enum) {
     };
 };
 
-pub const Scribble = union(enum) {
-    stroke: Stroke,
-    fill: Fill,
+pub const Scribble = struct {
+    shape: Shape,
+    action: Action,
 
-    pub const Stroke = struct {
-        thickness: f32,
-        color: Rgba,
-        shape: Shape,
-    };
+    pub const Action = union(enum) {
+        stroke: Stroke,
+        fill: Fill,
 
-    pub const Fill = struct {
-        color: Rgba,
-        shape: Shape,
+        pub const Stroke = struct {
+            thickness: f32,
+            color: Rgba,
+        };
+
+        pub const Fill = struct {
+            color: Rgba,
+        };
     };
 };
 
diff --git a/crates/haku2/src/vm.zig b/crates/haku2/src/vm.zig
index 96eb3cd..42556fa 100644
--- a/crates/haku2/src/vm.zig
+++ b/crates/haku2/src/vm.zig
@@ -1,8 +1,10 @@
 const std = @import("std");
 const mem = std.mem;
+const debug = std.debug;
 const testAllocator = std.testing.allocator;
 
 const bytecode = @import("bytecode.zig");
+const Canvas = @import("canvas.zig");
 const system = @import("system.zig");
 const value = @import("value.zig");
 const Value = value.Value;
@@ -15,7 +17,6 @@ call_stack: []CallFrame,
 call_stack_top: u32 = 0,
 defs: []Value,
 fuel: u32,
-exception_buffer: [1024]u8 = [_]u8{0} ** 1024, // buffer for exception message
 exception: ?Exception = null,
 
 pub const Limits = struct {
@@ -31,7 +32,9 @@ pub const CallFrame = struct {
 };
 
 pub const Exception = struct {
-    message: []const u8,
+    len: usize,
+    format: *const fn (buf: []u8, args: *align(64) const anyopaque) []u8,
+    args: [40]u8 align(64), // increase the size if we ever throw a larger exception
 };
 
 /// All errors coming from inside the VM get turned into a single Exception type, which signals
@@ -51,11 +54,28 @@ pub fn init(a: mem.Allocator, defs: *const bytecode.Defs, limits: *const Limits)
 }
 
 pub fn throw(vm: *Vm, comptime fmt: []const u8, args: anytype) Error {
-    const message = std.fmt.bufPrint(vm.exception_buffer[0..], fmt, args) catch {
-        vm.exception = .{ .message = "[exception message is too long; format string: " ++ fmt ++ "]" };
-        return error.Exception;
+    const Args = @TypeOf(args);
+    const max_args_size = @sizeOf(@TypeOf(vm.exception.?.args));
+    if (@sizeOf(Args) > max_args_size) {
+        @compileError(std.fmt.comptimePrint(
+            "format arguments are too large; size={}, max={}",
+            .{ @sizeOf(Args), max_args_size },
+        ));
+    }
+
+    const Formatter = struct {
+        fn format(buf: []u8, erased_args: *align(64) const anyopaque) []u8 {
+            return std.fmt.bufPrint(buf, fmt, @as(*const Args, @ptrCast(erased_args)).*) catch unreachable;
+        }
     };
-    vm.exception = .{ .message = message };
+
+    var exn = Exception{
+        .len = @truncate(std.fmt.count(fmt, args)),
+        .format = Formatter.format,
+        .args = undefined,
+    };
+    @memcpy(exn.args[0..@sizeOf(Args)], mem.asBytes(&args));
+
     return error.Exception;
 }
 
@@ -452,7 +472,43 @@ pub fn run(
             try vm.consumeFuel(&fuel, 1);
             // NOTE: Not a validateBytecode call because this is zero-cost on the happy path,
             // so we don't need to disable it on release builds.
-            return vm.throw("corrupted bytecode (invalid opcode {})", .{invalid_opcode});
+            return vm.throw("corrupted bytecode: invalid opcode {}", .{invalid_opcode});
         },
     }
 }
+
+pub const Dotter = struct {
+    from: value.Vec4,
+    to: value.Vec4,
+    num: f32,
+};
+
+/// NOTE: Assumes the value at the top is a dotter reticle.
+pub fn runDotter(
+    vm: *Vm,
+    allocator: mem.Allocator,
+    from: value.Vec4,
+    to: value.Vec4,
+    num: f32,
+) Error!void {
+    const reticle = try vm.pop();
+    const draw = reticle.ref.reticle.dotter.draw; // parameter count checked on construction
+
+    const data = allocator.dupe(Value, &[_]Value{
+        .{ .vec4 = from },
+        .{ .vec4 = to },
+        .{ .number = num },
+    }) catch return vm.outOfMemory();
+    const ref = allocator.create(value.Ref) catch return vm.outOfMemory();
+    ref.* = value.Ref{ .closure = .{
+        .chunk = &system.record_dotter,
+        .start = 0,
+        .param_count = 1,
+        .local_count = 0,
+        .captures = data,
+    } };
+
+    const bottom = vm.stack_top;
+    try vm.push(.{ .ref = ref });
+    try vm.run(allocator, draw, bottom);
+}