Agent Skills: Makepad 2.0 Shader Skill

Makepad 2.0 Shader Skill

Version: makepad-widgets (dev branch) | Last Updated: 2026-03-03

Overview

Makepad uses a custom GPU shader system integrated into the widget property tree. Shaders are defined inline using pixel: fn() { ... } and vertex: fn() { ... } blocks within draw_bg, draw_text, or custom draw objects.

Documentation

Refer to the local files for detailed documentation:

• ./references/shader-reference.md - Shader syntax, variables, built-ins, custom functions
• ./references/sdf2d-reference.md - SDF2D primitives, combinators, drawing operations

Shader Basics

Pixel Shader Structure

draw_bg +: {
    // Declare variables
    instance hover: 0.0          // Animatable per-instance
    uniform accent: #4488ff      // Shared across all instances

    pixel: fn() {
        let sdf = Sdf2d.viewport(self.pos * self.rect_size)
        // ... SDF operations ...
        return sdf.result
    }
}

Variable Types

| Type | Declaration | Animatable | Scope | |------|-------------|-----------|-------| | instance | instance hover: 0.0 | Yes (via Animator) | Per-widget instance | | uniform | uniform color: #fff | No | Shared across instances | | texture_2d | texture_2d tex: none | No | Texture sampler | | varying | varying uv: vec2 | No | Vertex → fragment |

Built-in Variables

| Variable | Type | Description | |----------|------|-------------| | self.pos | vec2 | Normalized position (0.0 to 1.0) | | self.rect_size | vec2 | Widget size in pixels | | self.dpi_factor | float | Screen DPI factor | | self.draw_pass.time | float | Time in seconds |

CRITICAL: Premultiply Alpha

Every pixel shader MUST return premultiplied alpha color!

// WRONG - non-premultiplied
pixel: fn() {
    return vec4(1.0, 0.0, 0.0, 0.5)
}

// CORRECT - use Pal.premul()
pixel: fn() {
    return Pal.premul(vec4(1.0, 0.0, 0.0, 0.5))
}

// ALSO CORRECT - sdf.result is already premultiplied
pixel: fn() {
    let sdf = Sdf2d.viewport(self.pos * self.rect_size)
    sdf.circle(cx, cy, r)
    sdf.fill(#f00)
    return sdf.result
}

SDF2D Quick Reference

Setup

let sdf = Sdf2d.viewport(self.pos * self.rect_size)

Primitives

sdf.circle(cx, cy, radius)
sdf.rect(x, y, w, h)
sdf.box(x, y, w, h, border_radius)
sdf.hexagon(cx, cy, radius)
sdf.arc(cx, cy, radius, start_angle, end_angle, thickness)
sdf.move_to(x, y)
sdf.line_to(x, y)
sdf.close_path()

Drawing

sdf.fill(color)         // Filled shape
sdf.stroke(color, width) // Outlined shape
sdf.glow(color, amount)  // Glow effect
sdf.clear(color)         // Clear with color

Combinators

sdf.union()       // Add shapes together
sdf.intersect()   // Keep overlap only
sdf.subtract()    // Remove second from first
sdf.gloop(radius) // Smooth union
sdf.blend(amount) // Linear blend

Transforms

sdf.translate(x, y)
sdf.rotate(angle, cx, cy)
sdf.scale(factor, cx, cy)

Color Operations

// Mix two colors
mix(#f00, #00f, 0.5)           // 50% blend

// Premultiply alpha
Pal.premul(vec4(r, g, b, a))

// HSV conversions
Pal.hsv2rgb(vec4(h, s, v, 1.0))
Pal.rgb2hsv(color)

// Random
Math.random_2d(vec2(x, y))

Common Shader Patterns

Gradient Background

draw_bg +: {
    pixel: fn() {
        let grad = mix(#1a1a2e, #16213e, self.pos.y)
        return Pal.premul(vec4(grad.xyz, 1.0))
    }
}

Hover Color Change

draw_bg +: {
    instance hover: 0.0
    color: #333
    pixel: fn() {
        return Pal.premul(mix(self.color, self.color * 1.3, self.hover))
    }
}

Box Shadow

draw_bg +: {
    pixel: fn() {
        let sdf = Sdf2d.viewport(self.pos * self.rect_size)
        // Shadow
        sdf.box(2.0, 2.0, self.rect_size.x - 4.0, self.rect_size.y - 4.0, 8.0)
        sdf.fill(GaussShadow.box_shadow(sdf, 4.0, #0005))
        // Card
        sdf.box(0.0, 0.0, self.rect_size.x - 2.0, self.rect_size.y - 2.0, 8.0)
        sdf.fill(#2a2a3d)
        return sdf.result
    }
}

Rounded Button with States

draw_bg +: {
    instance hover: 0.0
    instance down: 0.0
    uniform color_bg: #4488ff
    uniform color_hover: #5599ff
    uniform color_down: #3377ee

    pixel: fn() {
        let sdf = Sdf2d.viewport(self.pos * self.rect_size)
        sdf.box(0.0, 0.0, self.rect_size.x, self.rect_size.y, 6.0)
        let color = mix(self.color_bg, self.color_hover, self.hover)
        let color = mix(color, self.color_down, self.down)
        sdf.fill(color)
        return sdf.result
    }
}

Custom Shader Functions

draw_bg +: {
    fn wave(pos: vec2, time: float) -> float {
        return sin(pos.x * 10.0 + time * 3.0) * 0.1
    }

    pixel: fn() {
        let w = self.wave(self.pos, self.draw_pass.time)
        let color = mix(#1a1a2e, #4488ff, self.pos.y + w)
        return Pal.premul(vec4(color.xyz, 1.0))
    }
}

Splash Shader Capability Boundary

Splash CAN:

• Override pixel: fn(), vertex: fn(), get_color: fn() on existing draw types via +:
• Define helper shader functions within +: blocks
• Set instance/uniform variables on existing draw types
• Use all SDF2D, color, math built-ins in shader functions

Splash CANNOT:

• Create new DrawQuad/DrawText/DrawSvg types (must define in Rust)
• Add new instance fields to existing shaders (GPU layout is compile-time)

Rule: Rust defines the draw type struct + registers it; Splash overrides how it draws.

See ./references/shader-reference.md "Splash Shader Capabilities & Boundaries" for the full pattern.

Custom Fullscreen Shader Pattern (learned 2026-03-26)

For standalone shader-driven widgets (e.g. particle fields, visualizers), follow the examples/shader pattern:

1. Custom Draw Type (Rust)

#[derive(Script, ScriptHook)]
#[repr(C)]  // CRITICAL: must be repr(C) for GPU layout
pub struct DrawMyShader {
    #[deref] draw_super: DrawQuad,  // inherits from DrawQuad
    #[live] my_param: f32,           // maps to shader variable
}

2. Register + Define Shader (script_mod!)

set_type_default() do #(DrawMyShader::script_shader(vm)){
    ..mod.draw.DrawQuad        // inherit DrawQuad defaults
    my_param: 0.5              // default value

    // Custom functions: property-style syntax, NOT fn name(self, ...)
    my_helper: fn(a: float, b: float) -> vec2 {
        return vec2(a * 2.0, b * 0.5)
    }

    pixel: fn() {
        let result = self.my_helper(self.pos.x, self.pos.y)
        return Pal.premul(vec4(result.x, result.y, 0.0, 1.0))
    }
}

3. Widget with Turtle Layout

fn draw_walk(&mut self, cx: &mut Cx2d, _: &mut Scope, walk: Walk) -> DrawStep {
    cx.begin_turtle(walk, self.layout);
    let rect = cx.turtle().rect();
    self.draw_bg.draw_abs(cx, rect);      // single fullscreen quad
    cx.end_turtle_with_area(&mut self.area);
    DrawStep::done()
}

4. Updating Shader Variables from Rust

// Direct field access (when draw type has #[live] fields):
self.draw_bg.my_param = 0.75;
self.area.redraw(cx);

// Via NextFrame for animation:
if let Event::NextFrame(ne) = event {
    if ne.set.contains(&self.next_frame) {
        self.draw_bg.my_param += 0.01;
        self.area.redraw(cx);
        self.next_frame = cx.new_next_frame();
    }
}

Instanced Particle Rendering (learned 2026-03-26)

For drawing thousands of independent particles (dots, stars, etc.):

Draw Shader

#[derive(Script, ScriptHook)]
#[repr(C)]
pub struct DrawDot {
    #[deref] draw_super: DrawQuad,
    #[live] dot_color: Vec3,  // per-instance color
}

// Shader: each instance is a small circle
pixel: fn() {
    let d = length(self.pos - vec2(0.5, 0.5))
    let alpha = 1.0 - smoothstep(0.35, 0.5, d)
    return Pal.premul(vec4(self.dot_color * alpha, alpha))
}

Rendering Loop

self.draw_dot.begin_many_instances(cx);  // start batch

for i in 0..particles.len() {
    let (x, y) = particles[i];
    self.draw_dot.dot_color = vec3(r, g, b);  // set per-instance data
    self.draw_dot.draw_abs(cx, Rect {
        pos: dvec2(x - radius, y - radius),
        size: dvec2(radius * 2.0, radius * 2.0),
    });
}

self.draw_dot.end_many_instances(cx);  // submit batch as one draw call

Physics Pattern (spring-back displacement)

// Per particle: store persistent displacement
displacements: Vec<(f64, f64)>,

// Each frame:
for i in 0..dots.len() {
    let (mut dx, mut dy) = displacements[i];

    // 1. Decay (spring back, 0.94 = ~2-3 sec return)
    dx *= 0.94;
    dy *= 0.94;

    // 2. Apply forces (cursor push, ripples, etc.)
    let dist = distance(dot_pos, mouse_pos);
    let t = (1.0 - dist / radius).max(0.0);
    let push = t * t * t * strength;  // cubic falloff
    dx += direction.x * push;
    dy += direction.y * push;

    displacements[i] = (dx, dy);
    // Draw at original_pos + displacement
}

Performance Notes

• 10,000 particles at 60fps: OK on macOS Metal (one draw call via instancing)
• CPU physics loop: 10K × 17 distance checks = ~170K ops/frame, negligible
• Key: begin_many_instances / end_many_instances batches into single GPU draw call

Shader Syntax Pitfalls (learned 2026-03-26)

| Pitfall | Error | Fix | |---------|-------|-----| | let x = 1.0; x = 2.0 | cannot assign to let binding | Use different names: let x2 = ... | | fn push(self, ...) -> vec2 | method not found on self | Use property syntax: push: fn(...) -> vec2 { } | | return vec4(r, g, b, a) without premul | Incorrect alpha blending | return Pal.premul(vec4(r, g, b, a)) | | Custom shader in Splash eval | Silent blank render | Must use compiled script_mod! path | | Missing #[repr(C)] on draw struct | GPU layout mismatch | Always add #[repr(C)] | | fn calc(self, x: float) syntax | cannot push to frozen vec | Use calc: fn(x: float) -> float { } |

Best Practices

1. Always premultiply - Use Pal.premul() or return sdf.result
2. Use instance for animation - Only instance variables work with Animator
3. Use uniform for shared values - Colors, sizes shared across instances
4. Use +: merge operator - Extend default shaders: draw_bg +: { ... }
5. Keep shaders simple - Complex shaders impact rendering performance
6. Use SDF for shapes - Much cleaner than manual math
7. Test with new_batch: true - Required when mixing shaders with text
8. Property-style functions - name: fn(args) -> type { }, call via self.name(args)
9. Immutable let - Shader let cannot be reassigned; use unique names per step
10. #[repr(C)] on draw structs - Required for GPU memory layout alignment

SDF Capsule (Pill) Shape Pattern (learned 2026-03-31)

sdf.box() with large border_radius breaks when radius approaches half the dimension — the formula size.xy - vec2(2*r, 2*r) goes negative, producing diamond/spiky shapes. Use this standard capsule SDF instead:

draw_bg +: {
    pixel: fn() {
        let w = self.rect_size.x
        let h = self.rect_size.y
        let r = h * 0.5
        let px = self.pos.x * w
        let py = self.pos.y * h
        // Standard capsule: clamp x to center segment, then circle distance
        let cx = clamp(px, r, max(r, w - r))
        let cy = h * 0.5
        let d = length(vec2(px - cx, py - cy)) - r
        let alpha = 1.0 - smoothstep(-1.0, 1.0, d)
        return Pal.premul(vec4(0.1, 0.1, 0.18, alpha * 0.82))
    }
}

Key points:

• clamp(px, r, w-r) constrains x to the center line segment between the two end circles
• max(r, w-r) prevents clamp range inversion when widget is very narrow
• Smoothstep (-1.0, 1.0, d) provides 2px anti-aliasing
• Works correctly at any width — dynamically adapts as width: Fit content changes
• No three-part union (circles + rect) needed — single formula, no seam artifacts

Pulsing Dot in Background Shader (learned 2026-03-31)

Embed animation directly in the background shader to avoid z-order issues with child widgets (LoadingSpinner/other widgets can cause bleed-through at capsule edges):

draw_bg +: {
    pixel: fn() {
        let w = self.rect_size.x
        let h = self.rect_size.y
        let r = h * 0.5
        let px = self.pos.x * w
        let py = self.pos.y * h

        // Capsule background
        let cx_bg = clamp(px, r, max(r, w - r))
        let cy = h * 0.5
        let d_bg = length(vec2(px - cx_bg, py - cy)) - r
        let bg_alpha = 1.0 - smoothstep(-1.0, 1.0, d_bg)
        let bg = vec4(0.1, 0.1, 0.18, bg_alpha * 0.82)

        // Pulsing dot (driven by draw_pass.time)
        let t = self.draw_pass.time
        let pulse = 0.5 + 0.5 * sin(t * 4.0)
        let dot_r = 4.0 + pulse * 3.0
        let dot_cx = r + 2.0
        let d_dot = length(vec2(px - dot_cx, py - cy)) - dot_r
        let dot_alpha = (1.0 - smoothstep(-1.0, 1.0, d_dot)) * bg_alpha
        let dot_color = mix(vec3(0.3, 0.6, 1.0), vec3(0.2, 0.9, 0.5), pulse)

        // Composite
        let final_rgb = mix(bg.xyz, dot_color, dot_alpha * 0.8)
        let final_a = bg.w + dot_alpha * 0.6 * (1.0 - bg.w)
        return Pal.premul(vec4(final_rgb, final_a))
    }
}

IMPORTANT: Must call self.ui.widget(cx, ids!(my_window)).redraw(cx) from handle_next_frame to keep draw_pass.time advancing. Without continuous redraw, time-based animation freezes.