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Prowl.Paper is an open-source, MIT-licensed immediate-mode UI library for .NET. It powers the Prowl Game Engine’s editor UI and is available as a standalone NuGet package for any .NET 6–10 or netstandard2.1 project. Rather than managing persistent widget trees, Paper lets you declare your entire UI on every frame — making your code simpler, more direct, and trivially reactive to any state change.

Quickstart

Build your first Paper UI in minutes with a working code example.

Installation

Add Prowl.Paper via NuGet and configure a renderer backend.

Core Concepts

Understand immediate-mode UI, elements, layout, and the frame loop.

API Reference

Full reference for every class, method, and property in the library.

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What makes Paper different

Paper follows the immediate-mode GUI (IMGUI) paradigm — there are no retained widget objects, no data-binding, and no virtual DOM. You write ordinary C# code that runs every frame: 
Paper.Box("MyButton")
    .Size(120, 40)
    .BackgroundColor(Color.ForestGreen)
    .Rounded(8)
    .Text("Click Me", myFont)
    .Hovered
        .BackgroundColor(Color.DarkGreen)
    .End()
    .Transition(GuiProp.BackgroundColor, 0.15f, Easing.SineInOut)
    .OnClick(_ => Console.WriteLine("clicked!"));
State changes automatically flow into the next rendered frame — no setState, no subscriptions required.

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Key features

Flexible Layout Engine

Row, Column, and absolute positioning with Pixels, Percentage, Stretch, and Auto sizing units.

Rich Animation System

AnimateBool, AnimateFloat, AnimateSpring, AnimateColor, Shake, Pulse, and more — all stateless from your code’s perspective.

State-Driven Styles

Inline .Hovered, .Active, .Focused, and .If() blocks for conditional styling without any extra state variables.

Hardware Vector Graphics

Anti-aliased paths, gradients, box shadows, backdrop blur, and custom shaders via Prowl.Quill.

Rich Text & Markdown

Render plain text, CommonMark Markdown, or tagged rich text with animation effects like typewriter and rainbow.

Pluggable Renderers

Implement ICanvasRenderer to target OpenTK, Raylib, WebGL/WASM, or any custom backend.

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Quick example

The snippet below shows a complete minimal app structure. Paper’s frame loop is three calls:
1

Initialize once

var paper = new Paper(myRenderer, screenWidth, screenHeight, fontAtlasSettings);
2

Feed input every frame

paper.SetPointerPosition(mouseX, mouseY);
paper.SetPointerState(PaperMouseBtn.Left, mouseX, mouseY, isLeftDown, false);
paper.SetPointerWheel(wheelDelta);
3

Declare UI between BeginFrame / EndFrame

paper.BeginFrame(deltaTime);

using (paper.Column("Root").Enter())
{
    paper.Box("Title")
        .Height(60)
        .BackgroundColor(50, 120, 200)
        .Text("My App", titleFont);

    paper.Box("Content");
}

paper.EndFrame();
Ready-to-run samples for OpenTK, Raylib, and WebAssembly/WebGL are included in the GitHub repository.

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