Mado: A Window System for Resource-Constrained Devices

Introduction

MADO (Minimalistic Application Display Orchestrator) is an open-source library that brings advanced window system features to smaller devices. With embedded systems now featuring high-resolution displays and powerful CPUs, sophisticated graphical user interfaces can be realized even in the smallest devices. Although CPU power has increased dramatically within a given power budget, these small systems still face severe memory constraints. This unique environment presents interesting challenges for graphical system design and implementation.

To address these challenges, Mado has been developed. Drawing from ideas used in modern window systems for larger environments, Mado provides overlapping translucent windows, anti-aliased graphics, and scalable fonts within a memory budget of several kilobytes. Mado embeds window management directly into the toolkit, supporting resizing, moving, and minimizing without external control. The toolkit automatically constructs suitable decorations for each window as regular toolkit objects, and the normal event dispatch mechanism directs window management activities.

Mado is a continuation of the work on TWIN, originally developed by Keith Packard. 'Mado' means 'window' in the language of the Tao people (also known as Yami), an Austronesian ethnic group native to the tiny outlying Orchid Island of Taiwan.

Principles of design

Focusing on specific hardware capabilities and limitations, Mado efficiently utilizes limited resources. Over-constraining requirements can limit potential target environments, but exploring reduced variation can be beneficial.

Mado assumes little-to-no graphics acceleration within the framebuffer, which is connected to the CPU via a slow link. Therefore, most drawing is done in local memory, not directly to the framebuffer, ensuring synchronized screen updates and potentially reducing power consumption by minimizing off-chip data references.

The system requires a color screen with fixed color mapping. This simplifies software by allowing anti-aliased drawing to generate nearly invisible errors instead of visibly incorrect results. Smooth color gradations eliminate the need for dithering or other color-approximating schemes.

Mado is designed to ensure respectable CPU performance, which mitigates the need for caching intermediate rendering results like glyph images for text. It supports only a single, general performance class of drawing operations, focusing exclusively on anti-aliased drawing. This focus is practical because the CPUs that Mado supports are sufficiently fast, rendering non-anti-aliased drawing obsolete. Moreover, Mado avoids reliance on a floating point unit (FPU) or extensive FPU emulation by employing its own fixed-point arithmetic for all necessary computations. This approach extends to nearly all of Mado's optional components as well. For specific tasks such as subpixel font layout and alpha blending, Mado utilizes fixed-point math, ensuring that it remains efficient even on CPUs without an FPU.

These environmental limitations enable Mado to provide significant functionality with minimal wasted code. Unlike window systems designed for varied platforms, Mado avoids unnecessary generalization and variability, benefiting the entire application stack.

Build and Verify

Prerequisites

Mado is built with a minimalist design in mind. However, its verification relies on certain third-party packages for full functionality and access to all its features. We encourage the development environment to be installed with all optional packages, including libjpeg and libpng.

• macOS: brew install jpeg libpng
• Ubuntu Linux / Debian: sudo apt install libjpeg-dev libpng-dev

The renderer implementation can either use the built-in pixel manipulation or be based on Pixman. The built-in renderer is simple and performs adequately on platforms without SIMD instructions, such as RISC-V processors without vector extensions. However, for modern CPU architectures with extensive ISA extensions, Pixman offers faster pixel manipulation. Install Pixman before selecting the corresponding renderer implementation.

• macOS: brew install pixman
• Ubuntu Linux / Debian: sudo apt install libpixman-1-dev

In the meantime, ensure that you choose a graphics backend and install the necessary packages beforehand.

For SDL backend, install the SDL2 library.

• macOS: brew install sdl2
• Ubuntu Linux / Debian: sudo apt install libsdl2-dev

For the VNC backend, please note that it has only been tested on GNU/Linux, and the prebuilt neatvnc package might be outdated. To ensure you have the latest version, you can build the required packages from source by running the script:

$ tools/build-neatvnc.sh

For Linux framebuffer backend, install libudev and libuuid:

• Ubuntu Linux / Debian: sudo apt install libudev-dev uuid-dev

Configuration

Configure via Kconfiglib, you should select either SDL video, the Linux framebuffer, or VNC as the graphics backend.

$ make config

Build and execution

Build the library and demo program:

$ make

To run demo program with SDL backend:

$ ./demo-sdl

Once the window appears, you should be able to move the windows and interact with the widgets.

To run demo program with the Linux framebuffer backend:

$ sudo ./demo-fbdev

Normal users don't have access to /dev/fb0 so require sudo. Alternatively, you can add the user to the video group to avoid typing sudo every time:

$ sudo usermod -a -G video $USERNAME

In addition, the framebuffer device can be assigned via the environment variable FRAMEBUFFER.

To run demo program with the VNC backend:

$ ./demo-vnc

This will start the VNC server. You can use any VNC client to connect using the specified IP address (default is 127.0.0.1) and port (default is 5900). The IP address can be set using the MADO_VNC_HOST environment variable, and the port can be configured using MADO_VNC_PORT.

License

Mado is available under a MIT-style license, permitting liberal commercial use. Use of this source code is governed by a MIT license that can be found in the LICENSE file.