Repaint: Free, Open-Source Offline Painting App with 16-Bit Color and AI Integration

A Painting Tool Over a Decade in the Making

A developer recently shared on Reddit a painting application they had spent years building — Repaint. The project quietly took shape in the developer's home starting in 2009, and has now officially launched as a free, open-source, offline, cross-platform tool.

Unlike the many painting applications that rely on cloud services or subscription models, Repaint was designed from the ground up around three core principles: completely free, open-source code, and offline availability. The developer openly admits the app is still "full of bugs and a bit clunky" — but that's exactly what an early-stage open-source project looks like in its most authentic form, and users can witness its entire journey from rough edges to polished product.

One particularly notable development was a key technical migration: Repaint moved from the original Qt framework to Raylib. Qt is a cross-platform C++ application framework maintained by The Qt Company — comprehensive but heavyweight, with a typical Qt deployment package exceeding 50MB and commercial licensing constraints. For a lightweight personal tool, Qt's footprint is often more burden than benefit. This migration freed Repaint from any heavy framework dependency, allowing the entire dependency chain to be statically compiled into a single executable — achieving true lightweight, standalone distribution. It's also a textbook example of the "de-Qt-ification" trend gaining momentum in the indie developer community.

Technical Architecture: C/C++ with Raylib and ImGui

Repaint is written in C/C++ at its core, uses Raylib — a lightweight game and multimedia library — for graphics rendering, and ImGui (Immediate Mode GUI) for its user interface.

Raylib was created in 2013 by Spanish developer Ramon Santamaria, originally designed for educational purposes with a philosophy of "simple, easy-to-use, no external dependencies." It wraps low-level OpenGL calls behind a unified cross-platform API, supporting Windows, Linux, macOS, Android, WebAssembly, and more. Compared to similar libraries like SDL and SFML, Raylib's API is remarkably lean — the entire library can be compiled as a single file, meaning Raylib-based applications require virtually no additional runtime dependencies when distributed.

Dear ImGui (commonly known as ImGui) is an immediate-mode GUI library open-sourced by Cédric Julien in 2014. Unlike traditional "retained-mode" GUIs, an immediate-mode GUI redraws all interface elements every frame, with UI state driven directly by application logic — no need to maintain a complex tree of widget objects. This approach dramatically simplifies development for tool-type software, allowing debug panels and parameter editors to be built with minimal code. It has become one of the de facto standards in game engines, scientific visualization, and graphics editor development.

This technical combination is thoughtfully chosen. Raylib excels at simplicity and efficiency, making it well-suited for performance-sensitive graphics applications; ImGui's immediate rendering and easy integration make it a go-to solution for tool software. Compared to the original Qt-based architecture, the new stack significantly reduces binary size and makes cross-platform distribution much more straightforward.

For the open-source community, developers familiar with C/C++ and Raylib can contribute without a steep onboarding curve.

Core Feature Highlights

Despite being a solo project, Repaint's feature design reflects considerable professional ambition. Here are its key capabilities.

Fully Modulatable Brush System

Every single brush parameter can be modulated. Users can map input signals from a drawing tablet — such as pen pressure and stroke speed — to any parameter slider, with support for range inversion. This flexible mapping mechanism gives brushes exceptionally rich dynamic behavior, approaching the level of professional digital painting software.

Real-Time Custom Brush Textures

Users can draw their own brush stamps directly within the application, no external tools required. This WYSIWYG customization approach significantly lowers the barrier to creating personalized brushes.

16-Bit Per Channel Color Depth

Repaint supports 16-bit per channel color depth. Standard digital images typically use 8-bit per channel color, where each channel represents 256 discrete levels. When performing high-transparency compositing, gradient blending, or color space conversions, this precision can produce visible "color banding" — jarring steps between adjacent tonal levels in smooth gradients. With 16-bit per channel, each channel offers 65,536 levels, boosting color precision by a factor of 256, so even ultra-low-opacity brushstrokes produce no banding. Dithering is also applied on export — introducing subtle randomized noise to simulate smoother transitions, a standard technique in professional image processing software — ensuring final output quality.

Non-Destructive Layer Transforms

Layers support non-destructive transforms — even if you scale a layer down to a single pixel and then scale it back up, the content remains fully intact. This is invaluable for artists who frequently adjust compositions.

Built-In Masking and Texture Creation

The application includes SDF (Signed Distance Field)-like masking effects. SDF is a mathematical representation that encodes geometric shapes as distance information: each point in the field stores its distance to the nearest boundary, with negative values inside and positive values outside — originally proposed by Valve for high-quality font rendering. In a painting context, SDF-based masks produce smooth, controllable edge effects that are scale-independent, avoiding the jagged edges that bitmap masks show when enlarged. Combined with seamless painting and layer operation support, Repaint is particularly well-suited for game textures, material maps, and other tiling workflows.

AI Integration: Offline Stable Diffusion

In an era of proliferating AI art tools, Repaint has integrated offline Stable Diffusion and other neural networks.

Stable Diffusion is an open-source image generation model released by Stability AI in 2022, built on a Latent Diffusion Model architecture with fully public model weights that users can run on local GPUs. Through community optimizations such as quantization techniques, it can generate images at acceptable speeds even on consumer-grade graphics cards — a fundamental distinction from commercial products like DALL-E and Midjourney that only offer cloud-based APIs.

Unlike the many AI painting applications that depend on cloud APIs, Repaint emphasizes fully offline operation. Sketches, reference images, and generated results never leave the user's local device, which fundamentally addresses the potential copyright and data privacy risks associated with commercial AI art services, while also eliminating any dependence on a network connection. For users who value data sovereignty and creative freedom, this is a particularly compelling design choice.

By combining the fine-grained control of traditional digital painting with local AI generation capabilities, Repaint aims to serve both hand-drawing artists and AI-assisted creators within a single tool.

A Reflection on Independent Open-Source Projects

Repaint's story is a microcosm of countless independent open-source projects: one person, over a decade of perseverance, skills built from scratch, and a finished work carved out of "the various chaos of life."

One statement from the developer stands out: "Either way, this is a moment in time you'll look back on — 'I remember when Repaint was buggy and clunky and unlike anything I'd ever seen.' " This willingness to openly embrace the imperfections of a project's early days is one of the most valuable aspects of the open-source spirit.

Interested users can download and try the app via the project's GitHub page, and can support the developer's ongoing work through Ko-fi. As a tool still in rapid development, Repaint may not yet be fully polished — but the depth of its features and the technical sincerity it demonstrates already make it worthy of attention from digital art enthusiasts and the open-source community alike.