leanUI

leanUI is a small immediate-mode UI library written in C99. It’s designed for games and lightweight applications that need a clean, polished interface without bringing in a full framework.

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Overview

• Immediate-mode API (rendering is also immediate, now command list to traverse, easier to integrate your own rendering)
• No allocations, no dependencies beyond the C standard library
• Single header / single source, easy to drop into any project
• Built-in easing and animation for a smooth, modern feel
• A focused set of well-designed widgets
• ~500 lines of code, compiles in seconds
• Low memory footprint ~3kb as the lib does not store any data for widget and don’t use hashmap

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Widgets

leanUI provides a small, cohesive set of widgets:

• Buttons — with hover and press animations

• Labels / value fields — for clean key–value layouts

• Toggles — animated switches inspired by iOS

• Segmented controls — for mode switching

• Sliders

Each widget is built with consistent spacing, alignment, and interaction patterns. The goal is not to cover everything, but to make a minimal set look and feel right out of the box.

Animation

Unlike most tiny immediate-mode UI libraries, leanUI has animation built in:

• Hover, press and toggle states are automatically eased over time.
• Animation framerate independent, uses delta time between frames.
• No per-widget state is stored — transitions are computed implicitly based on widget identity.

This gives small UIs a fluid, modern feel without adding complexity to your code.

Layout

Very simple and effective layout (no rows or columns)

• Horizontal alignment : left, center or right
• Manually call to ui_newline to make a carriage return

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Integration

You just need to add lean_ui.c/h to your project, set the callbacks at init and update input/delta_time each frame.

Callbacks

draw_box: The rectangle’s rounded corners are expected to fit entirely within the rectangle’s bounds; no part of the corner should extend beyond x, y, width, or height. Colors are defined in the sRGB color space. leanUI uses alpha blending.

draw_text: Draws text at (x, y). The y coordinate specifies the top, but the text will be centered vertically relative to the font height.

set_clip_rect: Sets a clipping rectangle so that all subsequent drawing is limited to (x, y, width, height). You can ignore this callback if no widget will be drawn outside the window frame.

text_width: Returns the width that the renderer would need to draw the specified text.

Inputs

• ui_update_mouse_pos: Should be called each time the mouse moves
• ui_update_mouse_button: Call this function whenever a mouse button changes state. Do not call with button_pressed param if the button remains pressed.

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Example

For a complete and compiling example see test/test.c

#include "lean_ui.h"

void frame(ui_context* ctx, float delta_time)
{
ui_begin_frame(ctx, 1.f/60.f);
ui_begin_window(ctx, "Alright let's play!", 800, 100, 600, 1200, window_resizable);

const char* list[] = {"One", "Two", "Three", "Four"};
static uint32_t selected = 2;
ui_segmented(ctx, list, 4, &selected);

ui_separator(ctx);

static float quantity = 5.f;
ui_slider(ctx, "Distance", 0.f, 100.f, 1.f, &quantity, "%3.2fkm");

if (ui_button(ctx, "Left Button", align_left))
{
// process
}

ui_end_window(ctx);
ui_end_frame(ctx);
}