# on-demand-rendering Specification

## Purpose

Schedules window repaints for the Game of Life canvas: frames are produced only when displayed content changes, and only the changed cells are drawn, so idle CPU usage is near zero while the visible output stays identical to a full repaint.

## Requirements

### Requirement: Frame is rendered only on content change
The system SHALL produce a new rendered frame only when displayed content has changed since the last rendered frame, and SHALL NOT redraw when it has not.

#### Scenario: Idle while paused
- **WHEN** the simulation is paused, no tool is active, and no input arrives
- **THEN** no new frame is rendered over consecutive display refreshes

#### Scenario: Tick changes grid
- **WHEN** a simulation tick changes the grid while the simulation is running
- **THEN** a new frame is rendered that reflects the new generation

#### Scenario: Edit marks content dirty
- **WHEN** the user paints cells or places a pattern
- **THEN** a new frame is rendered with the modified cells

#### Scenario: Preview follows pointer
- **WHEN** a tool is selected and the pointer moves to a different cell
- **THEN** a new frame is rendered with the pattern preview at the new position

#### Scenario: Preview-affecting action
- **WHEN** the user selects a tool, rotates the active tool, or toggles grid lines
- **THEN** a new frame is rendered reflecting the change

#### Scenario: Resize forces full redraw
- **WHEN** the canvas size or scale changes
- **THEN** a new frame is rendered over the whole canvas

### Requirement: Simulation advances independently of rendering
The system SHALL accumulate simulation time and apply ticks based on elapsed time, regardless of whether frames are rendered.

#### Scenario: Ticks accumulate while idle
- **WHEN** multiple tick intervals elapse without a rendered frame
- **THEN** every elapsed interval results in exactly one applied tick

### Requirement: Only changed cells are repainted
The system SHALL redraw only the cells whose state differs from the last rendered frame, drawing each changed cell's background, grid-line fragments, and cell state, and SHALL leave unchanged cells' pixels untouched.

#### Scenario: Single cell changes
- **WHEN** exactly one cell changes state between rendered frames
- **THEN** only that cell's pixel area is rewritten in the framebuffer

#### Scenario: Unchanged pixels are preserved
- **WHEN** a frame is rendered after a change
- **THEN** pixels of unchanged cells are identical to the previous frame

### Requirement: Damage is limited to the changed region
The system SHALL report surface damage covering only the area repainted in the current frame.

#### Scenario: Small change damages small region
- **WHEN** only a few cells changed
- **THEN** the reported damage is limited to the union of those cells' areas rather than the whole surface