Add Multi-Layered Avatar Generation with Per-Layer Color Customization

[Sheikh566]

Summary

This PR introduces a powerful multi-layered avatar generation feature to goavatar, enabling the creation of more complex and visually distinctive avatars. Users can now compose avatars from multiple layers (1-3), each with its own pattern derived from a unique hash and customizable foreground color.

Key Features

1. Multi-Layer Support

• New WithLayers(n) option to specify the number of avatar layers (1-3)
• Each layer is rendered sequentially, creating composite patterns
• Subsequent layers use derived hashes for unique pattern variation

2. Per-Layer Color Customization

• New WithLayerColor(layerIndex, r, g, b, a) option to set custom colors for specific layers
• Flexible color assignment: set colors for some or all layers
• Unspecified layers automatically inherit colors from their derived hash values

3. Performance Improvements

• Optimized pixel drawing using image/draw package's Draw() function with draw.Uniform and draw.Over
• Significantly faster bulk fill operations compared to per-pixel Set() calls
• Background pre-fill optimization using draw.Src composite operator

4. Code Quality Improvements

• Changed color storage from single color.RGBA to slice of color.NRGBA for proper alpha handling
• Improved type documentation (renamed optFunc to OptFunc for exported visibility)
• Enhanced test coverage for multi-layer scenarios
• Updated examples demonstrating 2-layer and 3-layer avatars with custom colors

Technical Details

Color Space Update:

• Migrated from color.RGBA to color.NRGBA for more accurate alpha channel handling
• NRGBA (Non-premultiplied alpha) is the standard Go image color model

Multi-Layer Architecture:

• Background is drawn once with the specified background color
• Each foreground layer:
  • Derives its hash from the previous layer's hash using generateHash()
  • Uses either user-specified or hash-derived colors
  • Draws using the current layer's hash pattern
  • Overwrites previous pixels when pixelOn is true (cumulative composition)

Drawing Optimization:

• Uses image.Rect() to define rectangular regions
• Employs draw.Draw() with image.Uniform source for efficient fills
• Reduces drawing operations from O(width × height) nested loops to O(1) fill

Examples

2-Layer Avatar:

image := goavatar.Make("MultiLayer2",
    goavatar.WithSize(512),
    goavatar.WithLayers(2),
)

3-Layer Avatar with Custom Colors:

image := goavatar.Make("MultiLayer3Custom",
    goavatar.WithSize(512),
    goavatar.WithLayers(3),
    goavatar.WithLayerColor(0, 255, 0, 0, 255),   // Red
    goavatar.WithLayerColor(1, 0, 255, 0, 255),   // Green
    goavatar.WithLayerColor(2, 0, 0, 255, 255),   // Blue
)

Files Changed

• goavatar.go - Core implementation of multi-layer logic and drawing optimizations
• goavatar_test.go - Enhanced test coverage for multi-layer scenarios
• example/main.go - New examples demonstrating multi-layer functionality
• arts/avatar_7.png, arts/avatar_8.png - Example outputs
• arts/.gitignore - Updated to track new example images

Backward Compatibility

✅ Fully backward compatible. Existing code continues to work without modifications. The WithLayers() and WithLayerColor() options are opt-in.

Testing

All tests updated to support the new multi-layer architecture while maintaining existing test behavior for single-layer avatars.