1. Definition

Compositing is the process of combining multiple images or visual elements into a single final image.

• Each element may be a layer, such as a background, foreground, characters, or effects.
• Widely used in film, animation, graphics, and user interfaces.
• It allows creation of complex scenes without modifying the original images.

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2. Concept

• Each image or layer can have color and transparency (alpha) values.

• Alpha blending determines how layers are combined:

  • Fully opaque layers cover layers beneath.
  • Semi-transparent layers are blended with underlying layers.

• The final pixel color (C_{out}) is computed as:

$$C_{out} = \alpha \cdot C_{foreground} + (1 - \alpha) \cdot C_{background}$$

Where:

• $C_{foreground}$ = color of the top layer pixel
• $C_{background}$ = color of the bottom layer pixel
• $\alpha$ = transparency factor (0 = fully transparent, 1 = fully opaque)

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3. Types of Compositing

Type	Description
Alpha Blending	Combines foreground and background based on alpha value
Matte Compositing	Uses a mask (matte) to select areas for blending
Depth Compositing	Combines images based on distance from camera (Z-buffer)
Multi-pass Compositing	Combines multiple layers iteratively for complex effects

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4. Steps in Compositing

1. Prepare layers

   • Ensure each image has color and optional alpha/mask data.

2. Determine blending order

   • Usually back-to-front (painter’s algorithm) or using depth information.

3. Apply blending

   • Compute pixel colors using alpha blending or mask-based techniques.

4. Render final image

   • Combine all layers into one final framebuffer or image.

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5. Applications

1. Film & Animation – Combine live-action footage with CGI elements.
2. 2D/3D Graphics – Add shadows, reflections, or special effects.
3. User Interfaces – Layered windows, pop-ups, and translucency effects.
4. Games & Simulations – Particle effects, HUD elements, and post-processing.

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6. Advantages

• Allows non-destructive editing of images.
• Supports complex scenes without modifying original assets.
• Compatible with hardware acceleration for real-time graphics.

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7. Summary Table

Aspect	Description
Purpose	Combine multiple images/layers into one
Key Technique	Alpha blending, mask/matte, depth compositing
Formula	$C_{out} = \alpha C_{fg} + (1-\alpha) C_{bg}$
Applications	Film, animation, graphics, UI, games

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Key Points:

• Compositing = combining multiple images/layers to produce a final visual output.
• Alpha values or masks control transparency and blending.
• Essential for visual effects, layered graphics, and realistic rendering.