1. Definition

Hidden Surface Removal (HSR) is the process of determining which surfaces (or parts of surfaces) of 3D objects are visible from a viewpoint and which are hidden behind other objects.

• Ensures that only the visible surfaces are rendered.
• Improves realism and avoids unnecessary computation.

The Z-buffer (Depth Buffer) technique is one of the most widely used methods for HSR.

2. Z-Buffer Method

Z-buffering stores depth information for each pixel on the screen to determine which surfaces are visible.

2.1 Concept

• Each pixel has a depth value (z-value) representing its distance from the viewer.

• When rendering a polygon:

  1. Compute the z-value for each pixel on the polygon.
  2. Compare it with the current z-value stored in the Z-buffer.
  3. Update the pixel color only if the new z-value is closer (smaller) than the stored z-value.

3. Steps of Z-Buffer Algorithm

1. Initialize Buffers

   • Framebuffer: stores the color of each pixel.
   • Z-buffer: stores the depth (z-value) of each pixel. Initialize all values to far plane depth.

2. Scan Each Polygon

   • For every pixel covered by the polygon: a. Compute interpolated z-value for that pixel. b. Compare with Z-buffer value.

3. Update Buffers

   • If the polygon pixel is closer than the current Z-buffer value:

     • Update the framebuffer with polygon color.
     • Update the Z-buffer with new z-value.

4. Result

   • After processing all polygons, the framebuffer contains only the visible surfaces.

4. Advantages

• Simple and general: Works for any 3D scene with overlapping objects.
• Handles arbitrary polygons.
• Compatible with hardware acceleration, making it fast for real-time graphics.

5. Disadvantages

• Memory-intensive: Requires a Z-buffer the same size as the framebuffer.
• No automatic transparency handling: Transparent surfaces need special handling.
• Precision issues: Limited depth precision may cause Z-fighting when surfaces are very close.

6. Summary Table

Key Points:

• Z-buffering is a per-pixel technique for hidden surface removal.
• It compares depth values of surfaces to determine visibility.
• Essential for realistic 3D rendering in games, CAD, and simulations.