Page 115 - DCAP313_LAB_ON_COMPUTER_GRAPHICS

Unit 7: Implementation of Hidden Surface in 2D

two groups: near (potentially visible) and reverse (invisible). The near polygons have an obtuse Notes
angle with the projection direction; and the reverse ones have an acute angle with the projection
direction. In the following step, we split the edges into three types, according to which polygons
they cross:
First type: A conjunction of two reverse polygons, called the invisible edge.
Second type: A conjunction of the near polygon and the reverse polygon, called the visible
contour edge.
Third type: A conjunction between two near polygons, called the potential visible edge.
Newell’s Algorithm is a 3D computer graphics procedure for elimination of
polygon cycles in the depth sorting required in hidden surface removal. It
was proposed in 1972 by brothers Martin Newell and Dick Newell, and Tom
Sancha, while all three were working at CAD Centre.
Self Assessment Questions
1. The process of hidden surface determination is sometimes called hiding.
(a) hider (b) hiding
(c) hidden surface elimination (d) visible surface determination
2. Hidden surface removal is the process of removing surfaces (lines) which are not visible
from the chosen viewing position.
(a) True (b) False
3. According to the ……………………… can be split depending on the form of output data.
(a) line algorithm (b) raster algorithm
(c) apple algorithm (d) visibility algorithms
4. …………………. works in the area of objects and is based on the fact that every edge is a
conjunction of two polygons.
(a) line algorithm (b) raster algorithm
(c) apple algorithm (d) visibility algorithms
5. Second type is a conjunction between two near polygons, called the potential visible edge.
(a) True (b) False

7.2 Hidden Surface Removal Algorithms

Considering the rendering pipeline, the projection, the clipping, and the rasterization steps are
handled differently by the following algorithms:
1. Z-buffering
2. Painter’s algorithm
3. Binary space partitioning (BSP)
4. Ray tracing
5. The Warnock algorithm
7.2.1 Z-buffering

The Z-buffer or depth memory is one of the most common methods for solving the problem of
an objects’ visibility in a scene. It has relatively large memory demands because in displaying
objects it will keep their z-coordinates in a special two-dimensional field with the dimensions
of a table (mostly a display box), where there are stored the coordinates of calculated points.
At the beginning, this field is filled in with a maximum value (e.g., maximum z-value of all
objects in the display box).

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