10.2. Need for hidden line and surface elimination algorithms
When we attempt to view a picture that contains some non-transparent things or surfaces, then those objects or surfaces are hidden from us i.e. we cannot view those parts that are behind the objects that we can see. We should remove such hidden surfaces to get a realistic view of an image. Hidden surface problem is a process in which we identify and remove the hidden surfaces of a picture as shown in figure 10.1. Objects in computer graphics are generally represented in the three-dimensional system. Usually most of the objects are not transparent, so we deal with the outer surfaces of the objects. The properties such as shape, color and texture affect the graphical representation …show more content…
If there is another object that should be rendered in the same pixel value, then 3-D graphics card will compare the two depth value and it will choose the one which is closer to the observer. The chosen depth value is then added to the z-buffer, replacing the old value.
The granularity of the z-buffer plays an important role in the scene quality: a 16-bit z-buffer usually results in artifacts (called "z-fighting") when two objects are too close to each other. A 24-bit or 32-bit z-buffer behaves comparatively better, although additional algorithms are required to completely eliminate the problem. An 8-bit z-buffer is never used because of its little precision.
In this algorithm, we use some coherence. Commonly used is point-by-point coherence because the z-value of adjacent horizontal positions across the scan line is easily calculated by adding some constant value to the previous z-value. Also, on every scan line, we can simply calculate z-value at the starting point using the previous starting point of the previous scan …show more content…
Polygon 4: Given:

Depth=4; vertex1=(2,0); vertex2=(2,3); vertex3=(5,3); vertex4=(5,0)

This polygon is drawn hiding some portion of the red and purple polygons, but is itself obscured by the yellow polygon.
Painter’s Algorithm
Painter’s algorithm is also called as depth sorting algorithm in which first the surfaces are sorted according to their depth values in the decreasing order and then we start the scan conversion with the surface having greater depth. Just like a painter paints the most far-off objects first and then paints the closer objects and foreground objects are painted over the background objects on the drawing canvas and rest of the objects which are of least interest are painted separately, similar approach is used in this algorithm. Figure 10.5 illustrates this approach of painter’s algorithm. To describe Painter’s Algorithm, we use the following pseudo-code: sort polygons by z; for all polygon p do for all pixel(x, y) ϵ p do paint polygon.colour; end for
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