A-Buffer Method

A-Buffer method in computer graphics is a general hidden face detection mechanism suited to medium scale virtual memory computers. This method is also known as anti-aliased or area-averaged or accumulation buffer. This method extends the algorithm of depth-buffer (or Z Buffer) method. As the depth buffer method can only be used for opaque object but not for transparent object, the A-buffer method provides advantage in this scenario. Although the A buffer method requires more memory, but different surface colors can be correctly composed using it. Being a descendent of the Z-buffer algorithm, each position in the buffer can reference a linked list of surfaces. The key data structure in the A buffer is the accumulation buffer.

Each position in the A buffer has 2 fields :

1) Depth field

2) Surface data field or Intensity field

A depth field stores a positive or negative real number. A surface data field can stores surface intensity information or a pointer to a linked list of surfaces that contribute to that pixel position.

As shown in the above figure, if the value of depth is >= 0, the number stored at that position is the depth of single surface overlapping the corresponding pixel area. The 2nd field, i.e, the intensity field then stores the RGB components of the surface color at that point and the percent of 

pixel coverage.

As shown in the above figure, multiple-surface contributions to the pixel intensity is indicated by depth < 0. The 2nd field, i.e, the intensity field then stores a pointer to a linked list of surface data.

A buffer method is slightly costly than Z-buffer method because it requires more memory in comparison to the Z-buffer method. It proceeds just like the depth buffer algorithm. Here, the depth and opacity are used to determine the final colour of the pixel. As shown in the figure below, the A buffer method can be used to show the transparent objects.

The surface buffer in the A buffer method includes :

1. Depth
2. Surface Identifier
3. Opacity Parameter
4. Percent of area coverage
5. RGB intensity components
6. Pointer to the next surface

Q: List the advantage and disadvantage of back-face detection  and A-buffer method. Write the algorithm for back-face detection. [AKTU 2015-16 15M] 

Ans:-

Advantage of back-face detection method:-

1. It is fast.
2. It is a simple object space method.

Disadvantage of back-face detection method:-

1. It can only be used on solid objects modelled as polygon mesh.
2. It works fine for convex polyhedra but not necessary for concave polyhedra.

Advantage of A-buffer method:-

1. More than one surface intensity can be taken into consideration at each pixel position.
2. It also anti-aliased the object edges.
3. In this, surface are divided into polygon mesh and clipped against the pixel boundaries.
4. The intensity of pixel is determined by considering opacity parameter and percentage of overlaps of the overlapping surface.

Disadvantage of A-buffer method:-

1. This algorithm processes multiple object at a time. The total number of polygon in a picture can be arbitrary large.