Painter’s Algorithm in Computer Graphics
Painter’s algorithm is the algorithm which is introduced by Hewells in 1972.
The techniques used by these algorithms are image space and object space.
The name of this algorithm is Painter’s because it’s working is like a painter who creating an oil painting. Just like an artist paints, he start his painting with an empty canvas, the first thing the artist will do is to create a background layer for the painting, after this layer he start creating another layers of objects one-by-one. In this way he completes his painting, by covering the previous layer partially or fully according to the requirement of the painting.
This algorithm is basically used to paint the polygons in the view plane by considering their distance from the viewer. The polygons which are at more distance from the viewer are painted first. After that the nearer polygons are started painted on or over more distant polygons according to the requirement.
In this algorithm the polygons or surfaces in the scene are firstly scanned or then painted in the frame buffer in the decreasing distance from view point of the viewer starting with the polygons of maximum depth or we can say minimum z-value.
Firstly the depth sort is performed in which the polygons are listed according to their visibility order or depth priority.
As this algorithm uses the concept of depth priority so it is also called as depth priority algorithm or priority algorithm.
The frame buffer is painted with the background color. After that the polygon which is farthest enter to the frame buffer. For this, the pixel information will get changed i.e. information of the background which has the farthest polygon get replaced with that of the background. This is going to be repeatedly changed as we move from one polygon to the other and end up with the nearest polygon.
Usually comparisons are performed whenever the polygons are going to overlap each other. The most common method used for the comparison is called as mini-max method. For this purpose, the rectangles are drawn around the polygons such that the rectangles exactly fit the polygons.
Then the rectangles are going to check to see whether they overlap each other or not. If the rectangles are observed as they do not overlap then we consider that the surfaces are also not overlap. If the rectangles are overlapped then the surfaces are also overlapped which is as shown in the following figure:
To find out which rectangle is to be overlap, we need to find the minimum and maximum x and y values to the rectangles which are going to test for overlapping. If the minimum value of y of one of the rectangles is larger than the maximum y value of the other rectangle then they are not going to be overlapped and as the rectangles are not overlapped then the surfaces are also not overlapped.
The same test is to be performed for the x-coordinates.
If the surfaces are overlapping, we do not know which surface should be present on the top of the other. To find out which surface is to be present on the top the principle of mini-max is used on the depth values of both the overlapping surfaces.
Algorithm for Painter’s method
The following are the steps of this algorithm:
- Sorting of the various surfaces which is on the basis of their decreasing depth or we can say the largest value of z.
- Now scanning to convert the various surfaces which is in the order starting with the surface which has greatest depth.
- Comparing is to be done on the basis of various overlapping surfaces so that the user will determine which surface is to be kept visible.
- In the refresh buffer enter the intensity value for the determined surface i.e. the surface which is determined to be visible.
- The above process is going to be repeat for all the available surfaces.
- However, if the overlapping is observed, then there is the need of the further tests. The following tests are required: