2. Detection for Hidden Area
Before generating orthoimage we need to detect the ground location of hidden areas caused by building in a master image. We, accordingly, can trace the ground location in a slave image to retrieve the gray value information for compensation.

The hidden areas are detected by the concept of Z-buffer. The Z-buffer is a matrix, in the image coordinate system, which stores the distance from the camera location to the objects. As from figure 2 we see that point X and Y on the ground are projected on the same position at the image space. Object point Y with shorter distance means that it could be view by the camera and the other point x is in the hidden area.


Figure 2 Hidden Area Detection
In the proposed algorithm, we calculate the projected position pixel by pixel along the DBM to the image space using the collinearity condition equation and store their distances into a matrix. Whenever two points are located at a same cell, the distance values are compared to each other. The shortest distance is remained, while the pixel with longer distance is marked as a hidden pixel and saved in another index map in the ground coordinate system.

Because the collinearity condition equation is a non-linear transformation, tracing from an equal grid DBM to the image space will create isolated points of lines. A post processing on the index map is necessary. In which, a median filter is included to remove the isolated points and lines.

3. Generating of Orthoimage
A traditional bottom-up back projection to generate orthoimage is utilized [Chen & Rau, 1993] [Chen & Lee, 1993]. Using the index map created in the previous section we have detected the hidden areas. Thus the gray value information could be recovered from a slave image. Considering to the different illumination condition, the distribution of gray value for the two images, the master and the slave image, may varied. Thus, directly fill in gray value from slave image will create false boundary around the hidden areas. In order to cope with this problem, a smoothing technique using weighing average approach is applied to reduce the incongruity.

We create a buffer zone along the borders of each hidden area. Within the buffer zone, the gray value is calculated using weighting average considering the master and the image. The weighting factor is determined by the width of the buffer zone that is created using a mathematical morphology operation called DILATION.

Figure 3 shows an example of an index map after dilation processing. In which the continuous change of weighting factor is shown surrounding the hidden areas. According to the proposed scheme, an example of orthoimage is provided in figure 4. For the sake of comparison, a result of bottom-up back projection for a master image is provided in figure 5. It is noticed that, in figure 5 a building is slant to the left and blocks a road, because the camera center is to its right hand side. However, as shown in figure 4.the road recovered using the proposed scheme. Be aware that there is no gray value discontinuity around the borders of hidden areas.


Figure 3, An example of index map.

Figure 4, An orthoimage generated by the proposed scheme.

Figure 5, A back-projection example.