Door Identification -- Object Extraction

Eyal Amir and Pedrito Maynard-Reid II

Given the B/W image produced by the filtering stage, we segment the white pixels into objects using a labeling algorithm. For this task we use the MATLAB function bwlabel that labels pixels by running a window of a given size over the image, labeling two points with the same label if they are in the same window. After labeling the pixels, we separate the largest objects using the MATLAB function imfeature (with the 'Area' function) which computes the "mass" of the object in terms of the number of pixels in it. (For efficiency, we then crop each object so that the images used in the rest of the algorithm are significantly smaller.) The output of the algorithm after processing the image from Figure 2 (those objects which are fed to the rest of the algorithm) is shown in the following figure:
Figure 5: Superimposed objects with mass >= 1000 (0.5*size)
Figure 6: Superimposed objects with mass >= 250 (0.5*size)

Although not significant in these figures, there are several problems that this process may encounter or introduce. The more common problem is that different objects may be "glued" together. This especially happens with noise from the floor which can get associated with the door.

A more serious problem may occur if for some reason an object gets split into several smaller objects. In this case our treatment is risky, because further computation will not associate features of the different parts together (see experiment 203b). Nevertheless, the assumption that the door is a continuous surface that is not obscured to such a dangerous extent seems plausible and postprocessing may correct "broken" doors. Last, one may want to adjust the "mass" threshold to different values for different environments. It seems that for our environment the values of 250-1000 are good enough and clear some of the noise away yet keeps even distant doors. The experiments we show at the experiments section we produced using the mass threshold of 250.

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Eyal Amir and Pedrito Maynard-Reid II.
Last modified: Tue Mar 16 19:04:41 PST 1999