Learning OpenCV

Threshold

Frequently we have done many layers of processing steps and want either to make a final decision about the pixels in an image or to categorically reject those pixels below or above some value while keeping the others. The OpenCV function cvThreshold() accomplishes these tasks (see survey [Sezgin04]). The basic idea is that an array is given, along with a threshold, and then something happens to every element of the array depending on whether it is below or above the threshold.

double cvThreshold(
   CvArr* src,
   CvArr* dst,
   double threshold,
   double max_value,
   int threshold_type
);

As shown in Table 5-5, each threshold type corresponds to a particular comparison operation between the ith source pixel (src_i) and the threshold (denoted in the table by T). Depending on the relationship between the source pixel and the threshold, the destination pixel dst_i may be set to 0, the src_i, or the max_value (denoted in the table by M).

Table 5-5. cvThreshold() threshold_type options

Threshold type	Operation
CV_THRESH_BINARY
CV_THRESH_BINARY_INV
CV_THRESH_TRUNC
CV_THRESH_TOZERO_INV
CV_THRESH_TOZERO

Figure 5-23 should help to clarify the exact implications of each ...