Magic or Undefined Value

The alternative method for handling bad pixels is the so-called magic value method, where a pixel is assigned a special flag when it has an undefined value--it corresponds to a dead element in a CCD chip, for example, or is the result of division by zero. This terminology should not be confused with the HDS ``undefined state'', where a data object exists, but has no value(s) assigned to it. In this document ``undefined'' means ``having a magic value'', unless explicitly stated. An undefined pixel will always be bad, unless repaired in some fashion, and so the data-quality technique is not applicable.

The method is efficient on space: it can always be applied without increasing the data-storage requirement because the flag or magic value replaces the unwanted data value. (For applications where it is important to retain pixel values, or where there is a degree of badness, data quality should be used.) The method enables an application to discover whether a given pixel is bad as soon as it is accessed.

Alternative techniques, based on a list of bad pixels, would be less efficient, because the list would have to be searched repeatedly to see whether given pixels are bad. Such methods would be especially inefficient if large areas of pixels were undefined.

Once a bad pixel has been detected, the application can take appropriate action - flagging the corresponding output pixel as bad, or attempting a repair, perhaps via a choice of interpolation methods.

The HDS undefined state must not be used to indicate bad pixels. If an application finds a data-object in this state, it must report an error, so that the malfunctioning application which created the object can be identified and corrected. The error is fatal.