Watermark detectors have a buffer in which a number of image tiles are
folded and accumulated prior to computing the correlation between buffer
contents and the watermark pattern being looked for. The intention of the
folding and accumulation process is to average out the video content
while accumulating the embedded watermark energy. This no longer appears
to hold for strongly compressed video, such as DIVX, which exhibits a lot
of artificial noise and undesired similarity (block patterns). As a
result thereof, correlation peaks are often below the threshold. In a
similar manner, the compression affects scale detection. According to
this invention, only frames (or parts thereof) that are not so heavily
compressed and therefore have a high probability of carrying enough
watermark energy are folded and accumulated. To this end, a quality
metric is calculated, the quality metric being indicative of the degree
of compression of the data. The quality metric may be calculated based on
the compressed data itself or derived from the decompressed base-band
data. An advantageous example is the number of non-zero DCT coefficients
of a (residue) frame. A determination is then made as to whether to
exclude the frame (or part thereof) from the watermark decode process.
The quality metric may also be used to select data for use in a scale
detection process.