Image Processing Reference
In-Depth Information
Besides it, embedding worsens consumer characteristics of received record that is not
always tolerable, and, at special importance of originality of digital record, can be
inadmissible.
Other way of authenticity ascertainment is identification on the basis of recording path
features, which are presented in a digital record.
1.3 Digital images creation in photo cameras
The image on a photosensitive matrix of a photocamera is formed after light passage
through a lens and the blurring filter (LF-filter), further postprocessing of digital signal
received from a matrix [21]. At the analysis of the given circuit it is possible to select the
following main sections of a recording path in digital photographic cameras which can be
used for identification on a basis of features induced in resultant images [22]. The lens and
bayonet joint form identifiable signs (low-frequency defects of the image, vignetting). Usage
of the given signs for the automated and automatic identification is inconvenient in view of
complexity of their extraction from context and built-in compensating circuits and
algorithms in a majority of the modern cameras.
LF-filter (“blurring filter”) is applied to lower moire formed due to space sampling by a
photomatrix of image components with frequencies near and above Nyquist frequency. The
filter forms average and high-frequency stable signs (the shade of the settled dust, filter spot
defects). In view of it placement and, in most cases, impossibility of replacement, the
features imported by it, are similar to the signs imported by the matrix. The photosensitive
matrix unit with ADC forms stable signs in broad band of frequencies (additive and
multiplicative noise of a matrix, defects of sensor elements - pointwise, cluster, column,
line). In the majority of digital photocameras for color image forming the Bayer's [7] method
is used, thus there is only one photosensitive sensor before which the lattice color filter
(color filter array - CFA) is placed. Bayer's grid uses layout of filters of three primary colors
allocated shown on a picture 1.3, where R , G and B accordingly filters of red, green and blue
colors. The number of pixels with filters of green color is twice more than number of pixels
for red and blue components, that reflects spectral sensitivity features of a human eye.
Along with base Bayer pattern there is a set of other variants of a Bayer's matrix, created for
the purpose of increasing sensitivity and color rendition accuracy, generally reached at the
expense of space resolution of chromaticity.
Algorithms of interpolation form average and high-frequency features (correlative
dependences of adjacent pixels, context-dependent interpolation heuristics).
The non-linear processing including noise reduction, color correction, levels correction
(brightness, saturation, contrast). Forms low-frequency (gamma correction) and high-
frequency (increase of contour sharpness), equalizing.
Compression stage features at the given stage are features of a used format (JPEG or other)
such as specific quantization matrixes, a set and placement of the metadada fields.
In the most general case for the analysis of the image received from the real camera, the only
accessible image is image in one of storage formats with lossy compression. On occasion
(cameras of the upper consumer segment, semiprofessional and professional) also the RAW-
version of the image subjected to correction of matrix defects, or compressed by lossless
compression methods (TIFF) the image which has transited all steps of processing, except
compression with quality loss can be the accessible.
Thus it is possible to formulate the requirements necessary for practically applicable systems
of image identification:
Search WWH ::




Custom Search