They are only two: RCT and ICT.
RCT means ‘Reversible Color Transform’.
It is a transform that maps an integer number domain into an integer number codomain and it is used for Lossless compression.
It is divided into Forward RCT e Inverse RCT.
ICT means ‘Irreversible Color Transform’; it is the same transform as the conventional YCBCR used for image and video representation.
The meaning of scalability is not easy to define without saying what scalability concerns.
As far as JPEG2000 is concerned, it is possible to have scalability both in resolution and distortion because of the used coding algorithms. This means that we can obtain a lower memory requirement either by reducing the image resolution or by increasing the distortion; but all this has to be done a posteriori, that is after the generation of the bit-stream.
A compressed bit-stream is scalable in resolution if it has some identifiable elements that represent the subbands at every resolution level, without any dependence on higher resolution levels.
In order to have a distortion scalable bit-stream, inside it there must be identifiable elements highlighting the quality of the compressed representation in a progressive way and that allow to establish the aimed distortion level a posteriori, thus allowing to re-organize the bit-stream in a new embedded bit-stream.
ROI stands for Region Of Interest.
The basic concept deals with the identification of the regions of the image that are more important than the other ones, so they will be encoded with a better quality.
There are two main methods to code the ROI into a JPEG2000 image:
· Block based mode
· Priorization by scaling
‘Block based’ method consists of the possibility of increasing the quality of some blocks that correspond to the most important region of interests.
This approach is not optimal for static ROI solutions, while it is more useful for dynamic ones.
‘Priorization by scaling’ method consists of an increase of the priority of the pixels of the region, so that they can own the most significant bits of the code.
JPEG2000 is based on a technology that is able to code only the most important areas of an image (ROI), which can be in arbitrary form. This system is called 'Maxshift method' and is able to achieve high levels of compression but with the drawback of a loss in the quality of the background.
Unlike other image coding methods, JPEG2000 uses the DWT; if we scale appropriately the wavelet coefficients of the ROI, so that the bits associated to it belong to a bit-plane that is more significant compared to the background bit-planes (fig. 1 and 2), the information associated to it is sent for first. Therefore, by changing the quantization and truncating the obtained code it is possible to get different quality levels for the ROI and the background (thus sacrificing the background image in order to obtain higher compression ratios without compromising the image quality).
Fig. 1 Quantized wavelet coefficients
Fig.2 Wavelet coefficients being quantized after being scaled