Editing JPEG 2000 (section)

===Color components transformation===
Initially images have to be transformed from the RGB [[color space]] to another color space, leading to three ''components'' that are handled separately. There are two possible choices:
# Irreversible Color Transform (ICT) uses the well known BT.601 [[YCbCr#JPEG_conversion|YC{{sub|B}}C{{sub|R}}]] color space. It is called "irreversible" because it has to be implemented in floating or fix-point and causes round-off errors. The ICT shall be used only with the 9/7 wavelet transform.
# Reversible Color Transform (RCT) uses a modified YUV color space (almost the same as [[YCoCg|YC{{sub|G}}C{{sub|O}}]]) that does not introduce quantization errors, so it is fully reversible. Proper implementation of the RCT requires that numbers be rounded as specified and cannot be expressed exactly in matrix form. The RCT shall be used only with the 5/3 wavelet transform. The transformations are:
::<math>
\begin{array}{rl}
Y &=& \left\lfloor \frac{R+2G+B}{4} \right\rfloor ;  \\
C_B &=& B - G ; \\
C_R &=& R - G ;
\end{array}
\qquad
\begin{array}{rl}
G &=& Y - \left\lfloor \frac{C_B + C_R}{4} \right\rfloor ; \\
R &=& C_R + G ; \\
B &=& C_B + G.
\end{array}
</math>
If R, G, and B are normalized to the same precision, then numeric precision of C{{sub|B}} and C{{sub|R}} is one bit greater than the precision of the original components. This increase in precision is necessary to ensure reversibility. The [[chrominance]] components can be, but do not necessarily have to be, downscaled in resolution; in fact, since the wavelet transformation already separates images into scales, downsampling is more effectively handled by dropping the finest wavelet scale. This step is called ''multiple component transformation'' in the JPEG&nbsp;2000 language since its usage is not restricted to the [[RGB color model]].<ref>{{Cite web |title=T.800 : Information technology - JPEG 2000 image coding system: Core coding system |url=https://www.itu.int/rec/T-REC-T.800-201511-S/en |access-date=2021-03-19 |website=ITU.int }}</ref>