Previous Next

                                                   85
          SECTION 3.3                                                                      Filters




                        TABLE 3.11 Optional parameter for the DCTDecode filter
KEY                 TYPE        VALUE

ColorTransform      integer     A code specifying the transformation to be performed on the sample values:
                                   0    No transformation.
                                   1    If the image has three color components, transform RGB values to
                                        YUV before encoding and from YUV to RGB after decoding. If the
                                        image has four components, transform CMYK values to YUVK be-
                                        fore encoding and from YUVK to CMYK after decoding. This option
                                        is ignored if the image has one or two color components.
                                Note: The RGB and YUV used here have nothing to do with the color spaces
                                defined as part of the Adobe imaging model. The purpose of converting from
                                RGB to YUV is to separate luminance and chrominance information (see be-
                                low).
                                The default value of ColorTransform is 1 if the image has three components
                                and 0 otherwise. In other words, conversion between RGB and YUV is per-
                                formed for all three-component images unless explicitly disabled by setting
                                ColorTransform to 0. Additionally, the encoding algorithm inserts an Adobe-
                                defined marker code in the encoded data, indicating the ColorTransform val-
                                ue used. If present, this marker code overrides the ColorTransform value giv-
                                en to DCTDecode. Thus it is necessary to specify ColorTransform only when
                                decoding data that does not contain the Adobe-defined marker code.

          The encoding algorithm can reduce the information loss by making the step size
          in the quantization smaller at the expense of reducing the amount of compression
          achieved by the algorithm. The compression achieved by the JPEG algorithm
          depends on the image being compressed and the amount of loss that is
          acceptable. In general, a compression of 15 : 1 can be achieved without perceptible
          loss of information, and 30 : 1 compression causes little impairment of the image.

          Better compression is often possible for color spaces that treat luminance and
          chrominance separately than for those that do not. The RGB-to-YUV conversion
          provided by the filters is one attempt to separate luminance and chrominance; it
          conforms to CCIR recommendation 601-1. Other color spaces, such as the CIE
          1976 L*a*b* space, may also achieve this objective. The chrominance
          components can then be compressed more than the luminance by using coarser
          sampling or quantization, with no degradation in quality.

Previous Next