Independently adjusting the quality levels for luminance and chrominance channels in a digital image

A method for compressing a digital image, comprising:

• mapping the quality numbers to sets of quantizers of the wavelet sub-bands;
• generating a luminance preview of the digital image for a selected quality number by:
• applying a discrete wavelet transform to the luminance channel;
• quantizing the discrete wavelet transform coefficients based on the selected quality number;
• de-quantizing the quantized digital image;
• and
• applying an inverse discrete wavelet transform to the de-quantized digital image.

A computer-readable storage medium storing instructions that when executed by a computer cause the computer to perform a method for compressing a digital image, the method comprising:

• obtaining the digital image in which the image components are de-correlated into separate channels containing luminance information and chrominance information;
• receiving a quality number for the luminance channel;
• receiving a quality number for the chrominance channels;
• using the quality number for the luminance channel to determine a target bitrate for the luminance channel;
• using the quality number for the chrominance channels to determine a target bitrate for the chrominance channels;
• and
• given the target bitrate for the luminance channel and the target bit rate for the chrominance channels, performing a rate-distortion optimization separately on the luminance and chrominance channels to determine which portions of the luminance bitstream and which portions of the chrominance bitstream are to be included in the final bitstream.

A computer-readable storage medium storing instructions that when executed by a computer cause the computer to perform a method for compressing a digital image, the method comprising:

• mapping the quality numbers to sets of quantizers of the wavelet sub-bands;
• generating a luminance preview of the digital image for a selected quality number by:
• applying a discrete wavelet transform to the luminance channel;
• quantizing the discrete wavelet transform coefficients based on the selected quality number;
• de-quantizing the quantized digital image;
• and
• applying an inverse discrete wavelet transform to the de-quantized digital image.

An apparatus for compressing a digital image, comprising:

• a compression mechanism which is configured to:
• obtain the digital image in which the image components are de-correlated into separate channels containing luminance information and chrominance information;
• receive a quality number for the luminance channel;
• receive a quality number for the chrominance channels;
• use the quality number for the luminance channel to determine a target bitrate for the luminance channel;
• use the quality number for the chrominance channels to determine a target bitrate for the chrominance channels;
• and
• given the target bitrate for the luminance channel and the target bit rate for the chrominance channels, to perform a rate-distortion optimization separately on the luminance and chrominance channels to determine which portions of the luminance bitstream and which portions of the chrominance bitstream are to be included in the final bitstream.