Scene description nodes to support improved chroma-key shape representation of coded arbitrary images and video objects

A method of decoding a bitstream in which the shape of an object has been implicitly encoded, comprising the steps of:

• receiving a first bitstream including coded data for pixels in a box portion of a frame, said box portion containing an object;
• receiving a scene description bitstream including a node containing the key color and chroma-key thresholds for the object;
• decoding the pixel data for said box including said object;
• determining the difference between the color of each of said decoded pixels and the key color;
• assigning a value signifying transparent to each pixel for which the difference is below or equal to a low threshold;
• and
• assigning a value signifying opaque to each pixel for which the difference is greater than a high threshold.

A method of implicitly encoding shape information for an object, comprising the steps of:

• receiving a video frame, including a video object;
• creating a box bounding the video object, the bounding box divided into a plurality of macroblocks, each macroblock comprising a plurality of pixels;
• creating a node including at least a key color and chroma-key thresholds for the video object;
• identifying which macroblocks are inside the object or on the object boundary;
• for each boundary macroblock, replacing each pixel outside the object with a key color;
• for boundary macroblocks and macroblocks inside the object, computing luminance and chrominance pixel difference values by subtracting motion compensated prediction signals from the corresponding luminance and chrominance pixel values;
• for boundary macroblocks and macroblocks inside the object, transforming the luminance and chrominance pixel difference values to frequency domain coefficients;
• scaling the frequency domain coefficients for macroblocks inside the object using a first quantizer;
• scaling the coefficients for boundary macroblocks using a second quantizer to provide a finer level of quantization for said boundary macroblocks as compared to said macroblocks inside the object;
• and
• outputting a first bitstream the scaled coefficients and information identifying the quantizers;
• output a scene description bitstream including said node.

A method of decoding a video bitstream in which the shape of a video object has been implicitly encoded, comprising the steps of:

• receiving a first bitstream representing a video object, the bitstream including scaled frequency domain coefficients for each of a plurality of macroblocks inside the object or on the object boundary;
• receiving a scene description bitstream including a node containing the key color and chroma-key thresholds for the video object;
• resealing the coefficients for macroblocks inside the object using a first quantizer;
• rescaling the coefficients for macroblocks on the object boundary using a second quantizer wherein the second quantizer is smaller than or equal to the first quantizer;
• inverse transforming the frequency domain coefficients to obtain chrominance pixel difference values;
• adding a prediction signal generated by a motion compensator to the luminance and chrominance pixel difference values to obtain the luminance and chrominance pixel values of a reconstructed video object;
• and
• recovering the approximate shape of the object by analyzing the luminance and chrominance values of at least the boundary macroblocks of the reconstructed video object.