Fixed bit rate, intraframe compression and decompression of video
View Patent ↗High quality intraframe-only compression of video can be achieved using rate distortion optimization and without resizing or bit depth modification. The compression process involves transforming portions of the image to generate frequency domain coefficients for each portion. A bit rate for each transformed portion using a plurality of scale factors is determined. Distortion for each portion is estimated according to the plurality of scale factors. A scale factor is selected for each portion to minimize the total distortion in the image to achieve a desired bit rate. A quantization matrix is selected according to the desired bit rate. The frequency domain coefficients for each portion are quantized using the selected plurality of quantizers as scaled by the selected scale factor for the portion. The quantized frequency domain coefficients are encoded using a variable length encoding to provide compressed data for each of the defined portions. The compressed data is output for each of the defined portions to provide a compressed bitstream at the desired bit rate.
1. A method for fixed bit rate, intraframe compression of video, including a sequence of images, comprising, for each image:
transforming portions of the image to generate frequency domain coefficients for each portion;
selecting a quantization matrix according to the desired bit rate;
determining a bit rate for each transformed portion using a plurality of scale factors;
estimating distortion for each portion according to the plurality of scale factors;
selecting a scale factor for each portion to minimize the total distortion in the image to achieve a desired bit rate;
quantizing the frequency domain coefficients for each portion using the selected quantization matrix as scaled by the selected scale factor for the portion;
entropy encoding the quantized frequency domain coefficients using a variable length encoding to provide compressed data for each of the defined portions; and
outputting the compressed data for each of the defined portions to provide a compressed bitstream at the desired bit rate;
wherein entropy encoding comprises:
for each nonzero amplitude coefficient not preceded by a run of zero valued coefficients, determining whether the nonzero amplitude coefficient is in a base range or an index range;
for each nonzero amplitude coefficient not preceded by a run of zero valued coefficients and in the base range, encoding the nonzero amplitude coefficient using a code word from a first set of code words;
for each nonzero amplitude coefficient not preceded by a run of zero valued coefficients and in the index range, determining an index and encoding the nonzero amplitude coefficient using a code word from a second set of code words, followed by the index;
for each nonzero amplitude coefficient preceded by a run of zero valued coefficients, determining whether the nonzero amplitude coefficient is in a base range or an index range;
for each nonzero amplitude coefficient preceded by a run of zero valued coefficients and in the base range, encoding the nonzero amplitude coefficient using a code word from a third set of code words and encoding the run of zero valued coefficients using a code word from a fifth set of code words and after the code word for the nonzero amplitude coefficient; and
for each nonzero amplitude coefficient preceded by a run of zero valued coefficients and in the index range, determining an index and encoding the nonzero amplitude coefficient using a code word from a fourth set of code words, followed by the index and encoding the run of zero valued coefficients using a code word from the fifth set of code words and after the code word for the nonzero amplitude coefficient.