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:
segmenting the image into a plurality of portions;
transforming the portions of the image to generate frequency domain coefficients for each portion;
selecting, from a plurality of stored quantization matrices each defined for a respective one of a plurality of bit rates, a quantization matrix for the image according to a desired bit rate from the plurality of bit rates for the sequence of images;
determining a bit rate for each transformed portion of the image 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, while achieving 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.
2. The method of claim 1 , wherein the distortion is estimated according to a square of the scale factor.
3. The method of claim 1 , wherein each of the plurality of scale factors is a power of two.
4. The method of claim 1 , wherein determining a bit rate comprises:
determining a maximum scale factor that will cause the image data to be completely quantized; and
interpolating to provide an estimated bit rate for one or more scale factors between the maximum scale factor and a largest scale factor for which a bit rate has been determined.
5. The method of claim 1 , wherein:
the quantization matrix includes a plurality of quantizers;
each quantizer corresponds to a frequency coefficient; and
quantizing the frequency coefficients includes quantizing each frequency coefficient using the corresponding quantizer so as to provide a deadzone having a width greater than a value of the quantizer.